path: root/openvpn.8

.\"  OpenVPN -- An application to securely tunnel IP networks
.\"             over a single TCP/UDP port, with support for SSL/TLS-based
.\"             session authentication and key exchange,
.\"             packet encryption, packet authentication, and
.\"             packet compression.
.\"
.\"  Copyright (C) 2002-2008 OpenVPN Technologies, Inc. <sales@openvpn.net>
.\"
.\"  This program is free software; you can redistribute it and/or modify
.\"  it under the terms of the GNU General Public License version 2
.\"  as published by the Free Software Foundation.
.\"
.\"  This program is distributed in the hope that it will be useful,
.\"  but WITHOUT ANY WARRANTY; without even the implied warranty of
.\"  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
.\"  GNU General Public License for more details.
.\"
.\"  You should have received a copy of the GNU General Public License
.\"  along with this program (see the file COPYING included with this
.\"  distribution); if not, write to the Free Software Foundation, Inc.,
.\"  59 Temple Place, Suite 330, Boston, MA  02111-1307  USA
.\"
.\" Manual page for openvpn
.\" SH section heading
.\" SS subsection heading
.\" LP paragraph
.\" IP indented paragraph
.\" TP hanging label
.TH openvpn 8 "3 August 2005"
.\"*********************************************************
.SH NAME
openvpn \- secure IP tunnel daemon.
.\"*********************************************************
.SH SYNOPSIS
.LP
.nh
.in +4
.ti -4
.B openvpn
[\ \fB\-\-help\fR\ ]
.in -4
.ti +4
.hy

.nh
.in +4
.ti -4
.B openvpn
[\ \fB\-\-config\fR\ \fIfile\fR\ ]
.in -4
.ti +4
.hy

.nh
.in +4
.ti -4
.B openvpn
[\ \fB\-\-genkey\fR\ ]
[\ \fB\-\-secret\fR\ \fIfile\fR\ ]
.in -4
.ti +4
.hy

.nh
.in +4
.ti -4
.B openvpn
[\ \fB\-\-mktun\fR\ ]
[\ \fB\-\-rmtun\fR\ ]
[\ \fB\-\-dev\fR\ \fItunX\ |\ tapX\fR\ ]
[\ \fB\-\-dev\-type\fR\ \fIdevice\-type\fR\ ]
[\ \fB\-\-dev\-node\fR\ \fInode\fR\ ]
[\ \fB\-\-lladdr\fR\ \fIaddress\fR\ ]
[\ \fB\-\-user\fR\ \fIuser\fR\ ]
[\ \fB\-\-group\fR\ \fIgroup\fR\ ]
.in -4
.ti +4
.hy

.nh
.in +4
.ti -4
.B openvpn
[\ \fB\-\-test\-crypto\fR\ ]
[\ \fB\-\-secret\fR\ \fIfile\fR\ ]
[\ \fB\-\-auth\fR\ \fIalg\fR\ ]
[\ \fB\-\-cipher\fR\ \fIalg\fR\ ]
[\ \fB\-\-engine\fR\ ]
[\ \fB\-\-keysize\fR\ \fIn\fR\ ]
[\ \fB\-\-no\-replay\fR\ ]
[\ \fB\-\-no\-iv\fR\ ]
.in -4
.ti +4
.hy

.nh
.in +4
.ti -4
.B openvpn
[\ \fB\-\-allow\-nonadmin\fR\ \fI[TAP\-adapter]\fR\ ]
[\ \fB\-\-allow\-pull\-fqdn\fR\ ]
[\ \fB\-\-askpass\fR\ \fI[file]\fR\ ]
[\ \fB\-\-auth\-nocache\fR\ ]
[\ \fB\-\-auth\-retry\fR\ \fItype\fR\ ]
[\ \fB\-\-auth\-user\-pass\-optional\fR\ ]
[\ \fB\-\-auth\-user\-pass\-verify\fR\ \fIscript\fR\ ]
[\ \fB\-\-auth\-user\-pass\fR\ \fIup\fR\ ]
[\ \fB\-\-auth\fR\ \fIalg\fR\ ]
[\ \fB\-\-auto\-proxy\fR\ ]
[\ \fB\-\-bcast\-buffers\fR\ \fIn\fR\ ]
[\ \fB\-\-ca\fR\ \fIfile\fR\ ]
[\ \fB\-\-ccd\-exclusive\fR\ ]
[\ \fB\-\-cd\fR\ \fIdir\fR\ ]
[\ \fB\-\-cert\fR\ \fIfile\fR\ ]
[\ \fB\-\-chroot\fR\ \fIdir\fR\ ]
[\ \fB\-\-cipher\fR\ \fIalg\fR\ ]
[\ \fB\-\-client\-cert\-not\-required\fR\ ]
[\ \fB\-\-client\-config\-dir\fR\ \fIdir\fR\ ]
[\ \fB\-\-client\-connect\fR\ \fIscript\fR\ ]
[\ \fB\-\-client\-disconnect\fR\ ]
[\ \fB\-\-client\-to\-client\fR\ ]
[\ \fB\-\-client\fR\ ]
[\ \fB\-\-comp\-lzo\fR\ \fI[mode]\fR\ ]
[\ \fB\-\-comp\-noadapt\fR\ ]
[\ \fB\-\-config\fR\ \fIfile\fR\ ]
[\ \fB\-\-connect\-freq\fR\ \fIn\ sec\fR\ ]
[\ \fB\-\-connect\-retry\fR\ \fIn\fR\ ]
[\ \fB\-\-connect\-retry\-max\fR\ \fIn\fR\ ]
[\ \fB\-\-crl\-verify\fR\ \fIcrl\fR\ ]
[\ \fB\-\-cryptoapicert\fR\ \fIselect\-string\fR\ ]
[\ \fB\-\-daemon\fR\ \fI[progname]\fR\ ]
[\ \fB\-\-dev\-node\fR\ \fInode\fR\ ]
[\ \fB\-\-dev\-type\fR\ \fIdevice\-type\fR\ ]
[\ \fB\-\-dev\fR\ \fItunX\ |\ tapX\ |\ null\fR\ ]
[\ \fB\-\-dev\fR\ \fItunX\ |\ tapX\fR\ ]
[\ \fB\-\-dh\fR\ \fIfile\fR\ ]
[\ \fB\-\-dhcp\-option\fR\ \fItype\ [parm]\fR\ ]
[\ \fB\-\-dhcp\-release\fR\ ]
[\ \fB\-\-dhcp\-renew\fR\ ]
[\ \fB\-\-disable\-occ\fR\ ]
[\ \fB\-\-disable\fR\ ]
[\ \fB\-\-down\-pre\fR\ ]
[\ \fB\-\-down\fR\ \fIcmd\fR\ ]
[\ \fB\-\-duplicate\-cn\fR\ ]
[\ \fB\-\-echo\fR\ \fI[parms...]\fR\ ]
[\ \fB\-\-engine\fR\ \fI[engine\-name]\fR\ ]
[\ \fB\-\-explicit\-exit\-notify\fR\ \fI[n]\fR\ ]
[\ \fB\-\-fast\-io\fR\ ]
[\ \fB\-\-float\fR\ ]
[\ \fB\-\-fragment\fR\ \fImax\fR\ ]
[\ \fB\-\-genkey\fR\ ]
[\ \fB\-\-group\fR\ \fIgroup\fR\ ]
[\ \fB\-\-hand\-window\fR\ \fIn\fR\ ]
[\ \fB\-\-hash\-size\fR\ \fIr\ v\fR\ ]
[\ \fB\-\-help\fR\ ]
[\ \fB\-\-http\-proxy\-option\fR\ \fItype\ [parm]\fR\ ]
[\ \fB\-\-http\-proxy\-retry\fR\ ]
[\ \fB\-\-http\-proxy\-timeout\fR\ \fIn\fR\ ]
[\ \fB\-\-http\-proxy\fR\ \fIserver\ port\ [authfile]\ [auth\-method]\fR\ ]
[\ \fB\-\-ifconfig\-noexec\fR\ ]
[\ \fB\-\-ifconfig\-nowarn\fR\ ]
[\ \fB\-\-ifconfig\-pool\-linear\fR\ ]
[\ \fB\-\-ifconfig\-pool\-persist\fR\ \fIfile\ [seconds]\fR\ ]
[\ \fB\-\-ifconfig\-pool\fR\ \fIstart\-IP\ end\-IP\ [netmask]\fR\ ]
[\ \fB\-\-ifconfig\-push\fR\ \fIlocal\ remote\-netmask\fR\ ]
[\ \fB\-\-ifconfig\fR\ \fIl\ rn\fR\ ]
[\ \fB\-\-inactive\fR\ \fIn\ [bytes]\fR\ ]
[\ \fB\-\-inetd\fR\ \fI[wait|nowait]\ [progname]\fR\ ]
[\ \fB\-\-ip\-win32\fR\ \fImethod\fR\ ]
[\ \fB\-\-ipchange\fR\ \fIcmd\fR\ ]
[\ \fB\-\-iproute\fR\ \fIcmd\fR\ ]
[\ \fB\-\-iroute\fR\ \fInetwork\ [netmask]\fR\ ]
[\ \fB\-\-keepalive\fR\ \fIn\ m\fR\ ]
[\ \fB\-\-key\-method\fR\ \fIm\fR\ ]
[\ \fB\-\-key\fR\ \fIfile\fR\ ]
[\ \fB\-\-keysize\fR\ \fIn\fR\ ]
[\ \fB\-\-learn\-address\fR\ \fIcmd\fR\ ]
[\ \fB\-\-link\-mtu\fR\ \fIn\fR\ ]
[\ \fB\-\-local\fR\ \fIhost\fR\ ]
[\ \fB\-\-log\-append\fR\ \fIfile\fR\ ]
[\ \fB\-\-log\fR\ \fIfile\fR\ ]
[\ \fB\-\-suppress-timestamps\fR\ ]
[\ \fB\-\-lport\fR\ \fIport\fR\ ]
[\ \fB\-\-management\-client\-auth\fR\ ]
[\ \fB\-\-management\-client\-group\fR\ \fIg\fR\ ]
[\ \fB\-\-management\-client\-pf\fR\ ]
[\ \fB\-\-management\-client\-user\fR\ \fIu\fR\ ]
[\ \fB\-\-management\-forget\-disconnect\fR\ ]
[\ \fB\-\-management\-hold\fR\ ]
[\ \fB\-\-management\-log\-cache\fR\ \fIn\fR\ ]
[\ \fB\-\-management\-signal\fR\ ]
[\ \fB\-\-management\-query\-passwords\fR\ ]
[\ \fB\-\-management\fR\ \fIIP\ port\ [pw\-file]\fR\ ]
[\ \fB\-\-max\-clients\fR\ \fIn\fR\ ]
[\ \fB\-\-max\-routes\-per\-client\fR\ \fIn\fR\ ]
[\ \fB\-\-mktun\fR\ ]
[\ \fB\-\-mlock\fR\ ]
[\ \fB\-\-mode\fR\ \fIm\fR\ ]
[\ \fB\-\-mssfix\fR\ \fImax\fR\ ]
[\ \fB\-\-mtu\-disc\fR\ \fItype\fR\ ]
[\ \fB\-\-mtu\-test\fR\ ]
[\ \fB\-\-mute\-replay\-warnings\fR\ ]
[\ \fB\-\-mute\fR\ \fIn\fR\ ]
[\ \fB\-\-nice\fR\ \fIn\fR\ ]
[\ \fB\-\-no\-iv\fR\ ]
[\ \fB\-\-no\-name\-remapping\fR\ ]
[\ \fB\-\-no\-replay\fR\ ]
[\ \fB\-\-bind\fR\ ]
[\ \fB\-\-nobind\fR\ ]
[\ \fB\-\-ns\-cert\-type\fR\ \fIclient|server\fR\ ]
[\ \fB\-\-passtos\fR\ ]
[\ \fB\-\-pause\-exit\fR\ ]
[\ \fB\-\-persist\-key\fR\ ]
[\ \fB\-\-persist\-local\-ip\fR\ ]
[\ \fB\-\-persist\-remote\-ip\fR\ ]
[\ \fB\-\-persist\-tun\fR\ ]
[\ \fB\-\-ping\-exit\fR\ \fIn\fR\ ]
[\ \fB\-\-ping\-restart\fR\ \fIn\fR\ ]
[\ \fB\-\-ping\-timer\-rem\fR\ ]
[\ \fB\-\-ping\fR\ \fIn\fR\ ]
[\ \fB\-\-pkcs11\-cert\-private\fR\ \fI[0|1]...\fR\ ]
[\ \fB\-\-pkcs11\-id\fR\ \fIname\fR\ ]
[\ \fB\-\-pkcs11\-id\-management\fR\ ]
[\ \fB\-\-pkcs11\-pin\-cache\fR\ \fIseconds\fR\ ]
[\ \fB\-\-pkcs11\-private\-mode\fR\ \fImode...\fR\ ]
[\ \fB\-\-pkcs11\-protected\-authentication\fR\ \fI[0|1]...\fR\ ]
[\ \fB\-\-pkcs11\-providers\fR\ \fIprovider...\fR\ ]
[\ \fB\-\-pkcs12\fR\ \fIfile\fR\ ]
[\ \fB\-\-plugin\fR\ \fImodule\-pathname\ init\-string\fR\ ]
[\ \fB\-\-port\fR\ \fIport\fR\ ]
[\ \fB\-\-port\-share\fR\ \fIhost\ port\fR\ ]
[\ \fB\-\-proto\fR\ \fIp\fR\ ]
[\ \fB\-\-pull\fR\ ]
[\ \fB\-\-push\-reset\fR\ ]
[\ \fB\-\-push\fR\ \fI"option"\fR\ ]
[\ \fB\-\-rcvbuf\fR\ \fIsize\fR\ ]
[\ \fB\-\-redirect\-gateway\fR\ \fIflags...\fR\ ]
[\ \fB\-\-remap\-usr1\fR\ \fIsignal\fR\ ]
[\ \fB\-\-remote\-random\fR\ ]
[\ \fB\-\-remote\fR\ \fIhost\ [port]\fR\ ]
[\ \fB\-\-remote\-cert\-ku\ \fIv...\fR\ ]
[\ \fB\-\-remote\-cert\-eku\ \fIoid\fR\ ]
[\ \fB\-\-remote\-cert\-tls\ \fIt\fR\ ]
[\ \fB\-\-reneg\-bytes\fR\ \fIn\fR\ ]
[\ \fB\-\-reneg\-pkts\fR\ \fIn\fR\ ]
[\ \fB\-\-reneg\-sec\fR\ \fIn\fR\ ]
[\ \fB\-\-replay\-persist\fR\ \fIfile\fR\ ]
[\ \fB\-\-replay\-window\fR\ \fIn\ [t]\fR\ ]
[\ \fB\-\-resolv\-retry\fR\ \fIn\fR\ ]
[\ \fB\-\-rmtun\fR\ ]
[\ \fB\-\-route\-delay\fR\ \fI[n]\ [w]\fR\ ]
[\ \fB\-\-route\-gateway\fR\ \fIgw\fR\ ]
[\ \fB\-\-route\-method\fR\ \fIm\fR\ ]
[\ \fB\-\-route\-metric\fR\ \fIm\fR\ ]
[\ \fB\-\-route\-noexec\fR\ ]
[\ \fB\-\-route\-nopull\fR\ ]
[\ \fB\-\-route\-up\fR\ \fIcmd\fR\ ]
[\ \fB\-\-route\fR\ \fInetwork\ [netmask]\ [gateway]\ [metric]\fR\ ]
[\ \fB\-\-rport\fR\ \fIport\fR\ ]
[\ \fB\-\-script\-security\fR\ \fIlevel\fR\ ]
[\ \fB\-\-secret\fR\ \fIfile\ [direction]\fR\ ]
[\ \fB\-\-secret\fR\ \fIfile\fR\ ]
[\ \fB\-\-server\-bridge\fR\ \fIgateway\ netmask\ pool\-start\-IP\ pool\-end\-IP\fR\ ]
[\ \fB\-\-server\fR\ \fInetwork\ netmask\fR\ ]
[\ \fB\-\-service\fR\ \fIexit\-event\ [0|1]\fR\ ]
[\ \fB\-\-setenv\fR\ \fIname\ value\fR\ ]
[\ \fB\-\-setenv\-safe\fR\ \fIname\ value\fR\ ]
[\ \fB\-\-shaper\fR\ \fIn\fR\ ]
[\ \fB\-\-show\-adapters\fR\ ]
[\ \fB\-\-show\-ciphers\fR\ ]
[\ \fB\-\-show\-digests\fR\ ]
[\ \fB\-\-show\-engines\fR\ ]
[\ \fB\-\-show\-pkcs11\-ids\fR\ \fIprovider\ [cert_private]\fR\ ]
[\ \fB\-\-show\-net\-up\fR\ ]
[\ \fB\-\-show\-net\fR\ ]
[\ \fB\-\-show\-tls\fR\ ]
[\ \fB\-\-show\-valid\-subnets\fR\ ]
[\ \fB\-\-single\-session\fR\ ]
[\ \fB\-\-sndbuf\fR\ \fIsize\fR\ ]
[\ \fB\-\-socks\-proxy\-retry\fR\ ]
[\ \fB\-\-socks\-proxy\fR\ \fIserver\ [port]\fR\ ]
[\ \fB\-\-status\fR\ \fIfile\ [n]\fR\ ]
[\ \fB\-\-status\-version\fR\ \fIn\fR\ ]
[\ \fB\-\-syslog\fR\ \fI[progname]\fR\ ]
[\ \fB\-\-tap\-sleep\fR\ \fIn\fR\ ]
[\ \fB\-\-tcp\-queue\-limit\fR\ \fIn\fR\ ]
[\ \fB\-\-test\-crypto\fR\ ]
[\ \fB\-\-tls\-auth\fR\ \fIfile\ [direction]\fR\ ]
[\ \fB\-\-tls\-cipher\fR\ \fIl\fR\ ]
[\ \fB\-\-tls\-client\fR\ ]
[\ \fB\-\-tls\-exit\fR\ ]
[\ \fB\-\-tls\-remote\fR\ \fIx509name\fR\ ]
[\ \fB\-\-tls\-server\fR\ ]
[\ \fB\-\-tls\-timeout\fR\ \fIn\fR\ ]
[\ \fB\-\-tls\-verify\fR\ \fIcmd\fR\ ]
[\ \fB\-\-tmp\-dir\fR\ \fIdir\fR\ ]
[\ \fB\-\-topology\fR\ \fImode\fR\ ]
[\ \fB\-\-tran\-window\fR\ \fIn\fR\ ]
[\ \fB\-\-tun\-ipv6\fR\ ]
[\ \fB\-\-tun\-mtu\-extra\fR\ \fIn\fR\ ]
[\ \fB\-\-tun\-mtu\fR\ \fIn\fR\ ]
[\ \fB\-\-txqueuelen\fR\ \fIn\fR\ ]
[\ \fB\-\-up\-delay\fR\ ]
[\ \fB\-\-up\-restart\fR\ ]
[\ \fB\-\-up\fR\ \fIcmd\fR\ ]
[\ \fB\-\-user\fR\ \fIuser\fR\ ]
[\ \fB\-\-username\-as\-common\-name\fR\ ]
[\ \fB\-\-verb\fR\ \fIn\fR\ ]
[\ \fB\-\-win\-sys\fR\ \fIpath|'env'\fR\ ]
[\ \fB\-\-writepid\fR\ \fIfile\fR\ ]
.in -4
.ti +4
.hy
.\"*********************************************************
.SH INTRODUCTION
.LP
OpenVPN is an open source VPN daemon by James Yonan.
Because OpenVPN tries to
be a universal VPN tool offering a great deal of flexibility,
there are a lot of options on this manual page.
If you're new to OpenVPN, you might want to skip ahead to the
examples section where you will see how to construct simple
VPNs on the command line without even needing a configuration file.

Also note that there's more documentation and examples on
the OpenVPN web site:
.I http://openvpn.net/

And if you would like to see a shorter version of this manual,
see the openvpn usage message which can be obtained by
running
.B openvpn
without any parameters.
.\"*********************************************************
.SH DESCRIPTION
.LP
OpenVPN is a robust and highly flexible VPN daemon.
OpenVPN supports SSL/TLS security, ethernet bridging,
TCP or UDP tunnel transport through proxies or NAT,
support for dynamic IP addresses and DHCP,
scalability to hundreds or thousands of users,
and portability to most major OS platforms.

OpenVPN is tightly bound to the OpenSSL library, and derives much
of its crypto capabilities from it.

OpenVPN supports
conventional encryption
using a pre-shared secret key
.B (Static Key mode)
or
public key security
.B (SSL/TLS mode)
using client & server certificates.
OpenVPN also
supports non-encrypted TCP/UDP tunnels.  

OpenVPN is designed to work with the
.B TUN/TAP
virtual networking interface that exists on most platforms.

Overall, OpenVPN aims to offer many of the key features of IPSec but
with a relatively lightweight footprint.
.\"*********************************************************
.SH OPTIONS
OpenVPN allows any option to be placed either on the command line
or in a configuration file.  Though all command line options are preceded
by a double-leading-dash ("--"), this prefix can be removed when
an option is placed in a configuration file.
.\"*********************************************************
.TP
.B --help
Show options.
.\"*********************************************************
.TP
.B --config file
Load additional config options from
.B file
where each line corresponds to one command line option,
but with the leading '--' removed.

If
.B --config file
is the only option to the openvpn command,
the
.B --config
can be removed, and the command can be given as
.B openvpn file

Note that
configuration files can be nested to a reasonable depth.

Double quotation or single quotation characters ("", '')
can be used to enclose single parameters containing whitespace,
and "#" or ";" characters in the first column
can be used to denote comments.

Note that OpenVPN 2.0 and higher performs backslash-based shell
escaping for characters not in single quotations,
so the following mappings should be observed:

.RS
.ft 3
.nf
.sp
\\\\       Maps to a single backslash character (\\).
\\"       Pass a literal doublequote character ("), don't
         interpret it as enclosing a parameter.
\\[SPACE] Pass a literal space or tab character, don't
         interpret it as a parameter delimiter.
.ft
.LP
.RE
.fi

For example on Windows, use double backslashes to
represent pathnames:

.RS
.ft 3
.nf
.sp
secret "c:\\\\OpenVPN\\\\secret.key"
.ft
.LP
.RE
.fi

For examples of configuration files,
see
.I http://openvpn.net/examples.html

Here is an example configuration file:
.RS
.ft 3
.nf
.sp
#
# Sample OpenVPN configuration file for
# using a pre-shared static key.
#
# '#' or ';' may be used to delimit comments.

# Use a dynamic tun device.
dev tun

# Our remote peer
remote mypeer.mydomain

# 10.1.0.1 is our local VPN endpoint
# 10.1.0.2 is our remote VPN endpoint
ifconfig 10.1.0.1 10.1.0.2

# Our pre-shared static key
secret static.key
.ft
.LP
.RE
.fi
.\"*********************************************************
.SS Tunnel Options:
.TP
.B --mode m
Set OpenVPN major mode.  By default, OpenVPN runs in
point-to-point mode ("p2p").  OpenVPN 2.0 introduces
a new mode ("server") which implements a multi-client
server capability.
.\"*********************************************************
.TP
.B --local host
Local host name or IP address for bind.
If specified, OpenVPN will bind to this address only.
If unspecified, OpenVPN will bind to all interfaces.
.\"*********************************************************
.TP
.B --remote host [port] [proto]
Remote host name or IP address.  On the client, multiple
.B --remote
options may be specified for redundancy, each referring
to a different OpenVPN server.  Specifying multiple
.B --remote
options for this purpose is a special case of the more
general connection-profile feature.  See the
.B <connection>
documentation below.

The OpenVPN client will try to connect to a server at
.B host:port
in the order specified by the list of
.B --remote
options.

.B proto
indicates the protocol to use when connecting with the
remote, and may be "tcp" or "udp".

The client will move on to the next host in the list,
in the event of connection failure.
Note that at any given time, the OpenVPN client
will at most be connected to
one server.

Note that since UDP is connectionless, connection failure
is defined by the
.B --ping
and
.B --ping-restart
options.

Note the following corner case:  If you use multiple
.B --remote
options, AND you are dropping root privileges on
the client with
.B --user
and/or
.B --group,
AND the client is running a non-Windows OS, if the client needs
to switch to a different server, and that server pushes
back different TUN/TAP or route settings, the client may lack
the necessary privileges to close and reopen the TUN/TAP interface.
This could cause the client to exit with a fatal error.

If
.B --remote
is unspecified, OpenVPN will listen
for packets from any IP address, but will not act on those packets unless
they pass all authentication tests.  This requirement for authentication
is binding on all potential peers, even those from known and supposedly
trusted IP addresses (it is very easy to forge a source IP address on
a UDP packet).

When used in TCP mode, 
.B --remote
will act as a filter, rejecting connections from any host which does
not match
.B host.

If
.B host
is a DNS name which resolves to multiple IP addresses,
one will be randomly
chosen, providing a sort of basic load-balancing and
failover capability.
.\"*********************************************************
.TP
.B <connection>
Define a client connection
profile.  Client connection profiles are groups of OpenVPN options that
describe how to connect to a given OpenVPN server.  Client connection
profiles are specified within an OpenVPN configuration file, and
each profile is bracketed by
.B <connection>
and
.B </connection>.

An OpenVPN client will try each connection profile sequentially
until it achieves a successful connection.  

.B --remote-random
can be used to initially "scramble" the connection
list.

Here is an example of connection profile usage:

.RS
.ft 3
.nf
.sp
client
dev tun

<connection>
remote 198.19.34.56 1194 udp
</connection>

<connection>
remote 198.19.34.56 443 tcp
</connection>

<connection>
remote 198.19.34.56 443 tcp
http-proxy 192.168.0.8 8080
http-proxy-retry
</connection>

<connection>
remote 198.19.36.99 443 tcp
http-proxy 192.168.0.8 8080
http-proxy-retry
</connection>

persist-key
persist-tun
pkcs12 client.p12
ns-cert-type server
verb 3
.ft
.LP
.RE
.fi

First we try to connect to a server at 198.19.34.56:1194 using UDP.
If that fails, we then try to connect to 198.19.34.56:443 using TCP.
If that also fails, then try connecting through an HTTP proxy at 
192.168.0.8:8080 to 198.19.34.56:443 using TCP.  Finally, try to
connect through the same proxy to a server at 198.19.36.99:443
using TCP.

The following OpenVPN options may be used inside of
a
.B <connection>
block:

.B bind,
.B connect-retry,
.B connect-retry-max,
.B connect-timeout,
.B float,
.B http-proxy,
.B http-proxy-option,
.B http-proxy-retry,
.B http-proxy-timeout,
.B local,
.B lport,
.B nobind,
.B port,
.B proto,
.B remote,
.B rport,
.B socks-proxy, and
.B socks-proxy-retry.

A defaulting mechanism exists for specifying options to apply to
all
.B <connection>
profiles.  If any of the above options (with the exception of
.B remote
) appear outside of a
.B <connection>
block, but in a configuration file which has one or more
.B <connection>
blocks, the option setting will be used as a default for
.B <connection>
blocks which follow it in the configuration file.

For example, suppose the
.B nobind
option were placed in the sample configuration file above, near
the top of the file, before the first
.B <connection>
block.  The effect would be as if
.B nobind
were declared in all
.B <connection>
blocks below it.

.\"*********************************************************
.TP
.B --remote-random
When multiple
.B --remote
address/ports are specified, or if connection profiles are being
used, initially randomize the order of the list
as a kind of basic load-balancing measure.
.\"*********************************************************
.TP
.B --proto p
Use protocol
.B p
for communicating with remote host.
.B p
can be
.B udp,
.B tcp-client,
or
.B tcp-server.

The default protocol is
.B udp
when
.B --proto
is not specified.

For UDP operation,
.B --proto udp
should be specified on both peers.

For TCP operation, one peer must use
.B --proto tcp-server
and the other must use
.B --proto tcp-client.
A peer started with
.B tcp-server
will wait indefinitely for an incoming connection.  A peer
started with
.B tcp-client
will attempt to connect, and if that fails, will sleep for 5
seconds (adjustable via the
.B --connect-retry
option) and try again infinite or up to N retries (adjustable via the
.B --connect-retry-max
option).  Both TCP client and server will simulate
a SIGUSR1 restart signal if either side resets the connection.

OpenVPN is designed to operate optimally over UDP, but TCP capability is provided
for situations where UDP cannot be used.
In comparison with UDP, TCP will usually be
somewhat less efficient and less robust when used over unreliable or congested
networks.

This article outlines some of problems with tunneling IP over TCP:

.I http://sites.inka.de/sites/bigred/devel/tcp-tcp.html

There are certain cases, however, where using TCP may be advantageous from
a security and robustness perspective, such as tunneling non-IP or
application-level UDP protocols, or tunneling protocols which don't
possess a built-in reliability layer.
.\"*********************************************************
.TP
.B --connect-retry n
For
.B --proto tcp-client,
take
.B n
as the
number of seconds to wait
between connection retries (default=5).
.\"*********************************************************
.TP
.B --connect-retry-max n
For
.B --proto tcp-client,
take
.B n
as the
number of retries of connection attempt (default=infinite).
.\"*********************************************************
.TP
.B --auto-proxy
Try to sense HTTP or SOCKS proxy settings automatically.
If no settings are present, a direct connection will be attempted.
If both HTTP and SOCKS settings are present, HTTP will be preferred.
If the HTTP proxy server requires a password, it will be queried from
stdin or the management interface.  If the underlying OS doesn't support an API for
returning proxy settings, a direct connection will be attempted.
Currently, only Windows clients support this option via the
InternetQueryOption API.
This option exists in OpenVPN 2.1 or higher.
.\"*********************************************************
.TP
.B --http-proxy server port [authfile|'auto'] [auth-method]
Connect to remote host through an HTTP proxy at address
.B server
and port
.B port.
If HTTP Proxy-Authenticate is required,
.B authfile
is a file containing a username and password on 2 lines, or
"stdin" to prompt from console.

.B auth-method
should be one of "none", "basic", or "ntlm".

The
.B auto
flag causes OpenVPN to automatically determine the
.B auth-method
and query stdin or the management interface for
username/password credentials, if required.  This flag
exists on OpenVPN 2.1 or higher.
.\"*********************************************************
.TP
.B --http-proxy-retry
Retry indefinitely on HTTP proxy errors.  If an HTTP proxy error
occurs, simulate a SIGUSR1 reset.
.\"*********************************************************
.TP
.B --http-proxy-timeout n
Set proxy timeout to
.B n
seconds, default=5.
.\"*********************************************************
.TP
.B --http-proxy-option type [parm]
Set extended HTTP proxy options.
Repeat to set multiple options.

.B VERSION version --
Set HTTP version number to
.B version
(default=1.0).

.B AGENT user-agent --
Set HTTP "User-Agent" string to
.B user-agent.
.\"*********************************************************
.TP
.B --socks-proxy server [port]
Connect to remote host through a Socks5 proxy at address
.B server
and port
.B port
(default=1080).
.\"*********************************************************
.TP
.B --socks-proxy-retry
Retry indefinitely on Socks proxy errors.  If a Socks proxy error
occurs, simulate a SIGUSR1 reset.
.\"*********************************************************
.TP
.B --resolv-retry n
If hostname resolve fails for
.B --remote,
retry resolve for
.B n
seconds before failing.

Set
.B n
to "infinite" to retry indefinitely.

By default,
.B --resolv-retry infinite
is enabled.  You can disable by setting n=0.
.\"*********************************************************
.TP
.B --float
Allow remote peer to change its IP address and/or port number, such as due to
DHCP (this is the default if
.B --remote
is not used).
.B --float
when specified with
.B --remote
allows an OpenVPN session to initially connect to a peer
at a known address, however if packets arrive from a new
address and pass all authentication tests, the new address
will take control of the session.  This is useful when
you are connecting to a peer which holds a dynamic address
such as a dial-in user or DHCP client.

Essentially,
.B --float
tells OpenVPN to accept authenticated packets
from any address, not only the address which was specified in the
.B --remote
option.
.\"*********************************************************
.TP
.B --ipchange cmd
Execute shell command
.B cmd
when our remote ip-address is initially authenticated or
changes.

Execute as:

.B cmd ip_address port_number

Don't use
.B --ipchange
in
.B --mode server
mode.  Use a
.B --client-connect
script instead.

See the "Environmental Variables" section below for
additional parameters passed as environmental variables.

Note that
.B cmd
can be a shell command with multiple arguments, in which
case all OpenVPN-generated arguments will be appended
to
.B cmd
to build a command line which will be passed to the script.

If you are running in a dynamic IP address environment where
the IP addresses of either peer could change without notice,
you can use this script, for example, to edit the
.I /etc/hosts
file with the current address of the peer.  The script will
be run every time the remote peer changes its IP address.

Similarly if
.I our
IP address changes due to DHCP, we should configure
our IP address change script (see man page for
.BR dhcpcd (8)
) to deliver a
.B SIGHUP
or
.B SIGUSR1
signal to OpenVPN.  OpenVPN will then
reestablish a connection with its most recently authenticated
peer on its new IP address.
.\"*********************************************************
.TP
.B --port port
TCP/UDP port number for both local and remote.  The current
default of 1194 represents the official IANA port number
assignment for OpenVPN and has been used since version 2.0-beta17.
Previous versions used port 5000 as the default.
.\"*********************************************************
.TP
.B --lport port
TCP/UDP port number for bind.
.\"*********************************************************
.TP
.B --rport port
TCP/UDP port number for remote.
.\"*********************************************************
.TP
.B --bind
Bind to local address and port. This is the default unless any of 
.B --proto tcp-client
,
.B --http-proxy
or
.B --socks-proxy
are used.
.\"*********************************************************
.TP
.B --nobind
Do not bind to local address and port.  The IP stack will allocate
a dynamic port for returning packets.  Since the value of the dynamic port
could not be known in advance by a peer, this option is only suitable for
peers which will be initiating connections by using the
.B --remote
option.
.\"*********************************************************
.TP
.B --dev tunX | tapX | null
TUN/TAP virtual network device (
.B X
can be omitted for a dynamic device.)

See examples section below
for an example on setting up a TUN device.

You must use either tun devices on both ends of the connection
or tap devices on both ends.  You cannot mix them, as they
represent different underlying protocols.

.B tun
devices encapsulate IPv4 or IPv6 while
.B tap
devices encapsulate Ethernet 802.3.
.\"*********************************************************
.TP
.B --dev-type device-type
Which device type are we using?
.B device-type
should be
.B tun
or
.B tap.
Use this option only if the TUN/TAP device used with
.B --dev
does not begin with
.B tun
or
.B tap.
.\"*********************************************************
.TP
.B --topology mode
Configure virtual addressing topology when running in
.B --dev tun
mode.  This directive has no meaning in
.B --dev tap
mode, which always uses a
.B subnet
topology.

If you set this directive on the server, the
.B --server
and
.B --server-bridge
directives will automatically push your chosen topology setting to clients
as well.  This directive can also be manually pushed to clients.  Like the
.B --dev
directive, this directive must always be compatible between client and server.

.B mode
can be one of:

.B net30 --
Use a point-to-point topology, by allocating one /30 subnet per client.
This is designed to allow point-to-point semantics when some
or all of the connecting clients might be Windows systems.  This is the
default on OpenVPN 2.0.

.B p2p --
Use a point-to-point topology where the remote endpoint of the client's
tun interface always points to the local endpoint of the server's tun interface.
This mode allocates a single IP address per connecting client.
Only use
when none of the connecting clients are Windows systems.  This mode
is functionally equivalent to the
.B --ifconfig-pool-linear
directive which is available in OpenVPN 2.0 and is now deprecated.

.B subnet --
Use a subnet rather than a point-to-point topology by
configuring the tun interface with a local IP address and subnet mask,
similar to the topology used in
.B --dev tap
and ethernet bridging mode.
This mode allocates a single IP address per connecting client and works on
Windows as well.  Only available when server and clients are OpenVPN 2.1 or
higher, or OpenVPN 2.0.x which has been manually patched with the
.B --topology
directive code.  When used on Windows, requires version 8.2 or higher
of the TAP-Win32 driver.  When used on *nix, requires that the tun
driver supports an
.BR ifconfig (8)
command which sets a subnet instead of a remote endpoint IP address.

This option exists in OpenVPN 2.1 or higher.
.\"*********************************************************
.TP
.B --tun-ipv6
Build a tun link capable of forwarding IPv6 traffic.
Should be used in conjunction with
.B --dev tun
or
.B --dev tunX.
A warning will be displayed
if no specific IPv6 TUN support for your OS has been compiled into OpenVPN.
.\"*********************************************************
.TP
.B --dev-node node
Explicitly set the device node rather than using
/dev/net/tun, /dev/tun, /dev/tap, etc.  If OpenVPN
cannot figure out whether
.B node
is a TUN or TAP device based on the name, you should
also specify
.B --dev-type tun
or
.B --dev-type tap.

On Windows systems, select the TAP-Win32 adapter which
is named
.B node
in the Network Connections Control Panel or the
raw GUID of the adapter enclosed by braces.
The
.B --show-adapters
option under Windows can also be used
to enumerate all available TAP-Win32
adapters and will show both the network
connections control panel name and the GUID for
each TAP-Win32 adapter.
.TP
.B --lladdr address
Specify the link layer address, more commonly known as the MAC address.
Only applied to TAP devices.
.\"*********************************************************
.TP
.B --iproute cmd
Set alternate command to execute instead of default iproute2 command.
May be used in order to execute OpenVPN in unprivileged environment.
.\"*********************************************************
.TP
.B --ifconfig l rn
Set TUN/TAP adapter parameters. 
.B l
is the IP address of the local VPN endpoint.
For TUN devices,
.B rn
is the IP address of the remote VPN endpoint.
For TAP devices,
.B rn
is the subnet mask of the virtual ethernet segment
which is being created or connected to.

For TUN devices, which facilitate virtual
point-to-point IP connections,
the proper usage of
.B --ifconfig
is to use two private IP addresses
which are not a member of any
existing subnet which is in use.
The IP addresses may be consecutive
and should have their order reversed
on the remote peer.  After the VPN
is established, by pinging
.B rn,
you will be pinging across the VPN.

For TAP devices, which provide
the ability to create virtual
ethernet segments,
.B --ifconfig
is used to set an IP address and
subnet mask just as a physical
ethernet adapter would be
similarly configured.  If you are
attempting to connect to a remote
ethernet bridge, the IP address
and subnet should be set to values
which would be valid on the
the bridged ethernet segment (note
also that DHCP can be used for the
same purpose).

This option, while primarily a proxy for the
.BR ifconfig (8)
command, is designed to simplify TUN/TAP
tunnel configuration by providing a
standard interface to the different
ifconfig implementations on different
platforms.

.B --ifconfig
parameters which are IP addresses can
also be specified as a DNS or /etc/hosts
file resolvable name.

For TAP devices,
.B --ifconfig
should not be used if the TAP interface will be
getting an IP address lease from a DHCP
server.
.\"*********************************************************
.TP
.B --ifconfig-noexec
Don't actually execute ifconfig/netsh commands, instead
pass
.B --ifconfig
parameters to scripts using environmental variables.
.\"*********************************************************
.TP
.B --ifconfig-nowarn
Don't output an options consistency check warning
if the
.B --ifconfig
option on this side of the
connection doesn't match the remote side.  This is useful
when you want to retain the overall benefits of the
options consistency check (also see
.B --disable-occ
option) while only disabling the ifconfig component of
the check.

For example,
if you have a configuration where the local host uses
.B --ifconfig
but the remote host does not, use
.B --ifconfig-nowarn
on the local host.

This option will also silence warnings about potential
address conflicts which occasionally annoy more experienced
users by triggering "false positive" warnings.
.\"*********************************************************
.TP
.B --route network/IP [netmask] [gateway] [metric]
Add route to routing table after connection is established.
Multiple routes can be specified.  Routes will be
automatically torn down in reverse order prior to
TUN/TAP device close.

This option is intended as
a convenience proxy for the
.BR route (8)
shell command,
while at the same time providing portable semantics
across OpenVPN's platform space.

.B netmask
default -- 255.255.255.255

.B gateway
default -- taken from
.B --route-gateway
or the second parameter to
.B --ifconfig
when
.B --dev tun
is specified.

.B metric
default -- taken from
.B --route-metric
otherwise 0.

The default can be specified by leaving an option blank or setting
it to "default".

The
.B network
and
.B gateway
parameters can
also be specified as a DNS or /etc/hosts
file resolvable name, or as one of three special keywords:

.B vpn_gateway
-- The remote VPN endpoint address
(derived either from
.B --route-gateway
or the second parameter to
.B --ifconfig
when
.B --dev tun
is specified).

.B net_gateway
-- The pre-existing IP default gateway, read from the routing
table (not supported on all OSes).

.B remote_host
-- The
.B --remote
address if OpenVPN is being run in client mode, and is undefined in server mode.
.\"*********************************************************
.TP
.B --route-gateway gw|'dhcp'
Specify a default gateway
.B gw
for use with
.B --route.

If
.B dhcp
is specified as the parameter,
the gateway address will be extracted from a DHCP
negotiation with the OpenVPN server-side LAN.
.\"*********************************************************
.TP
.B --route-metric m
Specify a default metric
.B m
for use with
.B --route.
.\"*********************************************************
.TP
.B --route-delay [n] [w]
Delay
.B n
seconds (default=0) after connection
establishment, before adding routes. If
.B n
is 0, routes will be added immediately upon connection
establishment.  If
.B --route-delay
is omitted, routes will be added immediately after TUN/TAP device
open and
.B --up
script execution, before any
.B --user
or 
.B --group
privilege downgrade (or
.B --chroot
execution.)

This option is designed to be useful in scenarios where DHCP is
used to set
tap adapter addresses.  The delay will give the DHCP handshake
time to complete before routes are added.

On Windows,
.B --route-delay
tries to be more intelligent by waiting
.B w
seconds (w=30 by default)
for the TAP-Win32 adapter to come up before adding routes.
.\"*********************************************************
.TP
.B --route-up cmd
Execute shell command
.B cmd
after routes are added, subject to
.B --route-delay.

See the "Environmental Variables" section below for
additional parameters passed as environmental variables.

Note that
.B cmd
can be a shell command with multiple arguments.
.\"*********************************************************
.TP
.B --route-noexec
Don't add or remove routes automatically.  Instead pass routes to
.B --route-up
script using environmental variables.
.\"*********************************************************
.TP
.B --route-nopull
When used with
.B --client
or
.B --pull,
accept options pushed by server EXCEPT for routes.

When used on the client, this option effectively bars the
server from adding routes to the client's routing table,
however note that this option still allows the server
to set the TCP/IP properties of the client's TUN/TAP interface.
.\"*********************************************************
.TP
.B --allow-pull-fqdn
Allow client to pull DNS names from server (rather than being limited
to IP address) for
.B --ifconfig,
.B --route,
and
.B --route-gateway.
.\"*********************************************************
.TP
.B --redirect-gateway flags...
(Experimental) Automatically execute routing commands to cause all outgoing IP traffic
to be redirected over the VPN.

This option performs three steps:

.B (1)
Create a static route for the
.B --remote
address which forwards to the pre-existing default gateway.
This is done so that
.B (3)
will not create a routing loop.

.B (2)
Delete the default gateway route.

.B (3)
Set the new default gateway to be the VPN endpoint address (derived either from
.B --route-gateway
or the second parameter to
.B --ifconfig
when
.B --dev tun
is specified).

When the tunnel is torn down, all of the above steps are reversed so
that the original default route is restored.

Option flags:

.B local --
Add the
.B local
flag if both OpenVPN servers are directly connected via a common subnet,
such as with wireless.  The
.B local
flag will cause step
.B 1
above to be omitted.

.B def1 --
Use this flag to override
the default gateway by using 0.0.0.0/1 and 128.0.0.0/1
rather than 0.0.0.0/0.  This has the benefit of overriding
but not wiping out the original default gateway. 

.B bypass-dhcp --
Add a direct route to the DHCP server (if it is non-local) which
bypasses the tunnel
(Available on Windows clients, may not be available
on non-Windows clients).

.B bypass-dns --
Add a direct route to the DNS server(s) (if they are non-local) which
bypasses the tunnel
(Available on Windows clients, may not be available
on non-Windows clients).

Using the def1 flag is highly recommended.
.\"*********************************************************
.TP
.B --link-mtu n
Sets an upper bound on the size of UDP packets which are sent
between OpenVPN peers.  It's best not to set this parameter unless
you know what you're doing.
.\"*********************************************************
.TP
.B --tun-mtu n
Take the TUN device MTU to be
.B n
and derive the link MTU
from it (default=1500).  In most cases, you will probably want to
leave this parameter set to its default value.

The MTU (Maximum Transmission Units) is
the maximum datagram size in bytes that can be sent unfragmented
over a particular network path.  OpenVPN requires that packets
on the control or data channels be sent unfragmented.

MTU problems often manifest themselves as connections which
hang during periods of active usage.

It's best to use the
.B --fragment
and/or
.B --mssfix
options to deal with MTU sizing issues.
.\"*********************************************************
.TP
.B --tun-mtu-extra n
Assume that the TUN/TAP device might return as many as
.B n
bytes more than the
.B --tun-mtu
size on read.  This parameter defaults to 0, which is sufficient for
most TUN devices.  TAP devices may introduce additional overhead in excess
of the MTU size, and a setting of 32 is the default when TAP devices are used.
This parameter only controls internal OpenVPN buffer sizing,
so there is no transmission overhead associated with using a larger value.
.\"*********************************************************
.TP
.B --mtu-disc type
Should we do Path MTU discovery on TCP/UDP channel?  Only supported on OSes such
as Linux that supports the necessary system call to set.

.B 'no'
-- Never send DF (Don't Fragment) frames
.br
.B 'maybe'
-- Use per-route hints
.br
.B 'yes'
-- Always DF (Don't Fragment)
.br
.\"*********************************************************
.TP
.B --mtu-test
To empirically measure MTU on connection startup,
add the
.B --mtu-test
option to your configuration.
OpenVPN will send ping packets of various sizes
to the remote peer and measure the largest packets
which were successfully received.  The
.B --mtu-test
process normally takes about 3 minutes to complete.
.\"*********************************************************
.TP
.B --fragment max
Enable internal datagram fragmentation so
that no UDP datagrams are sent which
are larger than
.B max
bytes.

The
.B max
parameter is interpreted in the same way as the
.B --link-mtu
parameter, i.e. the UDP packet size after encapsulation
overhead has been added in, but not including
the UDP header itself.

The
.B --fragment
option only makes sense when you are using the UDP protocol (
.B --proto udp
).

.B --fragment
adds 4 bytes of overhead per datagram.

See the
.B --mssfix
option below for an important related option to
.B --fragment.

It should also be noted that this option is not meant to replace
UDP fragmentation at the IP stack level.  It is only meant as a
last resort when path MTU discovery is broken.  Using this option
is less efficient than fixing path MTU discovery for your IP link and
using native IP fragmentation instead.

Having said that, there are circumstances where using OpenVPN's
internal fragmentation capability may be your only option, such
as tunneling a UDP multicast stream which requires fragmentation.
.\"*********************************************************
.TP
.B --mssfix max
Announce to TCP sessions running over the tunnel that they should limit
their send packet sizes such that after OpenVPN has encapsulated them,
the resulting UDP packet size that OpenVPN sends to its peer will not
exceed
.B max
bytes.

The
.B max
parameter is interpreted in the same way as the
.B --link-mtu
parameter, i.e. the UDP packet size after encapsulation
overhead has been added in, but not including
the UDP header itself.

The
.B --mssfix
option only makes sense when you are using the UDP protocol
for OpenVPN peer-to-peer communication, i.e.
.B --proto udp.

.B --mssfix
and
.B --fragment
can be ideally used together, where
.B --mssfix
will try to keep TCP from needing
packet fragmentation in the first place,
and if big packets come through anyhow
(from protocols other than TCP),
.B --fragment
will internally fragment them.

Both
.B --fragment
and
.B --mssfix
are designed to work around cases where Path MTU discovery
is broken on the network path between OpenVPN peers.

The usual symptom of such a breakdown is an OpenVPN
connection which successfully starts, but then stalls
during active usage.

If
.B --fragment
and
.B --mssfix
are used together,
.B --mssfix
will take its default
.B max
parameter from the
.B --fragment max
option.

Therefore, one could lower the maximum UDP packet size
to 1300 (a good first try for solving MTU-related
connection problems) with the following options:

.B --tun-mtu 1500 --fragment 1300 --mssfix
.\"*********************************************************
.TP
.B --sndbuf size
Set the TCP/UDP socket send buffer size.
Currently defaults to 65536 bytes.
.\"*********************************************************
.TP
.B --rcvbuf size
Set the TCP/UDP socket receive buffer size.
Currently defaults to 65536 bytes.
.\"*********************************************************
.TP
.B --socket-flags flags...
Apply the given flags to the OpenVPN transport socket.
Currently, only
.B TCP_NODELAY
is supported.

The
.B TCP_NODELAY
socket flag is useful in TCP mode, and causes the kernel
to send tunnel packets immediately over the TCP connection without
trying to group several smaller packets into a larger packet.
This can result in a considerably improvement in latency.

This option is pushable from server to client, and should be used
on both client and server for maximum effect.
.\"*********************************************************
.TP
.B --txqueuelen n
(Linux only) Set the TX queue length on the TUN/TAP interface.
Currently defaults to 100.
.\"*********************************************************
.TP
.B --shaper n
Limit bandwidth of outgoing tunnel data to
.B n
bytes per second on the TCP/UDP port.
If you want to limit the bandwidth
in both directions, use this option on both peers.

OpenVPN uses the following algorithm to implement
traffic shaping: Given a shaper rate of
.I n
bytes per second, after a datagram write of
.I b
bytes is queued on the TCP/UDP port, wait a minimum of
.I (b / n)
seconds before queuing the next write.

It should be noted that OpenVPN supports multiple
tunnels between the same two peers, allowing you
to construct full-speed and reduced bandwidth tunnels
at the same time,
routing low-priority data such as off-site backups
over the reduced bandwidth tunnel, and other data
over the full-speed tunnel.

Also note that for low bandwidth tunnels
(under 1000 bytes per second), you should probably
use lower MTU values as well (see above), otherwise
the packet latency will grow so large as to trigger
timeouts in the TLS layer and TCP connections running
over the tunnel.

OpenVPN allows
.B n
to be between 100 bytes/sec and 100 Mbytes/sec.
.\"*********************************************************
.TP
.B --inactive n [bytes]
Causes OpenVPN to exit after
.B n
seconds of inactivity on the TUN/TAP device.  The time length
of inactivity is measured since the last incoming tunnel packet.

If the optional
.B bytes
parameter is included,
exit after n seconds of activity on tun/tap device
produces a combined in/out byte count that is less than
.B bytes.
.\"*********************************************************
.TP
.B --ping n
Ping remote over the TCP/UDP control channel
if no packets have been sent for at least
.B n
seconds (specify
.B --ping
on both peers to cause ping packets to be sent in both directions since
OpenVPN ping packets are not echoed like IP ping packets).
When used in one of OpenVPN's secure modes (where
.B --secret, --tls-server,
or
.B --tls-client
is specified), the ping packet
will be cryptographically secure.

This option has two intended uses:

(1) Compatibility
with stateful firewalls.  The periodic ping will ensure that
a stateful firewall rule which allows OpenVPN UDP packets to
pass will not time out.

(2) To provide a basis for the remote to test the existence
of its peer using the
.B --ping-exit
option.
.\"*********************************************************
.TP
.B --ping-exit n
Causes OpenVPN to exit after
.B n
seconds pass without reception of a ping
or other packet from remote.
This option can be combined with
.B --inactive, --ping,
and
.B --ping-exit
to create a two-tiered inactivity disconnect.

For example,

.B openvpn [options...] --inactive 3600 --ping 10 --ping-exit 60

when used on both peers will cause OpenVPN to exit within 60
seconds if its peer disconnects, but will exit after one
hour if no actual tunnel data is exchanged.
.\"*********************************************************
.TP
.B --ping-restart n
Similar to
.B --ping-exit,
but trigger a
.B SIGUSR1
restart after
.B n
seconds pass without reception of a ping
or other packet from remote.

This option is useful in cases
where the remote peer has a dynamic IP address and
a low-TTL DNS name is used to track the IP address using
a service such as
.I http://dyndns.org/
+ a dynamic DNS client such
as
.B ddclient.

If the peer cannot be reached, a restart will be triggered, causing
the hostname used with
.B --remote
to be re-resolved (if
.B --resolv-retry
is also specified).

In server mode,
.B --ping-restart, --inactive,
or any other type of internally generated signal will always be
applied to
individual client instance objects, never to whole server itself.
Note also in server mode that any internally generated signal
which would normally cause a restart, will cause the deletion
of the client instance object instead.

In client mode, the
.B --ping-restart
parameter is set to 120 seconds by default.  This default will
hold until the client pulls a replacement value from the server, based on
the
.B --keepalive
setting in the server configuration.
To disable the 120 second default, set
.B --ping-restart 0
on the client.

See the signals section below for more information
on
.B SIGUSR1.

Note that the behavior of
.B SIGUSR1
can be modified by the
.B --persist-tun, --persist-key, --persist-local-ip,
and
.B --persist-remote-ip
options.

Also note that
.B --ping-exit
and
.B --ping-restart
are mutually exclusive and cannot be used together.
.\"*********************************************************
.TP
.B --keepalive n m
A helper directive designed to simplify the expression of
.B --ping
and
.B --ping-restart
in server mode configurations.

For example,
.B --keepalive 10 60
expands as follows:

.RS
.ft 3
.nf
.sp
 if mode server:
   ping 10
   ping-restart 120
   push "ping 10"
   push "ping-restart 60"
 else
   ping 10
   ping-restart 60
.ft
.LP
.RE
.fi
.\"*********************************************************
.TP
.B --ping-timer-rem
Run the
.B --ping-exit
/
.B --ping-restart
timer only if we have a remote address.  Use this option if you are
starting the daemon in listen mode (i.e. without an explicit
.B --remote
peer), and you don't want to start clocking timeouts until a remote
peer connects.
.\"*********************************************************
.TP
.B --persist-tun
Don't close and reopen TUN/TAP device or run up/down scripts
across
.B SIGUSR1
or
.B --ping-restart
restarts.

.B SIGUSR1
is a restart signal similar to
.B SIGHUP,
but which offers finer-grained control over
reset options.
.\"*********************************************************
.TP
.B --persist-key
Don't re-read key files across
.B SIGUSR1
or
.B --ping-restart.

This option can be combined with
.B --user nobody
to allow restarts triggered by the
.B SIGUSR1
signal.
Normally if you drop root privileges in OpenVPN,
the daemon cannot be restarted since it will now be unable to re-read protected
key files.

This option solves the problem by persisting keys across
.B SIGUSR1
resets, so they don't need to be re-read.
.\"*********************************************************
.TP
.B --persist-local-ip
Preserve initially resolved local IP address and port number
across
.B SIGUSR1
or
.B --ping-restart
restarts.
.\"*********************************************************
.TP
.B --persist-remote-ip
Preserve most recently authenticated remote IP address and port number
across
.B SIGUSR1
or
.B --ping-restart
restarts.
.\"*********************************************************
.TP
.B --mlock
Disable paging by calling the POSIX mlockall function.
Requires that OpenVPN be initially run as root (though
OpenVPN can subsequently downgrade its UID using the
.B --user
option).

Using this option ensures that key material and tunnel
data are never written to disk due to virtual
memory paging operations which occur under most
modern operating systems.  It ensures that even if an
attacker was able to crack the box running OpenVPN, he
would not be able to scan the system swap file to
recover previously used
ephemeral keys, which are used for a period of time
governed by the
.B --reneg
options (see below), then are discarded.

The downside
of using
.B --mlock
is that it will reduce the amount of physical
memory available to other applications.
.\"*********************************************************
.TP
.B --up cmd
Shell command to run after successful TUN/TAP device open
(pre
.B --user
UID change).  The up script is useful for specifying route
commands which route IP traffic destined for
private subnets which exist at the other
end of the VPN connection into the tunnel.

For
.B --dev tun
execute as:

.B cmd tun_dev tun_mtu link_mtu ifconfig_local_ip ifconfig_remote_ip [ init | restart ]

For
.B --dev tap
execute as:

.B cmd tap_dev tap_mtu link_mtu ifconfig_local_ip ifconfig_netmask [ init | restart ]

See the "Environmental Variables" section below for
additional parameters passed as environmental variables.

Note that
.B cmd
can be a shell command with multiple arguments, in which
case all OpenVPN-generated arguments will be appended
to
.B cmd
to build a command line which will be passed to the shell.

Typically,
.B cmd
will run a script to add routes to the tunnel.

Normally the up script is called after the TUN/TAP device is opened.
In this context, the last command line parameter passed to the script
will be
.I init.
If the
.B --up-restart
option is also used, the up script will be called for restarts as
well.  A restart is considered to be a partial reinitialization
of OpenVPN where the TUN/TAP instance is preserved (the
.B --persist-tun
option will enable such preservation).  A restart
can be generated by a SIGUSR1 signal, a
.B --ping-restart
timeout, or a connection reset when the TCP protocol is enabled
with the
.B --proto
option.  If a restart occurs, and
.B --up-restart
has been specified, the up script will be called with
.I restart
as the last parameter.

The following standalone example shows how the
.B --up
script can be called in both an initialization and restart context.
(NOTE: for security reasons, don't run the following example unless UDP port
9999 is blocked by your firewall.  Also, the example will run indefinitely,
so you should abort with control-c).

.B openvpn --dev tun --port 9999 --verb 4 --ping-restart 10 --up 'echo up' --down 'echo down' --persist-tun --up-restart

Note that OpenVPN also provides the
.B --ifconfig
option to automatically ifconfig the TUN device,
eliminating the need to define an
.B --up
script, unless you also want to configure routes
in the
.B --up
script.

If
.B --ifconfig
is also specified, OpenVPN will pass the ifconfig local
and remote endpoints on the command line to the
.B --up
script so that they can be used to configure routes such as:

.B route add -net 10.0.0.0 netmask 255.255.255.0 gw $5
.\"*********************************************************
.TP
.B --up-delay
Delay TUN/TAP open and possible
.B --up
script execution
until after TCP/UDP connection establishment with peer.

In
.B --proto udp
mode, this option normally requires the use of
.B --ping
to allow connection initiation to be sensed in the absence
of tunnel data, since UDP is a "connectionless" protocol.

On Windows, this option will delay the TAP-Win32 media state
transitioning to "connected" until connection establishment,
i.e. the receipt of the first authenticated packet from the peer.
.\"*********************************************************
.TP
.B --down cmd
Shell command to run after TUN/TAP device close
(post
.B --user
UID change and/or
.B --chroot
).  Called with the same parameters and environmental
variables as the
.B --up
option above.

Note that if you reduce privileges by using
.B --user
and/or
.B --group,
your
.B --down
script will also run at reduced privilege.
.\"*********************************************************
.TP
.B --down-pre
Call
.B --down
cmd/script before, rather than after, TUN/TAP close.
.\"*********************************************************
.TP
.B --up-restart
Enable the
.B --up
and
.B --down
scripts to be called for restarts as well as initial program start.
This option is described more fully above in the
.B --up
option documentation.
.\"*********************************************************
.TP
.B --setenv name value
Set a custom environmental variable
.B name=value
to pass to script.
.\"*********************************************************
.TP
.B --setenv-safe name value
Set a custom environmental variable
.B OPENVPN_name=value
to pass to script.

This directive is designed to be pushed by the server to clients,
and the prepending of "OPENVPN_" to the environmental variable
is a safety precaution to prevent a LD_PRELOAD style attack
from a malicious or compromised server.
.\"*********************************************************
.TP
.B --script-security level
This directive offers policy-level control over OpenVPN's usage of external programs
and scripts.  Lower values are more restrictive, higher values are more permissive.  Settings for
.B level:

.B 0 --
Strictly no calling of external programs.
.br
.B 1 --
(Default) Only call built-in executables such as ifconfig, ip, route, or netsh.
.br
.B 2 --
Allow calling of built-in executables and user-defined scripts.
.br
.B 3 --
Allow passwords to be passed to scripts via environmental variables (potentially unsafe).
.\"*********************************************************
.TP
.B --disable-occ
Don't output a warning message if option inconsistencies are detected between
peers.  An example of an option inconsistency would be where one peer uses
.B --dev tun
while the other peer uses
.B --dev tap.

Use of this option is discouraged, but is provided as
a temporary fix in situations where a recent version of OpenVPN must
connect to an old version.
.\"*********************************************************
.TP
.B --user user
Change the user ID of the OpenVPN process to
.B user
after initialization, dropping privileges in the process.
This option is useful to protect the system
in the event that some hostile party was able to gain control of
an OpenVPN session.  Though OpenVPN's security features make
this unlikely, it is provided as a second line of defense.

By setting
.B user
to
.I nobody
or somebody similarly unprivileged, the hostile party would be
limited in what damage they could cause.  Of course once
you take away privileges, you cannot return them
to an OpenVPN session.  This means, for example, that if
you want to reset an OpenVPN daemon with a
.B SIGUSR1
signal
(for example in response
to a DHCP reset), you should make use of one or more of the
.B --persist
options to ensure that OpenVPN doesn't need to execute any privileged
operations in order to restart (such as re-reading key files
or running
.BR ifconfig
on the TUN device).
.\"*********************************************************
.TP
.B --group group
Similar to the
.B --user
option,
this option changes the group ID of the OpenVPN process to
.B group
after initialization.
.\"*********************************************************
.TP
.B --cd dir
Change directory to
.B dir
prior to reading any files such as
configuration files, key files, scripts, etc.
.B dir
should be an absolute path, with a leading "/",
and without any references
to the current directory such as "." or "..".

This option is useful when you are running
OpenVPN in 
.B --daemon
mode, and you want to consolidate all of
your OpenVPN control files in one location.
.\"*********************************************************
.TP
.B --chroot dir
Chroot to
.B dir
after initialization.  
.B --chroot
essentially redefines
.B dir
as being the top
level directory tree (/).  OpenVPN will therefore
be unable to access any files outside this tree.
This can be desirable from a security standpoint.

Since the chroot operation is delayed until after
initialization, most OpenVPN options that reference
files will operate in a pre-chroot context.

In many cases, the
.B dir
parameter can point to an empty directory, however
complications can result when scripts or restarts
are executed after the chroot operation.
.\"*********************************************************
.TP
.B --daemon [progname]
Become a daemon after all initialization functions are completed.
This option will cause all message and error output to
be sent to the syslog file (such as /var/log/messages),
except for the output of shell scripts and
ifconfig commands,
which will go to /dev/null unless otherwise redirected.
The syslog redirection occurs immediately at the point
that
.B --daemon
is parsed on the command line even though
the daemonization point occurs later.  If one of the
.B --log
options is present, it will supercede syslog
redirection.

The optional
.B progname
parameter will cause OpenVPN to report its program name
to the system logger as
.B progname.
This can be useful in linking OpenVPN messages
in the syslog file with specific tunnels.
When unspecified,
.B progname
defaults to "openvpn".

When OpenVPN is run with the
.B --daemon
option, it will try to delay daemonization until the majority of initialization
functions which are capable of generating fatal errors are complete.  This means
that initialization scripts can test the return status of the
openvpn command for a fairly reliable indication of whether the command
has correctly initialized and entered the packet forwarding event loop.

In OpenVPN, the vast majority of errors which occur after initialization are non-fatal.
.\"*********************************************************
.TP
.B --syslog [progname]
Direct log output to system logger, but do not become a daemon.
See
.B --daemon
directive above for description of
.B progname
parameter.
.\"*********************************************************
.TP
.B --passtos
Set the TOS field of the tunnel packet to what the payload's TOS is.
.\"*********************************************************
.TP
.B --inetd [wait|nowait] [progname]
Use this option when OpenVPN is being run from the inetd or
.BR xinetd(8)
server.

The
.B wait/nowait
option must match what is specified in the inetd/xinetd
config file.  The
.B nowait
mode can only be used with
.B --proto tcp-server.
The default is
.B wait.
The
.B nowait
mode can be used to instantiate the OpenVPN daemon as a classic TCP server,
where client connection requests are serviced on a single
port number.  For additional information on this kind of configuration,
see the OpenVPN FAQ:
.I http://openvpn.net/faq.html#oneport

This option precludes the use of
.B --daemon, --local,
or
.B --remote.
Note that this option causes message and error output to be handled in the same
way as the
.B --daemon
option.  The optional
.B progname
parameter is also handled exactly as in
.B --daemon.

Also note that in
.B wait
mode, each OpenVPN tunnel requires a separate TCP/UDP port and
a separate inetd or xinetd entry.  See the OpenVPN 1.x HOWTO for an example
on using OpenVPN with xinetd:
.I http://openvpn.net/1xhowto.html
.\"*********************************************************
.TP
.B --log file
Output logging messages to
.B file,
including output to stdout/stderr which
is generated by called scripts.
If
.B file
already exists it will be truncated.
This option takes effect
immediately when it is parsed in the command line
and will supercede syslog output if
.B --daemon
or
.B --inetd
is also specified.
This option is persistent over the entire course of
an OpenVPN instantiation and will not be reset by SIGHUP,
SIGUSR1, or
.B --ping-restart.

Note that on Windows, when OpenVPN is started as a service,
logging occurs by default without the need to specify
this option.
.\"*********************************************************
.TP
.B --log-append file
Append logging messages to
.B file.
If
.B file
does not exist, it will be created.
This option behaves exactly like
.B --log
except that it appends to rather
than truncating the log file.
.\"*********************************************************
.TP
.B --suppress-timestamps
Avoid writing timestamps to log messages, even when they
otherwise would be prepended. In particular, this applies to
log messages sent to stdout.
.\"*********************************************************
.TP
.B --writepid file
Write OpenVPN's main process ID to
.B file.
.\"*********************************************************
.TP
.B --nice n
Change process priority after initialization
(
.B n
greater than 0 is lower priority,
.B n
less than zero is higher priority).
.\"*********************************************************
.\".TP
.\".B --nice-work n
.\"Change priority of background TLS work thread.  The TLS thread
.\"feature is enabled when OpenVPN is built
.\"with pthread support, and you are running OpenVPN
.\"in TLS mode (i.e. with
.\".B --tls-client
.\"or
.\".B --tls-server
.\"specified).
.\"
.\"Using a TLS thread offloads the CPU-intensive process of SSL/TLS-based
.\"key exchange to a background thread so that it does not become
.\"a latency bottleneck in the tunnel packet forwarding process.
.\"
.\"The parameter
.\".B n
.\"is interpreted exactly as with the
.\".B --nice
.\"option above, but in relation to the work thread rather
.\"than the main thread.
.\"*********************************************************
.TP
.B --fast-io
(Experimental) Optimize TUN/TAP/UDP I/O writes by avoiding
a call to poll/epoll/select prior to the write operation.  The purpose
of such a call would normally be to block until the device
or socket is ready to accept the write.  Such blocking is unnecessary
on some platforms which don't support write blocking on UDP sockets
or TUN/TAP devices.  In such cases, one can optimize the event loop
by avoiding the poll/epoll/select call, improving CPU efficiency
by 5% to 10%.

This option can only be used on non-Windows systems, when
.B --proto udp
is specified, and when
.B --shaper
is NOT specified.
.\"*********************************************************
.TP
.B --echo [parms...]
Echo
.B parms
to log output.

Designed to be used to send messages to a controlling application
which is receiving the OpenVPN log output.
.\"*********************************************************
.TP
.B --remap-usr1 signal
Control whether internally or externally
generated SIGUSR1 signals are remapped to
SIGHUP (restart without persisting state) or
SIGTERM (exit).

.B signal
can be set to "SIGHUP" or "SIGTERM".  By default, no remapping
occurs.
.\"*********************************************************
.TP
.B --verb n
Set output verbosity to
.B n
(default=1).  Each level shows all info from the previous levels.
Level 3 is recommended if you want a good summary
of what's happening without being swamped by output.

.B 0 --
No output except fatal errors.
.br
.B 1 to 4 --
Normal usage range.
.br
.B 5 --
Output
.B R
and
.B W
characters to the console for each packet read and write, uppercase is
used for TCP/UDP packets and lowercase is used for TUN/TAP packets.
.br
.B 6 to 11 --
Debug info range (see errlevel.h for additional
information on debug levels).
.\"*********************************************************
.TP
.B --status file [n]
Write operational status to
.B file
every
.B n
seconds.

Status can also be written to the syslog by sending a
.B SIGUSR2
signal.
.\"*********************************************************
.TP
.B --status-version [n]
Choose the status file format version number.  Currently
.B n
can be 1, 2, or 3 and defaults to 1.
.\"*********************************************************
.TP
.B --mute n
Log at most
.B n
consecutive messages in the same category.  This is useful to
limit repetitive logging of similar message types.
.\"*********************************************************
.TP
.B --comp-lzo [mode]
Use fast LZO compression -- may add up to 1 byte per
packet for incompressible data.
.B mode
may be "yes", "no", or "adaptive" (default).

In a server mode setup, it is possible to selectively turn
compression on or off for individual clients.

First, make sure the client-side config file enables selective
compression by having at least one
.B --comp-lzo
directive, such as
.B --comp-lzo no.
This will turn off compression by default,
but allow a future directive push from the server to
dynamically change the
on/off/adaptive setting.

Next in a
.B --client-config-dir
file, specify the compression setting for the client,
for example:

.RS
.ft 3
.nf
.sp
comp-lzo yes
push "comp-lzo yes"
.ft
.LP
.RE
.fi

The first line sets the
.B comp-lzo
setting for the server
side of the link, the second sets the client side.
.\"*********************************************************
.TP
.B --comp-noadapt
When used in conjunction with
.B --comp-lzo,
this option will disable OpenVPN's adaptive compression algorithm.
Normally, adaptive compression is enabled with
.B --comp-lzo.

Adaptive compression tries to optimize the case where you have
compression enabled, but you are sending predominantly uncompressible
(or pre-compressed) packets over the tunnel, such as an FTP or rsync transfer
of a large, compressed file.  With adaptive compression,
OpenVPN will periodically sample the compression process to measure its
efficiency.  If the data being sent over the tunnel is already compressed,
the compression efficiency will be very low, triggering openvpn to disable
compression for a period of time until the next re-sample test.
.\"*********************************************************
.TP
.B --management IP port [pw-file]
Enable a TCP server on
.B IP:port
to handle daemon management functions.
.B pw-file,
if specified,
is a password file (password on first line)
or "stdin" to prompt from standard input.  The password
provided will set the password which TCP clients will need
to provide in order to access management functions.

The management interface can also listen on a unix domain socket,
for those platforms that support it.  To use a unix domain socket, specify
the unix socket pathname in place of
.B IP
and set
.B port
to 'unix'.  While the default behavior is to create a unix domain socket
that may be connected to by any process, the
.B --management-client-user
and
.B --management-client-group
directives can be used to restrict access.

The management interface provides a special mode where the TCP
management link can operate over the tunnel itself.  To enable this mode,
set
.B IP
= "tunnel".  Tunnel mode will cause the management interface
to listen for a TCP connection on the local VPN address of the
TUN/TAP interface.

While the management port is designed for programmatic control
of OpenVPN by other applications, it is possible to telnet
to the port, using a telnet client in "raw" mode.  Once connected,
type "help" for a list of commands.

For detailed documentation on the management interface, see
the management-notes.txt file in the
.B management
folder of
the OpenVPN source distribution.

It is strongly recommended that
.B IP
be set to 127.0.0.1
(localhost) to restrict accessibility of the management
server to local clients. 
.\"*********************************************************
.TP
.B --management-query-passwords
Query management channel for private key password and
.B --auth-user-pass
username/password.  Only query the management channel
for inputs which ordinarily would have been queried from the
console.
.\"*********************************************************
.TP
.B --management-forget-disconnect
Make OpenVPN forget passwords when management session
disconnects.

This directive does not affect the
.B --http-proxy
username/password.  It is always cached.
.\"*********************************************************
.TP
.B --management-hold
Start OpenVPN in a hibernating state, until a client
of the management interface explicitly starts it
with the
.B hold release
command.
.\"*********************************************************
.TP
.B --management-signal
Send SIGUSR1 signal to OpenVPN if management session disconnects.
This is useful when you wish to disconnect an OpenVPN session on
user logoff.
.\"*********************************************************
.TP
.B --management-log-cache n
Cache the most recent
.B n
lines of log file history for usage
by the management channel.
.\"*********************************************************
.TP
.B --management-client-auth
Gives management interface client the responsibility
to authenticate clients after their client certificate
has been verified.  See management-notes.txt in OpenVPN
distribution for detailed notes.
.\"*********************************************************
.TP
.B --management-client-pf
Management interface clients must specify a packet
filter file for each connecting client.  See management-notes.txt
in OpenVPN distribution for detailed notes.
.\"*********************************************************
.TP
.B --management-client-user u
When the management interface is listening on a unix domain socket,
only allow connections from user
.B u.
.\"*********************************************************
.TP
.B --management-client-group g
When the management interface is listening on a unix domain socket,
only allow connections from group
.B g.
.\"*********************************************************
.TP
.B --plugin module-pathname [init-string]
Load plug-in module from the file
.B module-pathname,
passing
.B init-string
as an argument
to the module initialization function.  Multiple
plugin modules may be loaded into one OpenVPN
process.

For more information and examples on how to build OpenVPN
plug-in modules, see the README file in the
.B plugin
folder of the OpenVPN source distribution.

If you are using an RPM install of OpenVPN, see
/usr/share/openvpn/plugin.  The documentation is
in
.B doc
and the actual plugin modules are in
.B lib.

Multiple plugin modules can be cascaded, and modules can be
used in tandem with scripts.  The modules will be called by
OpenVPN in the order that they are declared in the config
file.  If both a plugin and script are configured for the same
callback, the script will be called last.  If the
return code of the module/script controls an authentication
function (such as tls-verify, auth-user-pass-verify, or
client-connect), then
every module and script must return success (0) in order for
the connection to be authenticated.
.\"*********************************************************
.SS Server Mode
Starting with OpenVPN 2.0, a multi-client TCP/UDP server mode
is supported, and can be enabled with the
.B --mode server
option.  In server mode, OpenVPN will listen on a single
port for incoming client connections.  All client
connections will be routed through a single tun or tap
interface.  This mode is designed for scalability and should
be able to support hundreds or even thousands of clients
on sufficiently fast hardware.  SSL/TLS authentication must
be used in this mode.
.\"*********************************************************
.TP
.B --server network netmask
A helper directive designed to simplify the configuration
of OpenVPN's server mode.  This directive will set up an
OpenVPN server which will allocate addresses to clients
out of the given network/netmask.  The server itself
will take the ".1" address of the given network
for use as the server-side endpoint of the local
TUN/TAP interface.

For example,
.B --server 10.8.0.0 255.255.255.0
expands as follows:

.RS
.ft 3
.nf
.sp
 mode server
 tls-server
 push "topology [topology]"

 if dev tun AND (topology == net30 OR topology == p2p):
   ifconfig 10.8.0.1 10.8.0.2 
   ifconfig-pool 10.8.0.4 10.8.0.251
   route 10.8.0.0 255.255.255.0
   if client-to-client:
     push "route 10.8.0.0 255.255.255.0"
   else if topology == net30:
     push "route 10.8.0.1"

 if dev tap OR (dev tun AND topology == subnet):
   ifconfig 10.8.0.1 255.255.255.0
   ifconfig-pool 10.8.0.2 10.8.0.254 255.255.255.0
   push "route-gateway 10.8.0.1"
.ft
.LP
.RE
.fi

Don't use
.B --server
if you are ethernet bridging.  Use
.B --server-bridge
instead.
.\"*********************************************************
.TP
.B --server-bridge [ gateway netmask pool-start-IP pool-end-IP ]

A helper directive similar to
.B --server
which is designed to simplify the configuration
of OpenVPN's server mode in ethernet bridging configurations.

If
.B --server-bridge
is used without any parameters, it will enable a DHCP-proxy
mode, where connecting OpenVPN clients will receive an IP
address for their TAP adapter from the DHCP server running
on the OpenVPN server-side LAN.
Note that only clients that support
the binding of a DHCP client with the TAP adapter (such as
Windows) can support this mode.

To configure ethernet bridging, you 
must first use your OS's bridging capability
to bridge the TAP interface with the ethernet
NIC interface.  For example, on Linux this is done
with the
.B brctl
tool, and with Windows XP it is done in the Network
Connections Panel by selecting the ethernet and
TAP adapters and right-clicking on "Bridge Connections".

Next you you must manually set the
IP/netmask on the bridge interface.  The
.B gateway
and
.B netmask
parameters to
.B --server-bridge
can be set to either the IP/netmask of the
bridge interface, or the IP/netmask of the
default gateway/router on the bridged
subnet.

Finally, set aside a IP range in the bridged
subnet,
denoted by
.B pool-start-IP
and
.B pool-end-IP,
for OpenVPN to allocate to connecting
clients.

For example,
.B server-bridge 10.8.0.4 255.255.255.0 10.8.0.128 10.8.0.254
expands as follows:

.RS
.ft 3
.nf
.sp
mode server
tls-server

ifconfig-pool 10.8.0.128 10.8.0.254 255.255.255.0
push "route-gateway 10.8.0.4"
.ft
.LP
.RE
.fi

In another example,
.B --server-bridge
(without parameters) expands as follows:

.RS
.ft 3
.nf
.sp
mode server
tls-server

push "route-gateway dhcp"
.ft
.LP
.RE
.fi
.\"*********************************************************
.TP
.B --push "option"
Push a config file option back to the client for remote
execution.  Note that
.B
option
must be enclosed in double quotes ("").  The client must specify
.B --pull
in its config file.  The set of options which can be
pushed is limited by both feasibility and security.
Some options such as those which would execute scripts
are banned, since they would effectively allow a compromised
server to execute arbitrary code on the client.
Other options such as TLS or MTU parameters
cannot be pushed because the client needs to know
them before the connection to the server can be initiated.

This is a partial list of options which can currently be pushed:
.B --route, --route-gateway, --route-delay, --redirect-gateway,
.B --ip-win32, --dhcp-option,
.B --inactive, --ping, --ping-exit, --ping-restart,
.B --setenv,
.B --persist-key, --persist-tun, --echo,
.B --comp-lzo,
.B --socket-flags,
.B --sndbuf, --rcvbuf
.\"*********************************************************
.TP
.B --push-reset
Don't inherit the global push list for a specific client instance.
Specify this option in a client-specific context such
as with a
.B --client-config-dir
configuration file.  This option will ignore
.B --push
options at the global config file level.
.\"*********************************************************
.TP
.B --disable
Disable a particular client (based on the common name)
from connecting.  Don't use this option to disable a client
due to key or password compromise.  Use a CRL (certificate
revocation list) instead (see the
.B --crl-verify
option).

This option must be associated with a specific client instance,
which means that it must be specified either in a client
instance config file using
.B --client-config-dir
or dynamically generated using a
.B --client-connect
script.
.\"*********************************************************
.TP
.B --ifconfig-pool start-IP end-IP [netmask]
Set aside a pool of subnets to be
dynamically allocated to connecting clients, similar
to a DHCP server.  For tun-style
tunnels, each client will be given a /30 subnet (for
interoperability with Windows clients).  For tap-style
tunnels, individual addresses will be allocated, and the
optional
.B netmask
parameter will also be pushed to clients.

.\"*********************************************************
.TP
.B --ifconfig-pool-persist file [seconds]
Persist/unpersist ifconfig-pool
data to
.B file,
at
.B seconds
intervals (default=600), as well as on program startup and
shutdown.

The goal of this option is to provide a long-term association
between clients (denoted by their common name) and the virtual
IP address assigned to them from the ifconfig-pool.
Maintaining a long-term
association is good for clients because it allows them
to effectively use the
.B --persist-tun
option.

.B file
is a comma-delimited ASCII file, formatted as
<Common-Name>,<IP-address>.

If
.B seconds
= 0,
.B file
will be treated as read-only.  This is useful if
you would like to treat
.B file
as a configuration file.

Note that the entries in this file are treated by OpenVPN as
suggestions only, based on past associations between
a common name and IP address.  They do not guarantee that the given common
name will always receive the given IP address.  If you want guaranteed
assignment, use
.B --ifconfig-push
.\"*********************************************************
.TP
.B --ifconfig-pool-linear
Modifies the
.B --ifconfig-pool
directive to
allocate individual TUN interface addresses for
clients rather than /30 subnets.  NOTE:  This option
is incompatible with Windows clients.

This option is deprecated, and should be replaced with
.B --topology p2p
which is functionally equivalent.
.\"*********************************************************
.TP
.B --ifconfig-push local remote-netmask
Push virtual IP endpoints for client tunnel,
overriding the --ifconfig-pool dynamic allocation.

The parameters
.B local
and
.B remote-netmask
are set according to the
.B --ifconfig
directive which you want to execute on the client machine to
configure the remote end of the tunnel.  Note that the parameters
.B local
and
.B remote-netmask
are from the perspective of the client, not the server.  They may be
DNS names rather than IP addresses, in which case they will be resolved
on the server at the time of client connection.

This option must be associated with a specific client instance,
which means that it must be specified either in a client
instance config file using
.B --client-config-dir
or dynamically generated using a
.B --client-connect
script.

Remember also to include a
.B --route
directive in the main OpenVPN config file which encloses
.B local,
so that the kernel will know to route it
to the server's TUN/TAP interface.

OpenVPN's internal client IP address selection algorithm works as
follows:

.B 1
-- Use
.B --client-connect script
generated file for static IP (first choice).
.br
.B 2
-- Use 
.B --client-config-dir
file for static IP (next choice).
.br
.B 3
-- Use
.B --ifconfig-pool
allocation for dynamic IP (last choice).
.br
.\"*********************************************************
.TP
.B --iroute network [netmask]
Generate an internal route to a specific
client. The
.B netmask
parameter, if omitted, defaults to 255.255.255.255.

This directive can be used to route a fixed subnet from
the server to a particular client, regardless
of where the client is connecting from.  Remember
that you must also add the route to the system
routing table as well (such as by using the
.B --route
directive).  The reason why two routes are needed
is that the
.B --route
directive routes the packet from the kernel
to OpenVPN.  Once in OpenVPN, the
.B --iroute
directive routes to the specific client.

This option must be specified either in a client
instance config file using
.B --client-config-dir
or dynamically generated using a
.B --client-connect
script.

The
.B --iroute
directive also has an important interaction with
.B --push
"route ...".
.B --iroute
essentially defines a subnet which is owned by a
particular client (we will call this client A).
If you would like other clients to be able to reach A's
subnet, you can use
.B --push
"route ..."
together with
.B --client-to-client
to effect this.  In order for all clients to see
A's subnet, OpenVPN must push this route to all clients
EXCEPT for A, since the subnet is already owned by A.
OpenVPN accomplishes this by not
not pushing a route to a client
if it matches one of the client's iroutes.
.\"*********************************************************
.TP
.B --client-to-client
Because the OpenVPN server mode handles multiple clients
through a single tun or tap interface, it is effectively
a router.  The
.B --client-to-client
flag tells OpenVPN to internally route client-to-client
traffic rather than pushing all client-originating traffic
to the TUN/TAP interface.

When this option is used, each client will "see" the other
clients which are currently connected.  Otherwise, each
client will only see the server.  Don't use this option
if you want to firewall tunnel traffic using
custom, per-client rules.
.\"*********************************************************
.TP
.B --duplicate-cn
Allow multiple clients with the same common name to concurrently connect.
In the absence of this option, OpenVPN will disconnect a client instance
upon connection of a new client having the same common name.
.\"*********************************************************
.TP
.B --client-connect script
Run
.B script
on client connection.  The script is passed the common name
and IP address of the just-authenticated client
as environmental variables (see environmental variable section
below).  The script is also passed
the pathname of a not-yet-created temporary file as $1
(i.e. the first command line argument), to be used by the script
to pass dynamically generated config file directives back to OpenVPN.

If the script wants to generate a dynamic config file
to be applied on the server when the client connects,
it should write it to the file named by $1.

See the
.B --client-config-dir
option below for options which
can be legally used in a dynamically generated config file.

Note that the return value of
.B script
is significant.  If
.B script
returns a non-zero error status, it will cause the client
to be disconnected.
.\"*********************************************************
.TP
.B --client-disconnect
Like
.B --client-connect
but called on client instance shutdown.  Will not be called
unless the
.B --client-connect
script and plugins (if defined)
were previously called on this instance with
successful (0) status returns.

The exception to this rule is if the
.B --client-disconnect
script or plugins are cascaded, and at least one client-connect
function succeeded, then ALL of the client-disconnect functions for
scripts and plugins will be called on client instance object deletion,
even in cases where some of the related client-connect functions returned
an error status.
.B 
.\"*********************************************************
.TP
.B --client-config-dir dir
Specify a directory
.B dir
for custom client config files.  After
a connecting client has been authenticated, OpenVPN will
look in this directory for a file having the same name
as the client's X509 common name.  If a matching file
exists, it will be opened and parsed for client-specific
configuration options.  If no matching file is found, OpenVPN
will instead try to open and parse a default file called
"DEFAULT", which may be provided but is not required.

This file can specify a fixed IP address for a given
client using
.B --ifconfig-push,
as well as fixed subnets owned by the client using
.B --iroute.

One of the useful properties of this option is that it
allows client configuration files to be conveniently
created, edited, or removed while the server is live,
without needing to restart the server.

The following
options are legal in a client-specific context:
.B --push, --push-reset, --iroute, --ifconfig-push,
and
.B --config.
.\"*********************************************************
.TP
.B --ccd-exclusive
Require, as a
condition of authentication, that a connecting client has a
.B --client-config-dir
file.
.\"*********************************************************
.TP
.B --tmp-dir dir
Specify a directory
.B dir
for temporary files.  This directory will be used by
.B --client-connect
scripts to dynamically generate client-specific
configuration files.
.\"*********************************************************
.TP
.B --hash-size r v
Set the size of the real address hash table to
.B r
and the virtual address table to
.B v.
By default, both tables are sized at 256 buckets.
.\"*********************************************************
.TP
.B --bcast-buffers n
Allocate
.B n
buffers for broadcast datagrams (default=256).
.\"*********************************************************
.TP
.B --tcp-queue-limit n
Maximum number of output packets queued before TCP (default=64).

When OpenVPN is tunneling data from a TUN/TAP device to a
remote client over a TCP connection, it is possible that the TUN/TAP device
might produce data at a faster rate than the TCP connection
can support.  When the number of output packets queued before sending to
the TCP socket reaches this limit for a given client connection,
OpenVPN will start to drop outgoing packets directed
at this client.
.\"*********************************************************
.TP
.B --max-clients n
Limit server to a maximum of
.B n
concurrent clients.
.\"*********************************************************
.TP
.B --max-routes-per-client n
Allow a maximum of
.B n
internal routes per client (default=256).
This is designed to
help contain DoS attacks where an authenticated client floods the
server with packets appearing to come from many unique MAC addresses,
forcing the server to deplete
virtual memory as its internal routing table expands.
This directive can be used in a
.B --client-config-dir
file or auto-generated by a
.B --client-connect
script to override the global value for a particular client.

Note that this
directive affects OpenVPN's internal routing table, not the
kernel routing table.
.\"*********************************************************
.TP
.B --connect-freq n sec
Allow a maximum of
.B n
new connections per
.B sec 
seconds from clients.  This is designed to contain DoS attacks which flood
the server with connection requests using certificates which
will ultimately fail to authenticate.

This is an imperfect solution however, because in a real
DoS scenario, legitimate connections might also be refused.

For the best protection against DoS attacks in server mode,
use
.B --proto udp
and
.B --tls-auth.
.\"*********************************************************
.TP
.B --learn-address cmd
Run script or shell command
.B cmd
to validate client virtual addresses or routes.

.B cmd
will be executed with 3 parameters:

.B [1] operation --
"add", "update", or "delete" based on whether or not
the address is being added to, modified, or deleted from
OpenVPN's internal routing table.
.br
.B [2] address --
The address being learned or unlearned.  This can be
an IPv4 address such as "198.162.10.14", an IPv4 subnet
such as "198.162.10.0/24", or an ethernet MAC address (when
.B --dev tap
is being used) such as "00:FF:01:02:03:04".
.br
.B [3] common name --
The common name on the certificate associated with the
client linked to this address.  Only present for "add"
or "update" operations, not "delete".

On "add" or "update" methods, if the script returns
a failure code (non-zero), OpenVPN will reject the address
and will not modify its internal routing table.

Normally, the
.B cmd
script will use the information provided above to set
appropriate firewall entries on the VPN TUN/TAP interface.
Since OpenVPN provides the association between virtual IP
or MAC address and the client's authenticated common name,
it allows a user-defined script to configure firewall access
policies with regard to the client's high-level common name,
rather than the low level client virtual addresses.
.\"*********************************************************
.TP
.B --auth-user-pass-verify script method
Require the client to provide a username/password (possibly
in addition to a client certificate) for authentication.

OpenVPN will execute
.B script
as a shell command to validate the username/password
provided by the client.

If
.B method
is set to "via-env", OpenVPN will call
.B script
with the environmental variables
.B username
and
.B password
set to the username/password strings provided by the client.
Be aware that this method is insecure on some platforms which
make the environment of a process publicly visible to other
unprivileged processes.

If
.B method
is set to "via-file", OpenVPN will write the username and
password to the first two lines of a temporary file.  The filename
will be passed as an argument to
.B script,
and the file will be automatically deleted by OpenVPN after
the script returns.  The location of the temporary file is
controlled by the
.B --tmp-dir
option, and will default to the current directory if unspecified.
For security, consider setting 
.B --tmp-dir
to a volatile storage medium such as
.B /dev/shm
(if available) to prevent the username/password file from touching the hard drive.

The script should examine the username
and password,
returning a success exit code (0) if the
client's authentication request is to be accepted, or a failure
code (1) to reject the client.

This directive is designed to enable a plugin-style interface
for extending OpenVPN's authentication capabilities.

To protect against a client passing a maliciously formed
username or password string, the username string must
consist only of these characters: alphanumeric, underbar
('_'), dash ('-'), dot ('.'), or at ('@').  The password
string can consist of any printable characters except for
CR or LF.  Any illegal characters in either the username
or password string will be converted to underbar ('_').

Care must be taken by any user-defined scripts to avoid
creating a security vulnerability in the way that these
strings are handled.  Never use these strings in such a way
that they might be escaped or evaluated by a shell interpreter.

For a sample script that performs PAM authentication, see
.B sample-scripts/auth-pam.pl
in the OpenVPN source distribution.
.\"*********************************************************
.TP
.B --auth-user-pass-optional
Allow connections by clients that do not specify a username/password.
Normally, when
.B --auth-user-pass-verify
or
.B --management-client-auth
is specified (or an authentication plugin module), the
OpenVPN server daemon will require connecting clients to specify a
username and password.  This option makes the submission of a username/password
by clients optional, passing the responsibility to the user-defined authentication
module/script to accept or deny the client based on other factors
(such as the setting of X509 certificate fields).  When this option is used,
and a connecting client does not submit a username/password, the user-defined
authentication module/script will see the username and password as being set
to empty strings ("").  The authentication module/script MUST have logic
to detect this condition and respond accordingly.
.\"*********************************************************
.TP
.B --client-cert-not-required
Don't require client certificate, client will authenticate
using username/password only.  Be aware that using this directive
is less secure than requiring certificates from all clients.

If you use this directive, the
entire responsibility of authentication will rest on your
.B --auth-user-pass-verify
script, so keep in mind that bugs in your script
could potentially compromise the security of your VPN.

If you don't use this directive, but you also specify an
.B --auth-user-pass-verify
script, then OpenVPN will perform double authentication.  The
client certificate verification AND the
.B --auth-user-pass-verify
script will need to succeed in order for a client to be
authenticated and accepted onto the VPN.
.\"*********************************************************
.TP
.B --username-as-common-name
For
.B --auth-user-pass-verify
authentication, use
the authenticated username as the common name,
rather than the common name from the client cert.
.\"*********************************************************
.TP
.B --no-name-remapping
Allow Common Name, X509 Subject, and username strings to include
any printable character including space, but excluding control
characters such as tab, newline, and carriage-return.

By default, OpenVPN will remap
any character other than alphanumeric, underbar ('_'), dash
('-'), dot ('.'), and slash ('/') to underbar ('_').  The X509
Subject string as returned by the
.B tls_id
environmental variable, can additionally contain colon (':') or
equal ('=').

While name remapping is performed for security reasons to reduce
the possibility of introducing string expansion security vulnerabilities
in user-defined authentication
scripts, this option is provided for those cases where it is desirable to
disable the remapping feature.  Don't use this option unless you 
know what you are doing!
.\"*********************************************************
.TP
.B --port-share host port
When run in TCP server mode, share the OpenVPN port with
another application, such as an HTTPS server.  If OpenVPN
senses a connection to its port which is using a non-OpenVPN
protocol, it will proxy the connection to the server at
.B host:port.
Currently only designed to work with HTTP/HTTPS,
though it would be theoretically possible to extend to
other protocols such as ssh.

Not implemented on Windows.
.\"*********************************************************
.SS Client Mode
Use client mode when connecting to an OpenVPN server
which has
.B --server, --server-bridge,
or
.B --mode server
in it's configuration.
.\"*********************************************************
.TP
.B --client
A helper directive designed to simplify the configuration
of OpenVPN's client mode.  This directive is equivalent to:

.RS
.ft 3
.nf
.sp
 pull
 tls-client
.ft
.LP
.RE
.fi
.\"*********************************************************
.TP
.B --pull
This option must be used on a client which is connecting
to a multi-client server.  It indicates to OpenVPN that it
should accept options pushed by the server, provided they
are part of the legal set of pushable options (note that the
.B --pull
option is implied by
.B --client
).

In particular,
.B --pull
allows the server to push routes to the client, so you should
not use
.B --pull
or
.B --client
in situations where you don't trust the server to have control
over the client's routing table.
.\"*********************************************************
.TP
.B --auth-user-pass [up]
Authenticate with server using username/password.
.B up
is a file containing username/password on 2 lines (Note: OpenVPN
will only read passwords from a file if it has been built
with the --enable-password-save configure option, or on Windows
by defining ENABLE_PASSWORD_SAVE in config-win32.h).

If
.B up
is omitted, username/password will be prompted from the
console.

The server configuration must specify an
.B --auth-user-pass-verify
script to verify the username/password provided by
the client.
.\"*********************************************************
.TP
.B --auth-retry type
Controls how OpenVPN responds to username/password verification
errors such as the client-side response to an AUTH_FAILED message from the server
or verification failure of the private key password.

Normally used to prevent auth errors from being fatal
on the client side, and to permit username/password requeries in case
of error.

An AUTH_FAILED message is generated by the server if the client
fails
.B --auth-user-pass
authentication, or if the server-side
.B --client-connect
script returns an error status when the client
tries to connect.

.B type
can be one of:

.B none --
Client will exit with a fatal error (this is the default).
.br
.B nointeract --
Client will retry the connection without requerying for an
.B --auth-user-pass
username/password.  Use this option for unattended clients.
.br
.B interact --
Client will requery for an
.B --auth-user-pass
username/password and/or private key password before attempting a reconnection.

Note that while this option cannot be pushed, it can be controlled
from the management interface.
.\"*********************************************************
.TP
.B --explicit-exit-notify [n]
In UDP client mode or point-to-point mode, send server/peer an exit notification
if tunnel is restarted or OpenVPN process is exited.  In client mode, on
exit/restart, this
option will tell the server to immediately close its client instance object
rather than waiting for a timeout.  The
.B n
parameter (default=1) controls the maximum number of retries that the client
will attempt to resend the exit notification message.
.\"*********************************************************
.SS Data Channel Encryption Options:
These options are meaningful for both Static & TLS-negotiated key modes
(must be compatible between peers).
.\"*********************************************************
.TP
.B --secret file [direction]
Enable Static Key encryption mode (non-TLS).
Use pre-shared secret
.B file
which was generated with
.B --genkey.

The optional
.B direction
parameter enables the use of 4 distinct keys
(HMAC-send, cipher-encrypt, HMAC-receive, cipher-decrypt), so that
each data flow direction has a different set of HMAC and cipher keys.
This has a number of desirable security properties including
eliminating certain kinds of DoS and message replay attacks.

When the
.B direction
parameter is omitted, 2 keys are used bidirectionally, one for HMAC
and the other for encryption/decryption.

The
.B direction
parameter should always be complementary on either side of the connection,
i.e. one side should use "0" and the other should use "1", or both sides
should omit it altogether.

The
.B direction
parameter requires that
.B file
contains a 2048 bit key.  While pre-1.5 versions of OpenVPN
generate 1024 bit key files, any version of OpenVPN which
supports the
.B direction
parameter, will also support 2048 bit key file generation
using the
.B --genkey
option.

Static key encryption mode has certain advantages,
the primary being ease of configuration.

There are no certificates
or certificate authorities or complicated negotiation handshakes and protocols.
The only requirement is that you have a pre-existing secure channel with
your peer (such as
.B ssh
) to initially copy the key.  This requirement, along with the
fact that your key never changes unless you manually generate a new one,
makes it somewhat less secure than TLS mode (see below).  If an attacker
manages to steal your key, everything that was ever encrypted with
it is compromised.  Contrast that to the perfect forward secrecy features of
TLS mode (using Diffie Hellman key exchange), where even if an attacker
was able to steal your private key, he would gain no information to help
him decrypt past sessions.

Another advantageous aspect of Static Key encryption mode is that
it is a handshake-free protocol 
without any distinguishing signature or feature
(such as a header or protocol handshake sequence) 
that would mark the ciphertext packets as being
generated by OpenVPN.  Anyone eavesdropping on the wire
would see nothing
but random-looking data.
.\"*********************************************************
.TP
.B --auth alg
Authenticate packets with HMAC using message
digest algorithm
.B alg.
(The default is
.B SHA1
).
HMAC is a commonly used message authentication algorithm (MAC) that uses
a data string, a secure hash algorithm, and a key, to produce
a digital signature.

OpenVPN's usage of HMAC is to first encrypt a packet, then HMAC the resulting ciphertext.

In static-key encryption mode, the HMAC key
is included in the key file generated by
.B --genkey.
In TLS mode, the HMAC key is dynamically generated and shared
between peers via the TLS control channel.  If OpenVPN receives a packet with
a bad HMAC it will drop the packet.
HMAC usually adds 16 or 20 bytes per packet.
Set
.B alg=none
to disable authentication.

For more information on HMAC see
.I http://www.cs.ucsd.edu/users/mihir/papers/hmac.html
.\"*********************************************************
.TP
.B --cipher alg
Encrypt packets with cipher algorithm
.B alg.
The default is
.B BF-CBC,
an abbreviation for Blowfish in Cipher Block Chaining mode.
Blowfish has the advantages of being fast, very secure, and allowing key sizes
of up to 448 bits.  Blowfish is designed to be used in situations where
keys are changed infrequently.

For more information on blowfish, see
.I http://www.counterpane.com/blowfish.html

To see other ciphers that are available with
OpenVPN, use the
.B --show-ciphers
option.

OpenVPN supports the CBC, CFB, and OFB cipher modes.

Set
.B alg=none
to disable encryption.
.\"*********************************************************
.TP
.B --keysize n
Size of cipher key in bits (optional).
If unspecified, defaults to cipher-specific default.  The
.B --show-ciphers
option (see below) shows all available OpenSSL ciphers,
their default key sizes, and whether the key size can
be changed.  Use care in changing a cipher's default
key size.  Many ciphers have not been extensively
cryptanalyzed with non-standard key lengths, and a
larger key may offer no real guarantee of greater
security, or may even reduce security.
.\"*********************************************************
.TP
.B --engine [engine-name]
Enable OpenSSL hardware-based crypto engine functionality.

If
.B engine-name
is specified,
use a specific crypto engine.  Use the
.B --show-engines
standalone option to list the crypto engines which are
supported by OpenSSL.
.\"*********************************************************
.TP
.B --no-replay
Disable OpenVPN's protection against replay attacks.
Don't use this option unless you are prepared to make
a tradeoff of greater efficiency in exchange for less
security.

OpenVPN provides datagram replay protection by default.

Replay protection is accomplished
by tagging each outgoing datagram with an identifier
that is guaranteed to be unique for the key being used.
The peer that receives the datagram will check for
the uniqueness of the identifier.  If the identifier
was already received in a previous datagram, OpenVPN
will drop the packet.  Replay protection is important
to defeat attacks such as a SYN flood attack, where
the attacker listens in the wire, intercepts a TCP
SYN packet (identifying it by the context in which
it occurs in relation to other packets), then floods
the receiving peer with copies of this packet.

OpenVPN's replay protection is implemented in slightly
different ways, depending on the key management mode
you have selected.

In Static Key mode
or when using an CFB or OFB mode cipher, OpenVPN uses a
64 bit unique identifier that combines a time stamp with
an incrementing sequence number.

When using TLS mode for key exchange and a CBC cipher
mode, OpenVPN uses only a 32 bit sequence number without
a time stamp, since OpenVPN can guarantee the uniqueness
of this value for each key.  As in IPSec, if the sequence number is
close to wrapping back to zero, OpenVPN will trigger
a new key exchange.

To check for replays, OpenVPN uses
the
.I sliding window
algorithm used
by IPSec.
.\"*********************************************************
.TP
.B --replay-window n [t]
Use a replay protection sliding-window of size
.B n
and a time window of
.B t
seconds.

By default
.B n
is 64 (the IPSec default) and
.B t
is 15 seconds.

This option is only relevant in UDP mode, i.e.
when either
.B --proto udp
is specifed, or no
.B --proto
option is specified.

When OpenVPN tunnels IP packets over UDP, there is the possibility that
packets might be dropped or delivered out of order.  Because OpenVPN, like IPSec,
is emulating the physical network layer,
it will accept an out-of-order packet sequence, and
will deliver such packets in the same order they were received to
the TCP/IP protocol stack, provided they satisfy several constraints.

.B (a)
The packet cannot be a replay (unless
.B --no-replay
is specified, which disables replay protection altogether).

.B (b)
If a packet arrives out of order, it will only be accepted if the difference
between its sequence number and the highest sequence number received
so far is less than
.B n.

.B (c)
If a packet arrives out of order, it will only be accepted if it arrives no later
than
.B t
seconds after any packet containing a higher sequence number.

If you are using a network link with a large pipeline (meaning that
the product of bandwidth and latency is high), you may want to use
a larger value for
.B n.
Satellite links in particular often require this.

If you run OpenVPN at
.B --verb 4,
you will see the message "Replay-window backtrack occurred [x]"
every time the maximum sequence number backtrack seen thus far
increases.  This can be used to calibrate
.B n.

There is some controversy on the appropriate method of handling packet
reordering at the security layer.

Namely, to what extent should the
security layer protect the encapsulated protocol from attacks which masquerade
as the kinds of normal packet loss and reordering that occur over IP networks?

The IPSec and OpenVPN approach is to allow packet reordering within a certain
fixed sequence number window.

OpenVPN adds to the IPSec model by limiting the window size in time as well as
sequence space.

OpenVPN also adds TCP transport as an option (not offered by IPSec) in which
case OpenVPN can adopt a very strict attitude towards message deletion and
reordering:  Don't allow it.  Since TCP guarantees reliability, any packet
loss or reordering event can be assumed to be an attack.

In this sense, it could be argued that TCP tunnel transport is preferred when
tunneling non-IP or UDP application protocols which might be vulnerable to a
message deletion or reordering attack which falls within the normal
operational parameters of IP networks.

So I would make the statement that one should never tunnel a non-IP protocol
or UDP application protocol over UDP, if the protocol might be vulnerable to a
message deletion or reordering attack that falls within the normal operating
parameters of what is to be expected from the physical IP layer.  The problem
is easily fixed by simply using TCP as the VPN transport layer.
.\"*********************************************************
.TP
.B --mute-replay-warnings
Silence the output of replay warnings, which are a common
false alarm on WiFi networks.  This option preserves
the security of the replay protection code without
the verbosity associated with warnings about duplicate
packets.
.\"*********************************************************
.TP
.B --replay-persist file
Persist replay-protection state across sessions using
.B file
to save and reload the state.

This option will strengthen protection against replay attacks,
especially when you are using OpenVPN in a dynamic context (such
as with
.B --inetd)
when OpenVPN sessions are frequently started and stopped. 

This option will keep a disk copy of the current replay protection
state (i.e. the most recent packet timestamp and sequence number
received from the remote peer), so that if an OpenVPN session
is stopped and restarted, it will reject any replays of packets
which were already received by the prior session.

This option only makes sense when replay protection is enabled
(the default) and you are using either
.B --secret
(shared-secret key mode) or TLS mode with
.B --tls-auth.
.\"*********************************************************
.TP
.B --no-iv
Disable OpenVPN's use of IV (cipher initialization vector).
Don't use this option unless you are prepared to make
a tradeoff of greater efficiency in exchange for less
security.

OpenVPN uses an IV by default, and requires it for CFB and
OFB cipher modes (which are totally insecure without it).
Using an IV is important for security when multiple
messages are being encrypted/decrypted with the same key.

IV is implemented differently depending on the cipher mode used.

In CBC mode, OpenVPN uses a pseudo-random IV for each packet.

In CFB/OFB mode, OpenVPN uses a unique sequence number and time stamp
as the IV.  In fact, in CFB/OFB mode, OpenVPN uses a datagram
space-saving optimization that uses the unique identifier for
datagram replay protection as the IV.
.\"*********************************************************
.TP
.B --test-crypto
Do a self-test of OpenVPN's crypto options by encrypting and
decrypting test packets using the data channel encryption options
specified above.  This option does not require a peer to function,
and therefore can be specified without
.B --dev
or
.B --remote.

The typical usage of
.B --test-crypto
would be something like this:

.B openvpn --test-crypto --secret key

or

.B openvpn --test-crypto --secret key --verb 9

This option is very useful to test OpenVPN after it has been ported to
a new platform, or to isolate problems in the compiler, OpenSSL
crypto library, or OpenVPN's crypto code.  Since it is a self-test mode,
problems with encryption and authentication can be debugged independently
of network and tunnel issues.
.\"*********************************************************
.SS TLS Mode Options:
TLS mode is the most powerful crypto mode of OpenVPN in both security and flexibility.
TLS mode works by establishing control and
data channels which are multiplexed over a single TCP/UDP port.  OpenVPN initiates
a TLS session over the control channel and uses it to exchange cipher
and HMAC keys to protect the data channel.  TLS mode uses a robust reliability
layer over the UDP connection for all control channel communication, while
the data channel, over which encrypted tunnel data passes, is forwarded without
any mediation.  The result is the best of both worlds: a fast data channel
that forwards over UDP with only the overhead of encrypt,
decrypt, and HMAC functions,
and a control channel that provides all of the security features of TLS,
including certificate-based authentication and Diffie Hellman forward secrecy.

To use TLS mode, each peer that runs OpenVPN should have its own local
certificate/key pair (
.B --cert
and
.B --key
), signed by the root certificate which is specified
in
.B --ca.

When two OpenVPN peers connect, each presents its local certificate to the
other.  Each peer will then check that its partner peer presented a
certificate which was signed by the master root certificate as specified in
.B --ca.

If that check on both peers succeeds, then the TLS negotiation
will succeed, both OpenVPN
peers will exchange temporary session keys, and the tunnel will begin
passing data.

The OpenVPN distribution contains a set of scripts for
managing RSA certificates & keys,
located in the
.I easy-rsa
subdirectory.

The easy-rsa package is also rendered in web form here:
.I http://openvpn.net/easyrsa.html
.\"*********************************************************
.TP
.B --tls-server
Enable TLS and assume server role during TLS handshake.  Note that
OpenVPN is designed as a peer-to-peer application.  The designation
of client or server is only for the purpose of negotiating the TLS
control channel.
.\"*********************************************************
.TP
.B --tls-client
Enable TLS and assume client role during TLS handshake.
.\"*********************************************************
.TP
.B --ca file
Certificate authority (CA) file in .pem format, also referred to as the
.I root
certificate.  This file can have multiple
certificates in .pem format, concatenated together.  You can construct your own
certificate authority certificate and private key by using a command such as:

.B openssl req -nodes -new -x509 -keyout ca.key -out ca.crt

Then edit your openssl.cnf file and edit the
.B certificate
variable to point to your new root certificate
.B ca.crt.

For testing purposes only, the OpenVPN distribution includes a sample
CA certificate (ca.crt).
Of course you should never use
the test certificates and test keys distributed with OpenVPN in a
production environment, since by virtue of the fact that
they are distributed with OpenVPN, they are totally insecure.
.\"*********************************************************
.TP
.B --dh file
File containing Diffie Hellman parameters
in .pem format (required for
.B --tls-server
only). Use

.B openssl dhparam -out dh1024.pem 1024

to generate your own, or use the existing dh1024.pem file
included with the OpenVPN distribution.  Diffie Hellman parameters
may be considered public.
.\"*********************************************************
.TP
.B --cert file
Local peer's signed certificate in .pem format -- must be signed
by a certificate authority whose certificate is in
.B --ca file.
Each peer in an OpenVPN link running in TLS mode should have its own
certificate and private key file.  In addition, each certificate should
have been signed by the key of a certificate
authority whose public key resides in the
.B --ca
certificate authority file.
You can easily make your own certificate authority (see above) or pay money
to use a commercial service such as thawte.com (in which case you will be
helping to finance the world's second space tourist :).
To generate a certificate,
you can use a command such as:

.B openssl req -nodes -new -keyout mycert.key -out mycert.csr

If your certificate authority private key lives on another machine, copy
the certificate signing request (mycert.csr) to this other machine (this can
be done over an insecure channel such as email).  Now sign the certificate
with a command such as:

.B openssl ca -out mycert.crt -in mycert.csr

Now copy the certificate (mycert.crt)
back to the peer which initially generated the .csr file (this
can be over a public medium).
Note that the
.B openssl ca
command reads the location of the certificate authority key from its
configuration file such as
.B /usr/share/ssl/openssl.cnf
-- note also
that for certificate authority functions, you must set up the files
.B index.txt
(may be empty) and
.B serial
(initialize to
.B 
01
).
.\"*********************************************************
.TP
.B --key file
Local peer's private key in .pem format.  Use the private key which was generated
when you built your peer's certificate (see
.B -cert file
above).
.\"*********************************************************
.TP
.B --pkcs12 file
Specify a PKCS #12 file containing local private key,
local certificate, and root CA certificate.
This option can be used instead of
.B --ca, --cert,
and
.B --key.
.\"*********************************************************
.TP
.B --pkcs11-cert-private [0|1]...
Set if access to certificate object should be performed after login.
Every provider has its own setting.
.\"*********************************************************
.TP
.B --pkcs11-id name
Specify the serialized certificate id to be used. The id can be gotten
by the standalone
.B --show-pkcs11-ids
option.
.\"*********************************************************
.TP
.B --pkcs11-id-management
Acquire PKCS#11 id from management interface. In this case a NEED-STR 'pkcs11-id-request'
real-time message will be triggered, application may use pkcs11-id-count command to
retrieve available number of certificates, and pkcs11-id-get command to retrieve certificate
id and certificate body.
.\"*********************************************************
.TP
.B --pkcs11-pin-cache seconds
Specify how many seconds the PIN can be cached, the default is until the token is removed.
.\"*********************************************************
.TP
.B --pkcs11-protected-authentication [0|1]...
Use PKCS#11 protected authentication path, useful for biometric and external
keypad devices.
Every provider has its own setting.
.\"*********************************************************
.TP
.B --pkcs11-providers provider...
Specify a RSA Security Inc. PKCS #11 Cryptographic Token Interface (Cryptoki) providers
to load.
This option can be used instead of
.B --cert, --key,
and
.B --pkcs12.
.\"*********************************************************
.TP
.B --pkcs11-private-mode mode...
Specify which method to use in order to perform private key operations.
A different mode can be specified for each provider.
Mode is encoded as hex number, and can be a mask one of the following:

.B 0
(default) -- Try to determind automatically.
.br
.B 1
-- Use sign.
.br
.B 2
-- Use sign recover.
.br
.B 4
-- Use decrypt.
.br
.B 8
-- Use unwrap.
.br
.\"*********************************************************
.TP
.B --cryptoapicert select-string
Load the certificate and private key from the
Windows Certificate System Store (Windows Only).

Use this option instead of
.B --cert
and
.B --key.

This makes
it possible to use any smart card, supported by Windows, but also any
kind of certificate, residing in the Cert Store, where you have access to
the private key.  This option has been tested with a couple of different
smart cards (GemSAFE, Cryptoflex, and Swedish Post Office eID) on the
client side, and also an imported PKCS12 software certificate on the
server side.

To select a certificate, based on a substring search in the
certificate's subject:

.B cryptoapicert
"SUBJ:Peter Runestig"

To select a certificate, based on certificate's thumbprint:

.B cryptoapicert
"THUMB:f6 49 24 41 01 b4 ..."

The thumbprint hex string can easily be copy-and-pasted from the Windows
Certificate Store GUI.

.\"*********************************************************
.TP
.B --key-method m
Use data channel key negotiation method
.B m.
The key method must match on both sides of the connection.

After OpenVPN negotiates a TLS session, a new set of keys
for protecting the tunnel data channel is generated and
exchanged over the TLS session.

In method 1 (the default for OpenVPN 1.x), both sides generate
random encrypt and HMAC-send keys which are forwarded to
the other host over the TLS channel.

In method 2, (the default for OpenVPN 2.0)
the client generates a random key.  Both client
and server also generate some random seed material.  All key source
material is exchanged over the TLS channel. The actual
keys are generated using the TLS PRF function, taking source
entropy from both client and server.  Method 2 is designed to
closely parallel the key generation process used by TLS 1.0.

Note that in TLS mode, two separate levels 
of keying occur:

(1) The TLS connection is initially negotiated, with both sides
of the connection producing certificates and verifying the certificate
(or other authentication info provided) of
the other side.  The
.B --key-method
parameter has no effect on this process.

(2) After the TLS connection is established, the tunnel session keys are
separately negotiated over the existing secure TLS channel.  Here,
.B --key-method
determines the derivation of the tunnel session keys.
.\"*********************************************************
.TP
.B --tls-cipher l
A list
.B l
of allowable TLS ciphers delimited by a colon (":").
If you require a high level of security,
you may want to set this parameter manually, to prevent a
version rollback attack where a man-in-the-middle attacker tries
to force two peers to negotiate to the lowest level
of security they both support.
Use
.B --show-tls
to see a list of supported TLS ciphers.
.\"*********************************************************
.TP
.B --tls-timeout n
Packet retransmit timeout on TLS control channel
if no acknowledgment from remote within
.B n
seconds (default=2).  When OpenVPN sends a control
packet to its peer, it will expect to receive an
acknowledgement within
.B n
seconds or it will retransmit the packet, subject
to a TCP-like exponential backoff algorithm.  This parameter
only applies to control channel packets.  Data channel
packets (which carry encrypted tunnel data) are never
acknowledged, sequenced, or retransmitted by OpenVPN because
the higher level network protocols running on top of the tunnel
such as TCP expect this role to be left to them.
.\"*********************************************************
.TP
.B --reneg-bytes n
Renegotiate data channel key after
.B n
bytes sent or received (disabled by default).
OpenVPN allows the lifetime of a key
to be expressed as a number of bytes encrypted/decrypted, a number of packets, or
a number of seconds.  A key renegotiation will be forced
if any of these three criteria are met by either peer.
.\"*********************************************************
.TP
.B --reneg-pkts n
Renegotiate data channel key after
.B n
packets sent and received (disabled by default).
.\"*********************************************************
.TP
.B --reneg-sec n
Renegotiate data channel key after
.B n
seconds (default=3600).

When using dual-factor authentication, note that this default value may
cause the end user to be challenged to reauthorize once per hour.

Also, keep in mind that this option can be used on both the client and server,
and whichever uses the lower value will be the one to trigger the renegotiation.
A common mistake is to set
.B --reneg-sec
to a higher value on either the client or server, while the other side of the connection
is still using the default value of 3600 seconds, meaning that the renegotiation will
still occur once per 3600 seconds.  The solution is to increase --reneg-sec on both the
client and server, or set it to 0 on one side of the connection (to disable), and to
your chosen value on the other side.
.\"*********************************************************
.TP
.B --hand-window n
Handshake Window -- the TLS-based key exchange must finalize within
.B n
seconds
of handshake initiation by any peer (default = 60 seconds).
If the handshake fails
we will attempt to reset our connection with our peer and try again.
Even in the event of handshake failure we will still use
our expiring key for up to
.B --tran-window
seconds to maintain continuity of transmission of tunnel
data.
.\"*********************************************************
.TP
.B --tran-window n
Transition window -- our old key can live this many seconds
after a new a key renegotiation begins (default = 3600 seconds).
This feature allows for a graceful transition from old to new
key, and removes the key renegotiation sequence from the critical
path of tunnel data forwarding.
.\"*********************************************************
.TP
.B --single-session
After initially connecting to a remote peer, disallow any new connections.
Using this
option means that a remote peer cannot connect, disconnect, and then
reconnect.

If the daemon is reset by a signal or
.B --ping-restart,
it will allow one new connection.

.B --single-session
can be used with
.B --ping-exit
or
.B --inactive
to create a single dynamic session that will exit when finished.
.\"*********************************************************
.TP
.B --tls-exit
Exit on TLS negotiation failure.
.\"*********************************************************
.TP
.B --tls-auth file [direction]
Add an additional layer of HMAC authentication on top of the TLS
control channel to protect against DoS attacks.

In a nutshell,
.B --tls-auth
enables a kind of "HMAC firewall" on OpenVPN's TCP/UDP port,
where TLS control channel packets
bearing an incorrect HMAC signature can be dropped immediately without
response.

.B file
(required) is a key file which can be in one of two formats:

.B (1)
An OpenVPN static key file generated by
.B --genkey
(required if
.B direction
parameter is used).

.B (2)
A freeform passphrase file.  In this case the HMAC key will
be derived by taking a secure hash of this file, similar to
the
.BR md5sum (1)
or
.BR sha1sum (1)
commands.

OpenVPN will first try format (1), and if the file fails to parse as
a static key file, format (2) will be used.

See the
.B --secret
option for more information on the optional
.B direction
parameter.

.B --tls-auth
is recommended when you are running OpenVPN in a mode where
it is listening for packets from any IP address, such as when
.B --remote
is not specified, or
.B --remote
is specified with
.B --float.

The rationale for
this feature is as follows.  TLS requires a multi-packet exchange
before it is able to authenticate a peer.  During this time
before authentication, OpenVPN is allocating resources (memory
and CPU) to this potential peer.  The potential peer is also
exposing many parts of OpenVPN and the OpenSSL library to the packets
it is sending.  Most successful network attacks today seek
to either exploit bugs in programs (such as buffer overflow attacks) or
force a program to consume so many resources that it becomes unusable.
Of course the first line of defense is always to produce clean,
well-audited code.  OpenVPN has been written with buffer overflow
attack prevention as a top priority.
But as history has shown, many of the most widely used
network applications have, from time to time,
fallen to buffer overflow attacks.

So as a second line of defense, OpenVPN offers
this special layer of authentication on top of the TLS control channel so that
every packet on the control channel is authenticated by an
HMAC signature and a unique ID for replay protection.
This signature will also help protect against DoS (Denial of Service) attacks.
An important rule of thumb in reducing vulnerability to DoS attacks is to
minimize the amount of resources a potential, but as yet unauthenticated,
client is able to consume.

.B --tls-auth
does this by signing every TLS control channel packet with an HMAC signature,
including packets which are sent before the TLS level has had a chance
to authenticate the peer.
The result is that packets without
the correct signature can be dropped immediately upon reception,
before they have a chance to consume additional system resources
such as by initiating a TLS handshake.
.B --tls-auth
can be strengthened by adding the
.B --replay-persist
option which will keep OpenVPN's replay protection state
in a file so that it is not lost across restarts.

It should be emphasized that this feature is optional and that the
passphrase/key file used with
.B --tls-auth
gives a peer nothing more than the power to initiate a TLS
handshake.  It is not used to encrypt or authenticate any tunnel data.
.\"*********************************************************
.TP
.B --askpass [file]
Get certificate password from console or
.B file
before we daemonize.

For the extremely
security conscious, it is possible to protect your private key with
a password.  Of course this means that every time the OpenVPN
daemon is started you must be there to type the password.  The
.B --askpass
option allows you to start OpenVPN from the command line.  It will
query you for a password before it daemonizes.  To protect a private
key with a password you should omit the
.B -nodes
option when you use the
.B openssl
command line tool to manage certificates and private keys.

If
.B file
is specified, read the password from the first line of
.B file.
Keep in mind that storing your password in a file
to a certain extent invalidates the extra security provided by
using an encrypted key (Note: OpenVPN
will only read passwords from a file if it has been built
with the --enable-password-save configure option, or on Windows
by defining ENABLE_PASSWORD_SAVE in config-win32.h).
.\"*********************************************************
.TP
.B --auth-nocache
Don't cache
.B --askpass
or
.B --auth-user-pass
username/passwords in virtual memory.

If specified, this directive will cause OpenVPN to immediately
forget username/password inputs after they are used.  As a result,
when OpenVPN needs a username/password, it will prompt for input
from stdin, which may be multiple times during the duration of an
OpenVPN session.

This directive does not affect the
.B --http-proxy
username/password.  It is always cached.
.\"*********************************************************
.TP
.B --tls-verify cmd
Execute shell command
.B cmd
to verify the X509 name of a
pending TLS connection that has otherwise passed all other
tests of certification (except for revocation via
.B --crl-verify
directive; the revocation test occurs after the
.B --tls-verify
test).

.B cmd
should return 0 to allow the TLS handshake to proceed, or 1 to fail.
.B cmd
is executed as

.B cmd certificate_depth X509_NAME_oneline

This feature is useful if the peer you want to trust has a certificate
which was signed by a certificate authority who also signed many
other certificates, where you don't necessarily want to trust all of them,
but rather be selective about which
peer certificate you will accept.  This feature allows you to write a script
which will test the X509 name on a certificate and decide whether or
not it should be accepted.  For a simple perl script which will test
the common name field on the certificate, see the file
.B verify-cn
in the OpenVPN distribution.

See the "Environmental Variables" section below for
additional parameters passed as environmental variables.

Note that
.B cmd
can be a shell command with multiple arguments, in which
case all OpenVPN-generated arguments will be appended
to
.B cmd
to build a command line which will be passed to the script.
.\"*********************************************************
.TP
.B --tls-remote name
Accept connections only from a host with X509 name
or common name equal to
.B name.
The remote host must also pass all other tests
of verification.

Name can also be a common name prefix, for example if you
want a client to only accept connections to "Server-1",
"Server-2", etc., you can simply use
.B --tls-remote Server

Using a common name prefix is a useful alternative to managing
a CRL (Certificate Revocation List) on the client, since it allows the client
to refuse all certificates except for those associated
with designated servers.

.B --tls-remote
is a useful replacement for the
.B --tls-verify
option to verify the remote host, because
.B --tls-remote
works in a
.B --chroot
environment too.
.\"*********************************************************
.TP
.B --ns-cert-type client|server
Require that peer certificate was signed with an explicit
.B nsCertType
designation of "client" or "server".

This is a useful security option for clients, to ensure that
the host they connect with is a designated server.

See the easy-rsa/build-key-server script for an example
of how to generate a certificate with the
.B nsCertType
field set to "server".

If the server certificate's nsCertType field is set
to "server", then the clients can verify this with
.B --ns-cert-type server.

This is an important security precaution to protect against
a man-in-the-middle attack where an authorized client
attempts to connect to another client by impersonating the server.
The attack is easily prevented by having clients verify
the server certificate using any one of
.B --ns-cert-type, --tls-remote,
or
.B --tls-verify.
.\"*********************************************************
.TP
.B --remote-cert-ku v...
Require that peer certificate was signed with an explicit
.B key usage.

This is a useful security option for clients, to ensure that
the host they connect to is a designated server.

The key usage should be encoded in hex, more than one key
usage can be specified.
.\"*********************************************************
.TP
.B --remote-cert-eku oid
Require that peer certificate was signed with an explicit
.B extended key usage.

This is a useful security option for clients, to ensure that
the host they connect to is a designated server.

The extended key usage should be encoded in oid notation, or
OpenSSL symbolic representation.
.\"*********************************************************
.TP
.B --remote-cert-tls client|server
Require that peer certificate was signed with an explicit
.B key usage
and
.B extended key usage
based on RFC3280 TLS rules.

This is a useful security option for clients, to ensure that
the host they connect to is a designated server.

The
.B --remote-cert-tls client
option is equivalent to
.B
--remote-cert-ku 80 08 88 --remote-cert-eku "TLS Web Client Authentication"

The key usage is digitalSignature and/or keyAgreement.

The
.B --remote-cert-tls server
option is equivalent to
.B
--remote-cert-ku a0 88 --remote-cert-eku "TLS Web Server Authentication"

The key usage is digitalSignature and ( keyEncipherment or keyAgreement ).

This is an important security precaution to protect against
a man-in-the-middle attack where an authorized client
attempts to connect to another client by impersonating the server.
The attack is easily prevented by having clients verify
the server certificate using any one of
.B --remote-cert-tls, --tls-remote,
or
.B --tls-verify.
.\"*********************************************************
.TP
.B --crl-verify crl
Check peer certificate against the file
.B crl
in PEM format.

A CRL (certificate revocation list) is used when a particular key is
compromised but when the overall PKI is still intact.

Suppose you had a PKI consisting of a CA, root certificate, and a number of
client certificates.  Suppose a laptop computer containing a client key and
certificate was stolen.  By adding the stolen certificate to the CRL file,
you could reject any connection which attempts to use it, while preserving the
overall integrity of the PKI.

The only time when it would be necessary to rebuild the entire PKI from scratch would be
if the root certificate key itself was compromised.
.\"*********************************************************
.SS SSL Library information:
.\"*********************************************************
.TP
.B --show-ciphers
(Standalone)
Show all cipher algorithms to use with the
.B --cipher
option.
.\"*********************************************************
.TP
.B --show-digests
(Standalone)
Show all message digest algorithms to use with the
.B --auth
option.
.\"*********************************************************
.TP
.B --show-tls
(Standalone)
Show all TLS ciphers (TLS used only as a control channel).  The TLS
ciphers will be sorted from highest preference (most secure) to
lowest.
.\"*********************************************************
.TP
.B --show-engines
(Standalone)
Show currently available hardware-based crypto acceleration
engines supported by the OpenSSL library.
.\"*********************************************************
.SS Generate a random key:
Used only for non-TLS static key encryption mode.
.\"*********************************************************
.TP
.B --genkey
(Standalone)
Generate a random key to be used as a shared secret,
for use with the
.B --secret
option.  This file must be shared with the
peer over a pre-existing secure channel such as
.BR scp (1)
.
.\"*********************************************************
.TP
.B --secret file
Write key to
.B file.
.\"*********************************************************
.SS TUN/TAP persistent tunnel config mode:
Available with linux 2.4.7+.  These options comprise a standalone mode
of OpenVPN which can be used to create and delete persistent tunnels.
.\"*********************************************************
.TP
.B --mktun
(Standalone)
Create a persistent tunnel on platforms which support them such
as Linux.  Normally TUN/TAP tunnels exist only for
the period of time that an application has them open.  This option
takes advantage of the TUN/TAP driver's ability to build persistent
tunnels that live through multiple instantiations of OpenVPN and die
only when they are deleted or the machine is rebooted.

One of the advantages of persistent tunnels is that they eliminate the
need for separate
.B --up
and
.B --down
scripts to run the appropriate
.BR ifconfig (8)
and
.BR route (8)
commands.  These commands can be placed in the the same shell script
which starts or terminates an OpenVPN session.

Another advantage is that open connections through the TUN/TAP-based tunnel
will not be reset if the OpenVPN peer restarts.  This can be useful to
provide uninterrupted connectivity through the tunnel in the event of a DHCP
reset of the peer's public IP address (see the
.B --ipchange
option above).

One disadvantage of persistent tunnels is that it is harder to automatically
configure their MTU value (see
.B --link-mtu
and
.B --tun-mtu
above).

On some platforms such as Windows, TAP-Win32 tunnels are persistent by
default.
.\"*********************************************************
.TP
.B --rmtun
(Standalone)
Remove a persistent tunnel.
.\"*********************************************************
.TP
.B --dev tunX | tapX
TUN/TAP device
.\"*********************************************************
.TP
.B --user user
Optional user to be owner of this tunnel.
.\"*********************************************************
.TP
.B --group group
Optional group to be owner of this tunnel.
.\"*********************************************************
.SS Windows-Specific Options:
.\"*********************************************************
.TP
.B --win-sys path|'env'
Set the Windows system directory pathname to use when looking for system
executables such as
.B route.exe
and
.B netsh.exe.
By default, if this directive is
not specified, the pathname will be set to "C:\\WINDOWS"

The special string
.B 'env'
indicates that the pathname should be read from the
.B SystemRoot
environmental variable.
.\"*********************************************************
.TP
.B --ip-win32 method
When using
.B --ifconfig
on Windows, set the TAP-Win32 adapter
IP address and netmask using
.B method.
Don't use this option unless you are also using
.B --ifconfig.

.B manual --
Don't set the IP address or netmask automatically.
Instead output a message
to the console telling the user to configure the
adapter manually and indicating the IP/netmask which
OpenVPN expects the adapter to be set to.

.B dynamic [offset] [lease-time] --
Automatically set the IP address and netmask by replying to
DHCP query messages generated by the kernel.  This mode is
probably the "cleanest" solution
for setting the TCP/IP properties since it uses the well-known
DHCP protocol.  There are, however, two prerequisites for using
this mode: (1) The TCP/IP properties for the TAP-Win32
adapter must be set to "Obtain an IP address automatically," and
(2) OpenVPN needs to claim an IP address in the subnet for use
as the virtual DHCP server address.  By default in
.B --dev tap
mode, OpenVPN will
take the normally unused first address in the subnet.  For example,
if your subnet is 192.168.4.0 netmask 255.255.255.0, then
OpenVPN will take the IP address 192.168.4.0 to use as the
virtual DHCP server address.  In
.B --dev tun
mode, OpenVPN will cause the DHCP server to masquerade as if it were
coming from the remote endpoint.  The optional offset parameter is
an integer which is > -256 and < 256 and which defaults to 0.
If offset is positive, the DHCP server will masquerade as the IP
address at network address + offset.
If offset is negative, the DHCP server will masquerade as the IP
address at broadcast address + offset.  The Windows
.B ipconfig /all
command can be used to show what Windows thinks the DHCP server
address is.  OpenVPN will "claim" this address, so make sure to
use a free address.  Having said that, different OpenVPN instantiations,
including different ends of the same connection, can share the same
virtual DHCP server address.  The
.B lease-time
parameter controls the lease time of the DHCP assignment given to
the TAP-Win32 adapter, and is denoted in seconds.
Normally a very long lease time is preferred
because it prevents routes involving the TAP-Win32 adapter from
being lost when the system goes to sleep.  The default
lease time is one year.

.B netsh --
Automatically set the IP address and netmask using
the Windows command-line "netsh"
command.  This method appears to work correctly on
Windows XP but not Windows 2000.

.B ipapi --
Automatically set the IP address and netmask using the
Windows IP Helper API.  This approach
does not have ideal semantics, though testing has indicated
that it works okay in practice.  If you use this option,
it is best to leave the TCP/IP properties for the TAP-Win32
adapter in their default state, i.e. "Obtain an IP address
automatically."

.B adaptive --
(Default) Try
.B dynamic
method initially and fail over to
.B netsh
if the DHCP negotiation with the TAP-Win32 adapter does
not succeed in 20 seconds.  Such failures have been known
to occur when certain third-party firewall packages installed
on the client machine block the DHCP negotiation used by
the TAP-Win32 adapter.
Note that if the
.B netsh
failover occurs, the TAP-Win32 adapter
TCP/IP properties will be reset from DHCP to static, and this
will cause future OpenVPN startups using the
.B adaptive
mode to use
.B netsh
immediately, rather than trying
.B dynamic
first.  To "unstick" the
.B adaptive
mode from using
.B netsh,
run OpenVPN at least once using the
.B dynamic
mode to restore the TAP-Win32 adapter TCP/IP properties
to a DHCP configuration.
.\"*********************************************************
.TP
.B --route-method m
Which method
.B m
to use for adding routes on Windows?

.B adaptive
(default) -- Try IP helper API first.  If that fails, fall
back to the route.exe shell command.
.br
.B ipapi
-- Use IP helper API.
.br
.B exe
-- Call the route.exe shell command.
.\"*********************************************************
.TP
.B --dhcp-option type [parm]
Set extended TAP-Win32 TCP/IP properties, must
be used with
.B --ip-win32 dynamic
or
.B --ip-win32 adaptive.
This option can be used to set additional TCP/IP properties
on the TAP-Win32 adapter, and is particularly useful for
configuring an OpenVPN client to access a Samba server
across the VPN.

.B DOMAIN name --
Set Connection-specific DNS Suffix.

.B DNS addr --
Set primary domain name server address.  Repeat
this option to set secondary DNS server addresses.

.B WINS addr --
Set primary WINS server address (NetBIOS over TCP/IP Name Server).
Repeat this option to set secondary WINS server addresses.

.B NBDD addr --
Set primary NBDD server address (NetBIOS over TCP/IP Datagram Distribution Server)
Repeat this option
to set secondary NBDD server addresses.

.B NTP addr --
Set primary NTP server address (Network Time Protocol).
Repeat this option
to set secondary NTP server addresses.

.B NBT type --
Set NetBIOS over TCP/IP Node type.  Possible options:
.B 1
= b-node (broadcasts),
.B 2
= p-node (point-to-point
name queries to a WINS server),
.B 4
= m-node (broadcast
then query name server), and
.B 8
= h-node (query name server, then broadcast).

.B NBS scope-id --
Set NetBIOS over TCP/IP Scope. A NetBIOS Scope ID provides an extended
naming service for the NetBIOS over TCP/IP (Known as NBT) module. The
primary purpose of a NetBIOS scope ID is to isolate NetBIOS traffic on
a single network to only those nodes with the same NetBIOS scope ID.
The NetBIOS scope ID is a character string that is appended to the NetBIOS
name. The NetBIOS scope ID on two hosts must match, or the two hosts
will not be able to communicate. The NetBIOS Scope ID also allows
computers to use the same computer name, as they have different
scope IDs. The Scope ID becomes a part of the NetBIOS name, making the name unique.
(This description of NetBIOS scopes courtesy of NeonSurge@abyss.com)

.B DISABLE-NBT --
Disable Netbios-over-TCP/IP.

Note that if
.B --dhcp-option
is pushed via
.B --push
to a non-windows client, the option will be saved in the client's
environment before the up script is called, under
the name "foreign_option_{n}".
.\"*********************************************************
.TP
.B --tap-sleep n
Cause OpenVPN to sleep for
.B n
seconds immediately after the TAP-Win32 adapter state
is set to "connected".

This option is intended to be used to troubleshoot problems
with the
.B --ifconfig
and
.B --ip-win32
options, and is used to give
the TAP-Win32 adapter time to come up before
Windows IP Helper API operations are applied to it.
.\"*********************************************************
.TP
.B --show-net-up
Output OpenVPN's view of the system routing table and network
adapter list to the syslog or log file after the TUN/TAP adapter
has been brought up and any routes have been added.
.\"*********************************************************
.TP
.B --dhcp-renew
Ask Windows to renew the TAP adapter lease on startup.
This option is normally unnecessary, as Windows automatically
triggers a DHCP renegotiation on the TAP adapter when it
comes up, however if you set the TAP-Win32 adapter
Media Status property to "Always Connected", you may need this
flag.
.\"*********************************************************
.TP
.B --dhcp-release
Ask Windows to release the TAP adapter lease on shutdown.
This option has the same caveats as
.B --dhcp-renew
above.
.\"*********************************************************
.TP
.B --pause-exit
Put up a "press any key to continue" message on the console prior
to OpenVPN program exit.  This option is automatically used by the
Windows explorer when OpenVPN is run on a configuration
file using the right-click explorer menu.
.\"*********************************************************
.TP
.B --service exit-event [0|1]
Should be used when OpenVPN is being automatically executed by another
program in such
a context that no interaction with the user via display or keyboard
is possible.  In general, end-users should never need to explicitly
use this option, as it is automatically added by the OpenVPN service wrapper
when a given OpenVPN configuration is being run as a service.

.B exit-event
is the name of a Windows global event object, and OpenVPN will continuously
monitor the state of this event object and exit when it becomes signaled.

The second parameter indicates the initial state of
.B exit-event
and normally defaults to 0.

Multiple OpenVPN processes can be simultaneously executed with the same
.B exit-event
parameter.  In any case, the controlling process can signal
.B exit-event,
causing all such OpenVPN processes to exit.

When executing an OpenVPN process using the
.B --service
directive, OpenVPN will probably not have a console
window to output status/error
messages, therefore it is useful to use
.B --log
or
.B --log-append
to write these messages to a file.
.\"*********************************************************
.TP
.B --show-adapters
(Standalone)
Show available TAP-Win32 adapters which can be selected using the
.B --dev-node
option.  On non-Windows systems, the
.BR ifconfig (8)
command provides similar functionality.
.\"*********************************************************
.TP
.B --allow-nonadmin [TAP-adapter]
(Standalone)
Set
.B TAP-adapter
to allow access from non-administrative accounts.  If
.B TAP-adapter
is omitted, all TAP adapters on the system will be configured to allow
non-admin access.
The non-admin access setting will only persist for the length of time that
the TAP-Win32 device object and driver remain loaded, and will need
to be re-enabled after a reboot, or if the driver is unloaded
and reloaded.
This directive can only be used by an administrator.
.\"*********************************************************
.TP
.B --show-valid-subnets
(Standalone)
Show valid subnets for
.B --dev tun
emulation.  Since the TAP-Win32 driver
exports an ethernet interface to Windows, and since TUN devices are
point-to-point in nature, it is necessary for the TAP-Win32 driver
to impose certain constraints on TUN endpoint address selection.

Namely, the point-to-point endpoints used in TUN device emulation
must be the middle two addresses of a /30 subnet (netmask 255.255.255.252).
.\"*********************************************************
.TP
.B --show-net
(Standalone)
Show OpenVPN's view of the system routing table and network
adapter list.
.\"*********************************************************
.SS PKCS#11 Standalone Options:
.\"*********************************************************
.TP
.B --show-pkcs11-ids provider [cert_private]
(Standalone)
Show PKCS#11 token object list. Specify cert_private as 1
if certificates are stored as private objects.

.B --verb
option can be used BEFORE this option to produce debugging information.
.\"*********************************************************
.SH SCRIPTING AND ENVIRONMENTAL VARIABLES
OpenVPN exports a series
of environmental variables for use by user-defined scripts.
.\"*********************************************************
.SS Script Order of Execution
.\"*********************************************************
.TP
.B --up
Executed after TCP/UDP socket bind and TUN/TAP open.
.\"*********************************************************
.TP
.B --tls-verify
Executed when we have a still untrusted remote peer.
.\"*********************************************************
.TP
.B --ipchange
Executed after connection authentication, or remote IP address change.
.\"*********************************************************
.TP
.B --client-connect
Executed in
.B --mode server
mode immediately after client authentication.
.\"*********************************************************
.TP
.B --route-up
Executed after connection authentication, either
immediately after, or some number of seconds after
as defined by the
.B --route-delay
option.
.\"*********************************************************
.TP
.B --client-disconnect
Executed in
.B --mode server
mode on client instance shutdown.
.\"*********************************************************
.TP
.B --down
Executed after TCP/UDP and TUN/TAP close.
.\"*********************************************************
.TP
.B --learn-address
Executed in
.B --mode server
mode whenever an IPv4 address/route or MAC address is added to OpenVPN's
internal routing table.
.\"*********************************************************
.TP
.B --auth-user-pass-verify
Executed in
.B --mode server
mode on new client connections, when the client is
still untrusted.
.\"*********************************************************
.SS String Types and Remapping
In certain cases, OpenVPN will perform remapping of characters
in strings.  Essentially, any characters outside the set of
permitted characters for each string type will be converted
to underbar ('_').

.B Q:
Why is string remapping necessary?

.B A:
It's an important security feature to prevent the malicious coding of
strings from untrusted sources to be passed as parameters to scripts,
saved in the environment, used as a common name, translated to a filename,
etc.

Here is a brief rundown of OpenVPN's current string types and the 
permitted character class for each string:

.B X509 Names:
Alphanumeric, underbar ('_'), dash ('-'), dot ('.'), at 
('@'), colon (':'), slash ('/'), and equal ('=').  Alphanumeric is defined 
as a character which will cause the C library isalnum() function to return 
true.

.B Common Names:
Alphanumeric, underbar ('_'), dash ('-'), dot ('.'), and at                
('@').

.B --auth-user-pass username:
Same as Common Name, with one exception: starting with OpenVPN 2.0.1,
the username is passed to the OPENVPN_PLUGIN_AUTH_USER_PASS_VERIFY plugin in its raw form,
without string remapping.

.B --auth-user-pass password:
Any "printable" character except CR or LF.  
Printable is defined to be a character which will cause the C library 
isprint() function to return true.

.B --client-config-dir filename as derived from common name or username:
Alphanumeric, underbar ('_'), dash ('-'), and dot ('.') except for "." or 
".." as standalone strings.  As of 2.0.1-rc6, the at ('@') character has
been added as well for compatibility with the common name character class.

.B Environmental variable names:
Alphanumeric or underbar ('_').

.B Environmental variable values:
Any printable character.

For all cases, characters in a string which are not members of the legal 
character class for that string type will be remapped to underbar ('_').
.\"*********************************************************
.SS Environmental Variables
Once set, a variable is persisted
indefinitely until it is reset by a new value or a restart,

As of OpenVPN 2.0-beta12, in server mode, environmental
variables set by OpenVPN
are scoped according to the client objects
they are
associated with, so there should not be any issues with
scripts having access to stale, previously set variables
which refer to different client instances.
.\"*********************************************************
.TP
.B bytes_received
Total number of bytes received from client during VPN session.
Set prior to execution of the
.B --client-disconnect
script.
.\"*********************************************************
.TP
.B bytes_sent
Total number of bytes sent to client during VPN session.
Set prior to execution of the
.B --client-disconnect
script.
.\"*********************************************************
.TP
.B common_name
The X509 common name of an authenticated client.
Set prior to execution of
.B --client-connect, --client-disconnect,
and
.B --auth-user-pass-verify
scripts.
.\"*********************************************************
.TP
.B config
Name of first
.B --config
file.
Set on program initiation and reset on SIGHUP.
.\"*********************************************************
.TP
.B daemon
Set to "1" if the
.B --daemon
directive is specified, or "0" otherwise.
Set on program initiation and reset on SIGHUP.
.\"*********************************************************
.TP
.B daemon_log_redirect
Set to "1" if the
.B --log
or
.B --log-append
directives are specified, or "0" otherwise.
Set on program initiation and reset on SIGHUP.
.\"*********************************************************
.TP
.B dev
The actual name of the TUN/TAP device, including
a unit number if it exists.
Set prior to
.B --up
or
.B --down
script execution.
.\"*********************************************************
.TP
.B foreign_option_{n}
An option pushed via
.B --push
to a client which does not natively support it,
such as
.B --dhcp-option
on a non-Windows system, will be recorded to this
environmental variable sequence prior to
.B --up
script execution.
.\"*********************************************************
.TP
.B ifconfig_broadcast
The broadcast address for the virtual
ethernet segment which is derived from the
.B --ifconfig
option when
.B --dev tap
is used.
Set prior to OpenVPN calling the
.I ifconfig
or
.I netsh
(windows version of ifconfig) commands which
normally occurs prior to
.B --up
script execution.
.\"*********************************************************
.TP
.B ifconfig_local
The local VPN endpoint IP address specified in the
.B --ifconfig
option (first parameter).
Set prior to OpenVPN calling the
.I ifconfig
or
.I netsh
(windows version of ifconfig) commands which
normally occurs prior to
.B --up
script execution.
.\"*********************************************************
.TP
.B ifconfig_remote
The remote VPN endpoint IP address specified in the
.B --ifconfig
option (second parameter) when
.B --dev tun
is used.
Set prior to OpenVPN calling the
.I ifconfig
or
.I netsh
(windows version of ifconfig) commands which
normally occurs prior to
.B --up
script execution.
.\"*********************************************************
.TP
.B ifconfig_netmask
The subnet mask of the virtual ethernet segment
that is specified as the second parameter to
.B --ifconfig
when
.B --dev tap
is being used.
Set prior to OpenVPN calling the
.I ifconfig
or
.I netsh
(windows version of ifconfig) commands which
normally occurs prior to
.B --up
script execution.
.\"*********************************************************
.TP
.B ifconfig_pool_local_ip
The local
virtual IP address for the TUN/TAP tunnel taken from an
.B --ifconfig-push
directive if specified, or otherwise from
the ifconfig pool (controlled by the
.B --ifconfig-pool
config file directive).
Only set for
.B --dev tun
tunnels.
This option is set on the server prior to execution
of the
.B --client-connect
and
.B --client-disconnect
scripts.
.\"*********************************************************
.TP
.B ifconfig_pool_netmask
The
virtual IP netmask for the TUN/TAP tunnel taken from an
.B --ifconfig-push
directive if specified, or otherwise from
the ifconfig pool (controlled by the
.B --ifconfig-pool
config file directive).
Only set for
.B --dev tap
tunnels.
This option is set on the server prior to execution
of the
.B --client-connect
and
.B --client-disconnect
scripts.
.\"*********************************************************
.TP
.B ifconfig_pool_remote_ip
The remote
virtual IP address for the TUN/TAP tunnel taken from an
.B --ifconfig-push
directive if specified, or otherwise from
the ifconfig pool (controlled by the
.B --ifconfig-pool
config file directive).
This option is set on the server prior to execution
of the
.B --client-connect
and
.B --client-disconnect
scripts.
.\"*********************************************************
.TP
.B link_mtu
The maximum packet size (not including the IP header)
of tunnel data in UDP tunnel transport mode.
Set prior to
.B --up
or
.B --down
script execution.
.\"*********************************************************
.TP
.B local
The
.B --local
parameter.
Set on program initiation and reset on SIGHUP.
.\"*********************************************************
.TP
.B local_port
The local port number, specified by
.B --port
or
.B --lport.
Set on program initiation and reset on SIGHUP.
.\"*********************************************************
.TP
.B password
The password provided by a connecting client.
Set prior to
.B --auth-user-pass-verify
script execution only when the
.B via-env
modifier is specified, and deleted from the environment
after the script returns.
.\"*********************************************************
.TP
.B proto
The
.B --proto
parameter.
Set on program initiation and reset on SIGHUP.
.\"*********************************************************
.TP
.B remote_{n}
The
.B --remote
parameter.
Set on program initiation and reset on SIGHUP.
.\"*********************************************************
.TP
.B remote_port_{n}
The remote port number, specified by
.B --port
or
.B --rport.
Set on program initiation and reset on SIGHUP.
.\"*********************************************************
.TP
.B route_net_gateway
The pre-existing default IP gateway in the system routing
table.
Set prior to
.B --up
script execution.
.\"*********************************************************
.TP
.B route_vpn_gateway
The default gateway used by
.B --route
options, as specified in either the
.B --route-gateway
option or the second parameter to
.B --ifconfig
when
.B --dev tun
is specified.
Set prior to
.B --up
script execution.
.\"*********************************************************
.TP
.B route_{parm}_{n}
A set of variables which define each route to be added, and
are set prior to
.B --up
script execution.

.B parm
will be one of "network", "netmask", "gateway", or "metric".

.B n
is the OpenVPN route number, starting from 1.

If the network or gateway are resolvable DNS names,
their IP address translations will be recorded rather
than their names as denoted on the command line
or configuration file.
.\"*********************************************************
.TP
.B script_context
Set to "init" or "restart" prior to up/down script execution.
For more information, see
documentation for
.B --up.
.\"*********************************************************
.TP
.B script_type
One of
.B up, down, ipchange, route-up, tls-verify, auth-user-pass-verify,
.B client-connect, client-disconnect, 
or
.B learn-address.
Set prior to execution of any script.
.\"*********************************************************
.TP
.B signal
The reason for exit or restart.  Can be one of
.B sigusr1, sighup, sigterm, sigint, inactive
(controlled by
.B --inactive
option),
.B ping-exit
(controlled by
.B --ping-exit
option),
.B ping-restart
(controlled by
.B --ping-restart
option),
.B connection-reset
(triggered on TCP connection reset),
.B error,
or
.B unknown
(unknown signal).  This variable is set just prior to down script execution.
.\"*********************************************************
.TP
.B time_ascii
Client connection timestamp, formatted as a human-readable
time string.
Set prior to execution of the
.B --client-connect
script.
.\"*********************************************************
.TP
.B time_duration
The duration (in seconds) of the client session which is now
disconnecting.
Set prior to execution of the
.B --client-disconnect
script.
.\"*********************************************************
.TP
.B time_unix
Client connection timestamp, formatted as a unix integer
date/time value.
Set prior to execution of the
.B --client-connect
script.
.\"*********************************************************
.TP
.B tls_id_{n}
A series of certificate fields from the remote peer,
where
.B n
is the verification level.  Only set for TLS connections.  Set prior
to execution of
.B --tls-verify
script.
.\"*********************************************************
.TP
.B tls_serial_{n}
The serial number of the certificate from the remote peer,
where
.B n
is the verification level.  Only set for TLS connections.  Set prior
to execution of
.B --tls-verify
script.
.\"*********************************************************
.TP
.B tun_mtu
The MTU of the TUN/TAP device.
Set prior to
.B --up
or
.B --down
script execution.
.\"*********************************************************
.TP
.B trusted_ip
Actual IP address of connecting client or peer which has been authenticated.
Set prior to execution of
.B --ipchange, --client-connect,
and
.B --client-disconnect
scripts.
.\"*********************************************************
.TP
.B trusted_port
Actual port number of connecting client or peer which has been authenticated.
Set prior to execution of
.B --ipchange, --client-connect,
and
.B --client-disconnect
scripts.
.\"*********************************************************
.TP
.B untrusted_ip
Actual IP address of connecting client or peer which has not been authenticated
yet.  Sometimes used to
.B nmap
the connecting host in a
.B --tls-verify
script to ensure it is firewalled properly.
Set prior to execution of
.B --tls-verify
and
.B --auth-user-pass-verify
scripts.
.\"*********************************************************
.TP
.B untrusted_port
Actual port number of connecting client or peer which has not been authenticated
yet.
Set prior to execution of
.B --tls-verify
and
.B --auth-user-pass-verify
scripts.
.\"*********************************************************
.TP
.B username
The username provided by a connecting client.
Set prior to
.B --auth-user-pass-verify
script execution only when the
.B via-env
modifier is specified.
.\"*********************************************************
.TP
.B X509_{n}_{subject_field}
An X509 subject field from the remote peer certificate,
where
.B n
is the verification level.  Only set for TLS connections.  Set prior
to execution of
.B --tls-verify
script.  This variable is similar to
.B tls_id_{n}
except the component X509 subject fields are broken out, and
no string remapping occurs on these field values (except for remapping
of control characters to "_").
For example, the following variables would be set on the
OpenVPN server using the sample client certificate
in sample-keys (client.crt).
Note that the verification level is 0 for the client certificate
and 1 for the CA certificate.
.RS
.ft 3
.nf
.sp
X509_0_emailAddress=me@myhost.mydomain
X509_0_CN=Test-Client
X509_0_O=OpenVPN-TEST
X509_0_ST=NA
X509_0_C=KG
X509_1_emailAddress=me@myhost.mydomain
X509_1_O=OpenVPN-TEST
X509_1_L=BISHKEK
X509_1_ST=NA
X509_1_C=KG
.ft
.LP
.RE
.fi
.\"*********************************************************
.SH SIGNALS
.TP
.B SIGHUP
Cause OpenVPN to close all TUN/TAP and
network connections,
restart, re-read the configuration file (if any),
and reopen TUN/TAP and network connections.
.\"*********************************************************
.TP
.B SIGUSR1
Like 
.B SIGHUP,
except don't re-read configuration file, and possibly don't close and reopen TUN/TAP
device, re-read key files, preserve local IP address/port, or preserve most recently authenticated
remote IP address/port based on
.B --persist-tun, --persist-key, --persist-local-ip,
and
.B --persist-remote-ip
options respectively (see above).

This signal may also be internally generated by a timeout condition, governed
by the
.B --ping-restart
option.

This signal, when combined with
.B --persist-remote-ip,
may be
sent when the underlying parameters of the host's network interface change
such as when the host is a DHCP client and is assigned a new IP address.
See
.B --ipchange
above for more information.
.\"*********************************************************
.TP
.B SIGUSR2
Causes OpenVPN to display its current statistics (to the syslog
file if
.B --daemon
is used, or stdout otherwise).
.\"*********************************************************
.TP
.B SIGINT, SIGTERM
Causes OpenVPN to exit gracefully.
.\"*********************************************************
.SH TUN/TAP DRIVER SETUP
If you are running Linux 2.4.7 or higher, you probably have the TUN/TAP driver
already installed.  If so, there are still a few things you need to do:

Make device:
.B mknod /dev/net/tun c 10 200

Load driver:
.B modprobe tun

If you have Linux 2.2 or earlier, you should obtain version 1.1 of the
TUN/TAP driver from
.I http://vtun.sourceforge.net/tun/
and follow the installation instructions.
.\"*********************************************************
.SH EXAMPLES
Prior to running these examples, you should have OpenVPN installed on two
machines with network connectivity between them.  If you have not
yet installed OpenVPN, consult the INSTALL file included in the OpenVPN
distribution.
.\"*********************************************************
.SS TUN/TAP Setup:
If you are using Linux 2.4 or higher,
make the tun device node and load the tun module:
.IP
.B mknod /dev/net/tun c 10 200
.LP
.IP
.B modprobe tun
.LP
If you installed from RPM, the
.B mknod
step may be omitted, because the RPM install does that for you.

If you have Linux 2.2, you should obtain version 1.1 of the
TUN/TAP driver from
.I http://vtun.sourceforge.net/tun/
and follow the installation instructions.

For other platforms, consult the INSTALL file at
.I http://openvpn.net/install.html
for more information.
.\"*********************************************************
.SS Firewall Setup:
If firewalls exist between
the two machines, they should be set to forward UDP port 1194
in both directions.  If you do not have control over the firewalls
between the two machines, you may still be able to use OpenVPN by adding
.B --ping 15
to each of the
.B openvpn
commands used below in the examples (this will cause each peer to send out
a UDP ping to its remote peer once every 15 seconds which will cause many
stateful firewalls to forward packets in both directions
without an explicit firewall rule).

If you are using a Linux iptables-based firewall, you may need to enter
the following command to allow incoming packets on the TUN device:
.IP
.B iptables -A INPUT -i tun+ -j ACCEPT
.LP
See the firewalls section below for more information on configuring firewalls
for use with OpenVPN.
.\"*********************************************************
.SS VPN Address Setup:
For purposes
of our example, our two machines will be called
.B may.kg
and
.B june.kg.
If you are constructing a VPN over the internet, then replace
.B may.kg
and
.B june.kg
with the internet hostname or IP address that each machine will use
to contact the other over the internet.

Now we will choose the tunnel endpoints.  Tunnel endpoints are
private IP addresses that only have meaning in the context of
the VPN.  Each machine will use the tunnel endpoint of the other
machine to access it over the VPN.  In our example,
the tunnel endpoint for may.kg
will be 10.4.0.1 and for june.kg, 10.4.0.2.

Once the VPN is established, you have essentially
created a secure alternate path between the two hosts
which is addressed by using the tunnel endpoints.  You can
control which network
traffic passes between the hosts 
(a) over the VPN or (b) independently of the VPN, by choosing whether to use
(a) the VPN endpoint address or (b) the public internet address,
to access the remote host. For example if you are on may.kg and you wish to connect to june.kg
via
.B ssh
without using the VPN (since
.B ssh
has its own built-in security) you would use the command
.B ssh june.kg.
However in the same scenario, you could also use the command
.B telnet 10.4.0.2
to create a telnet session with june.kg over the VPN, that would
use the VPN to secure the session rather than
.B ssh.

You can use any address you wish for the
tunnel endpoints
but make sure that they are private addresses
(such as those that begin with 10 or 192.168) and that they are
not part of any existing subnet on the networks of
either peer, unless you are bridging.  If you use an address that is part of
your local subnet for either of the tunnel endpoints,
you will get a weird feedback loop.
.\"*********************************************************
.SS Example 1: A simple tunnel without security
.LP
On may:
.IP
.B openvpn --remote june.kg --dev tun1 --ifconfig 10.4.0.1 10.4.0.2 --verb 9
.LP
On june:
.IP
.B openvpn --remote may.kg --dev tun1 --ifconfig 10.4.0.2 10.4.0.1 --verb 9
.LP
Now verify the tunnel is working by pinging across the tunnel.
.LP
On may:
.IP
.B ping 10.4.0.2
.LP
On june:
.IP
.B ping 10.4.0.1
.LP
The
.B --verb 9
option will produce verbose output, similar to the
.BR tcpdump (8)
program.  Omit the
.B --verb 9
option to have OpenVPN run quietly.
.\"*********************************************************
.SS Example 2: A tunnel with static-key security (i.e. using a pre-shared secret)
First build a static key on may.
.IP
.B openvpn --genkey --secret key
.LP
This command will build a random key file called
.B key
(in ascii format).
Now copy
.B key
to june over a secure medium such as by
using the
.BR scp (1)
program.
.LP
On may:
.IP
.B openvpn --remote june.kg --dev tun1 --ifconfig 10.4.0.1 10.4.0.2 --verb 5 --secret key
.LP
On june:
.IP
.B openvpn --remote may.kg --dev tun1 --ifconfig 10.4.0.2 10.4.0.1 --verb 5 --secret key
.LP
Now verify the tunnel is working by pinging across the tunnel.
.LP
On may:
.IP
.B ping 10.4.0.2
.LP
On june:
.IP
.B ping 10.4.0.1
.\"*********************************************************
.SS Example 3: A tunnel with full TLS-based security
For this test, we will designate
.B may
as the TLS client and
.B june
as the TLS server.
.I Note that client or server designation only has meaning for the TLS subsystem.  It has no bearing on OpenVPN's peer-to-peer, UDP-based communication model.

First, build a separate certificate/key pair
for both may and june (see above where
.B --cert
is discussed for more info).  Then construct
Diffie Hellman parameters (see above where
.B --dh
is discussed for more info).  You can also use the
included test files client.crt, client.key,
server.crt, server.key and ca.crt.
The .crt files are certificates/public-keys, the .key
files are private keys, and ca.crt is a certification
authority who has signed both
client.crt and server.crt.  For Diffie Hellman
parameters you can use the included file dh1024.pem.
.I Note that all client, server, and certificate authority certificates and keys included in the OpenVPN distribution are totally insecure and should be used for testing only.
.LP
On may:
.IP
.B openvpn --remote june.kg --dev tun1 --ifconfig 10.4.0.1 10.4.0.2 --tls-client --ca ca.crt --cert client.crt --key client.key --reneg-sec 60 --verb 5
.LP
On june:
.IP
.B openvpn --remote may.kg --dev tun1 --ifconfig 10.4.0.2 10.4.0.1 --tls-server --dh dh1024.pem --ca ca.crt --cert server.crt --key server.key --reneg-sec 60 --verb 5
.LP
Now verify the tunnel is working by pinging across the tunnel.
.LP
On may:
.IP
.B ping 10.4.0.2
.LP
On june:
.IP
.B ping 10.4.0.1
.LP
Notice the
.B --reneg-sec 60
option we used above.  That tells OpenVPN to renegotiate
the data channel keys every minute.
Since we used
.B --verb 5
above, you will see status information on each new key negotiation.

For production operations, a key renegotiation interval of 60 seconds
is probably too frequent.  Omit the
.B --reneg-sec 60
option to use OpenVPN's default key renegotiation interval of one hour.
.\"*********************************************************
.SS Routing:
Assuming you can ping across the tunnel,
the next step is to route a real subnet over
the secure tunnel.  Suppose that may and june have two network
interfaces each, one connected
to the internet, and the other to a private
network.  Our goal is to securely connect
both private networks.  We will assume that may's private subnet
is 10.0.0.0/24 and june's is 10.0.1.0/24.
.LP
First, ensure that IP forwarding is enabled on both peers.
On Linux, enable routing:
.IP
.B echo 1 > /proc/sys/net/ipv4/ip_forward
.LP
and enable TUN packet forwarding through the firewall:
.IP
.B iptables -A FORWARD -i tun+ -j ACCEPT
.LP
On may:
.IP
.B route add -net 10.0.1.0 netmask 255.255.255.0 gw 10.4.0.2
.LP
On june:
.IP
.B route add -net 10.0.0.0 netmask 255.255.255.0 gw 10.4.0.1
.LP
Now any machine on the 10.0.0.0/24 subnet can
access any machine on the 10.0.1.0/24 subnet
over the secure tunnel (or vice versa).

In a production environment, you could put the route command(s)
in a shell script and execute with the
.B --up
option.
.\"*********************************************************
.SH FIREWALLS
OpenVPN's usage of a single UDP port makes it fairly firewall-friendly.
You should add an entry to your firewall rules to allow incoming OpenVPN
packets.  On Linux 2.4+:
.IP
.B iptables -A INPUT -p udp -s 1.2.3.4 --dport 1194 -j ACCEPT
.LP
This will allow incoming packets on UDP port 1194 (OpenVPN's default UDP port)
from an OpenVPN peer at 1.2.3.4.

If you are using HMAC-based packet authentication (the default in any of
OpenVPN's secure modes), having the firewall filter on source
address can be considered optional, since HMAC packet authentication
is a much more secure method of verifying the authenticity of
a packet source.  In that case:
.IP
.B iptables -A INPUT -p udp --dport 1194 -j ACCEPT
.LP
would be adequate and would not render the host inflexible with
respect to its peer having a dynamic IP address.

OpenVPN also works well on stateful firewalls.  In some cases, you may
not need to add any static rules to the firewall list if you are
using a stateful firewall that knows how to track UDP connections.
If you specify
.B --ping n,
OpenVPN will be guaranteed
to send a packet to its peer at least once every
.B n
seconds.  If
.B n
is less than the stateful firewall connection timeout, you can
maintain an OpenVPN connection indefinitely without explicit
firewall rules.

You should also add firewall rules to allow incoming IP traffic on
TUN or TAP devices such as:
.IP
.B iptables -A INPUT -i tun+ -j ACCEPT
.LP
to allow input packets from tun devices,
.IP
.B iptables -A FORWARD -i tun+ -j ACCEPT
.LP
to allow input packets from tun devices to be forwarded to
other hosts on the local network,
.IP
.B iptables -A INPUT -i tap+ -j ACCEPT
.LP
to allow input packets from tap devices, and
.IP
.B iptables -A FORWARD -i tap+ -j ACCEPT
.LP
to allow input packets from tap devices to be forwarded to
other hosts on the local network.

These rules are secure if you use packet authentication,
since no incoming packets will arrive on a TUN or TAP
virtual device
unless they first pass an HMAC authentication test.
.\"*********************************************************
.SH FAQ
.I http://openvpn.net/faq.html
.\"*********************************************************
.SH HOWTO
For a more comprehensive guide to setting up OpenVPN
in a production setting, see the OpenVPN HOWTO at
.I http://openvpn.net/howto.html
.\"*********************************************************
.SH PROTOCOL
For a description of OpenVPN's underlying protocol,
see 
.I http://openvpn.net/security.html
.\"*********************************************************
.SH WEB
OpenVPN's web site is at
.I http://openvpn.net/

Go here to download the latest version of OpenVPN, subscribe
to the mailing lists, read the mailing list
archives, or browse the CVS repository.
.\"*********************************************************
.SH BUGS
Report all bugs to the OpenVPN users list <openvpn-users@lists.sourceforge.net>.
To subscribe to the list or see the archives, go to
.I http://openvpn.net/mail.html
.\"*********************************************************
.SH "SEE ALSO"
.BR dhcpcd (8),
.BR ifconfig (8),
.BR openssl (1),
.BR route (8),
.BR scp (1)
.BR ssh (1)
.\"*********************************************************
.SH NOTES 
.LP
This product includes software developed by the
OpenSSL Project (
.I http://www.openssl.org/
)

For more information on the TLS protocol, see
.I http://www.ietf.org/rfc/rfc2246.txt

For more information on the LZO real-time compression library see
.I http://www.oberhumer.com/opensource/lzo/
.\"*********************************************************
.SH COPYRIGHT
Copyright (C) 2002-2008 OpenVPN Technologies, Inc. This program is free software;
you can redistribute it and/or modify
it under the terms of the GNU General Public License version 2
as published by the Free Software Foundation.
.\"*********************************************************
.SH AUTHORS
James Yonan <jim@yonan.net>