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-This document describes how to use the kernel's L2TP drivers to
-provide L2TP functionality. L2TP is a protocol that tunnels one or
-more sessions over an IP tunnel. It is commonly used for VPNs
-(L2TP/IPSec) and by ISPs to tunnel subscriber PPP sessions over an IP
-network infrastructure. With L2TPv3, it is also useful as a Layer-2
-tunneling infrastructure.
-
-Features
-========
-
-L2TPv2 (PPP over L2TP (UDP tunnels)).
-L2TPv3 ethernet pseudowires.
-L2TPv3 PPP pseudowires.
-L2TPv3 IP encapsulation.
-Netlink sockets for L2TPv3 configuration management.
-
-History
-=======
-
-The original pppol2tp driver was introduced in 2.6.23 and provided
-L2TPv2 functionality (rfc2661). L2TPv2 is used to tunnel one or more PPP
-sessions over a UDP tunnel.
-
-L2TPv3 (rfc3931) changes the protocol to allow different frame types
-to be passed over an L2TP tunnel by moving the PPP-specific parts of
-the protocol out of the core L2TP packet headers. Each frame type is
-known as a pseudowire type. Ethernet, PPP, HDLC, Frame Relay and ATM
-pseudowires for L2TP are defined in separate RFC standards. Another
-change for L2TPv3 is that it can be carried directly over IP with no
-UDP header (UDP is optional). It is also possible to create static
-unmanaged L2TPv3 tunnels manually without a control protocol
-(userspace daemon) to manage them.
-
-To support L2TPv3, the original pppol2tp driver was split up to
-separate the L2TP and PPP functionality. Existing L2TPv2 userspace
-apps should be unaffected as the original pppol2tp sockets API is
-retained. L2TPv3, however, uses netlink to manage L2TPv3 tunnels and
-sessions.
-
-Design
-======
-
-The L2TP protocol separates control and data frames. The L2TP kernel
-drivers handle only L2TP data frames; control frames are always
-handled by userspace. L2TP control frames carry messages between L2TP
-clients/servers and are used to setup / teardown tunnels and
-sessions. An L2TP client or server is implemented in userspace.
-
-Each L2TP tunnel is implemented using a UDP or L2TPIP socket; L2TPIP
-provides L2TPv3 IP encapsulation (no UDP) and is implemented using a
-new l2tpip socket family. The tunnel socket is typically created by
-userspace, though for unmanaged L2TPv3 tunnels, the socket can also be
-created by the kernel. Each L2TP session (pseudowire) gets a network
-interface instance. In the case of PPP, these interfaces are created
-indirectly by pppd using a pppol2tp socket. In the case of ethernet,
-the netdevice is created upon a netlink request to create an L2TPv3
-ethernet pseudowire.
-
-For PPP, the PPPoL2TP driver, net/l2tp/l2tp_ppp.c, provides a
-mechanism by which PPP frames carried through an L2TP session are
-passed through the kernel's PPP subsystem. The standard PPP daemon,
-pppd, handles all PPP interaction with the peer. PPP network
-interfaces are created for each local PPP endpoint. The kernel's PPP
-subsystem arranges for PPP control frames to be delivered to pppd,
-while data frames are forwarded as usual.
-
-For ethernet, the L2TPETH driver, net/l2tp/l2tp_eth.c, implements a
-netdevice driver, managing virtual ethernet devices, one per
-pseudowire. These interfaces can be managed using standard Linux tools
-such as "ip" and "ifconfig". If only IP frames are passed over the
-tunnel, the interface can be given an IP addresses of itself and its
-peer. If non-IP frames are to be passed over the tunnel, the interface
-can be added to a bridge using brctl. All L2TP datapath protocol
-functions are handled by the L2TP core driver.
-
-Each tunnel and session within a tunnel is assigned a unique tunnel_id
-and session_id. These ids are carried in the L2TP header of every
-control and data packet. (Actually, in L2TPv3, the tunnel_id isn't
-present in data frames - it is inferred from the IP connection on
-which the packet was received.) The L2TP driver uses the ids to lookup
-internal tunnel and/or session contexts to determine how to handle the
-packet. Zero tunnel / session ids are treated specially - zero ids are
-never assigned to tunnels or sessions in the network. In the driver,
-the tunnel context keeps a reference to the tunnel UDP or L2TPIP
-socket. The session context holds data that lets the driver interface
-to the kernel's network frame type subsystems, i.e. PPP, ethernet.
-
-Userspace Programming
-=====================
-
-For L2TPv2, there are a number of requirements on the userspace L2TP
-daemon in order to use the pppol2tp driver.
-
-1. Use a UDP socket per tunnel.
-
-2. Create a single PPPoL2TP socket per tunnel bound to a special null
- session id. This is used only for communicating with the driver but
- must remain open while the tunnel is active. Opening this tunnel
- management socket causes the driver to mark the tunnel socket as an
- L2TP UDP encapsulation socket and flags it for use by the
- referenced tunnel id. This hooks up the UDP receive path via
- udp_encap_rcv() in net/ipv4/udp.c. PPP data frames are never passed
- in this special PPPoX socket.
-
-3. Create a PPPoL2TP socket per L2TP session. This is typically done
- by starting pppd with the pppol2tp plugin and appropriate
- arguments. A PPPoL2TP tunnel management socket (Step 2) must be
- created before the first PPPoL2TP session socket is created.
-
-When creating PPPoL2TP sockets, the application provides information
-to the driver about the socket in a socket connect() call. Source and
-destination tunnel and session ids are provided, as well as the file
-descriptor of a UDP socket. See struct pppol2tp_addr in
-include/linux/if_pppol2tp.h. Note that zero tunnel / session ids are
-treated specially. When creating the per-tunnel PPPoL2TP management
-socket in Step 2 above, zero source and destination session ids are
-specified, which tells the driver to prepare the supplied UDP file
-descriptor for use as an L2TP tunnel socket.
-
-Userspace may control behavior of the tunnel or session using
-setsockopt and ioctl on the PPPoX socket. The following socket
-options are supported:-
-
-DEBUG - bitmask of debug message categories. See below.
-SENDSEQ - 0 => don't send packets with sequence numbers
- 1 => send packets with sequence numbers
-RECVSEQ - 0 => receive packet sequence numbers are optional
- 1 => drop receive packets without sequence numbers
-LNSMODE - 0 => act as LAC.
- 1 => act as LNS.
-REORDERTO - reorder timeout (in millisecs). If 0, don't try to reorder.
-
-Only the DEBUG option is supported by the special tunnel management
-PPPoX socket.
-
-In addition to the standard PPP ioctls, a PPPIOCGL2TPSTATS is provided
-to retrieve tunnel and session statistics from the kernel using the
-PPPoX socket of the appropriate tunnel or session.
-
-For L2TPv3, userspace must use the netlink API defined in
-include/linux/l2tp.h to manage tunnel and session contexts. The
-general procedure to create a new L2TP tunnel with one session is:-
-
-1. Open a GENL socket using L2TP_GENL_NAME for configuring the kernel
- using netlink.
-
-2. Create a UDP or L2TPIP socket for the tunnel.
-
-3. Create a new L2TP tunnel using a L2TP_CMD_TUNNEL_CREATE
- request. Set attributes according to desired tunnel parameters,
- referencing the UDP or L2TPIP socket created in the previous step.
-
-4. Create a new L2TP session in the tunnel using a
- L2TP_CMD_SESSION_CREATE request.
-
-The tunnel and all of its sessions are closed when the tunnel socket
-is closed. The netlink API may also be used to delete sessions and
-tunnels. Configuration and status info may be set or read using netlink.
-
-The L2TP driver also supports static (unmanaged) L2TPv3 tunnels. These
-are where there is no L2TP control message exchange with the peer to
-setup the tunnel; the tunnel is configured manually at each end of the
-tunnel. There is no need for an L2TP userspace application in this
-case -- the tunnel socket is created by the kernel and configured
-using parameters sent in the L2TP_CMD_TUNNEL_CREATE netlink
-request. The "ip" utility of iproute2 has commands for managing static
-L2TPv3 tunnels; do "ip l2tp help" for more information.
-
-Debugging
-=========
-
-The driver supports a flexible debug scheme where kernel trace
-messages may be optionally enabled per tunnel and per session. Care is
-needed when debugging a live system since the messages are not
-rate-limited and a busy system could be swamped. Userspace uses
-setsockopt on the PPPoX socket to set a debug mask.
-
-The following debug mask bits are available:
-
-PPPOL2TP_MSG_DEBUG verbose debug (if compiled in)
-PPPOL2TP_MSG_CONTROL userspace - kernel interface
-PPPOL2TP_MSG_SEQ sequence numbers handling
-PPPOL2TP_MSG_DATA data packets
-
-If enabled, files under a l2tp debugfs directory can be used to dump
-kernel state about L2TP tunnels and sessions. To access it, the
-debugfs filesystem must first be mounted.
-
-# mount -t debugfs debugfs /debug
-
-Files under the l2tp directory can then be accessed.
-
-# cat /debug/l2tp/tunnels
-
-The debugfs files should not be used by applications to obtain L2TP
-state information because the file format is subject to change. It is
-implemented to provide extra debug information to help diagnose
-problems.) Users should use the netlink API.
-
-/proc/net/pppol2tp is also provided for backwards compaibility with
-the original pppol2tp driver. It lists information about L2TPv2
-tunnels and sessions only. Its use is discouraged.
-
-Unmanaged L2TPv3 Tunnels
-========================
-
-Some commercial L2TP products support unmanaged L2TPv3 ethernet
-tunnels, where there is no L2TP control protocol; tunnels are
-configured at each side manually. New commands are available in
-iproute2's ip utility to support this.
-
-To create an L2TPv3 ethernet pseudowire between local host 192.168.1.1
-and peer 192.168.1.2, using IP addresses 10.5.1.1 and 10.5.1.2 for the
-tunnel endpoints:-
-
-# modprobe l2tp_eth
-# modprobe l2tp_netlink
-
-# ip l2tp add tunnel tunnel_id 1 peer_tunnel_id 1 udp_sport 5000 \
- udp_dport 5000 encap udp local 192.168.1.1 remote 192.168.1.2
-# ip l2tp add session tunnel_id 1 session_id 1 peer_session_id 1
-# ifconfig -a
-# ip addr add 10.5.1.2/32 peer 10.5.1.1/32 dev l2tpeth0
-# ifconfig l2tpeth0 up
-
-Choose IP addresses to be the address of a local IP interface and that
-of the remote system. The IP addresses of the l2tpeth0 interface can be
-anything suitable.
-
-Repeat the above at the peer, with ports, tunnel/session ids and IP
-addresses reversed. The tunnel and session IDs can be any non-zero
-32-bit number, but the values must be reversed at the peer.
-
-Host 1 Host2
-udp_sport=5000 udp_sport=5001
-udp_dport=5001 udp_dport=5000
-tunnel_id=42 tunnel_id=45
-peer_tunnel_id=45 peer_tunnel_id=42
-session_id=128 session_id=5196755
-peer_session_id=5196755 peer_session_id=128
-
-When done at both ends of the tunnel, it should be possible to send
-data over the network. e.g.
-
-# ping 10.5.1.1
-
-
-Sample Userspace Code
-=====================
-
-1. Create tunnel management PPPoX socket
-
- kernel_fd = socket(AF_PPPOX, SOCK_DGRAM, PX_PROTO_OL2TP);
- if (kernel_fd >= 0) {
- struct sockaddr_pppol2tp sax;
- struct sockaddr_in const *peer_addr;
-
- peer_addr = l2tp_tunnel_get_peer_addr(tunnel);
- memset(&sax, 0, sizeof(sax));
- sax.sa_family = AF_PPPOX;
- sax.sa_protocol = PX_PROTO_OL2TP;
- sax.pppol2tp.fd = udp_fd; /* fd of tunnel UDP socket */
- sax.pppol2tp.addr.sin_addr.s_addr = peer_addr->sin_addr.s_addr;
- sax.pppol2tp.addr.sin_port = peer_addr->sin_port;
- sax.pppol2tp.addr.sin_family = AF_INET;
- sax.pppol2tp.s_tunnel = tunnel_id;
- sax.pppol2tp.s_session = 0; /* special case: mgmt socket */
- sax.pppol2tp.d_tunnel = 0;
- sax.pppol2tp.d_session = 0; /* special case: mgmt socket */
-
- if(connect(kernel_fd, (struct sockaddr *)&sax, sizeof(sax) ) < 0 ) {
- perror("connect failed");
- result = -errno;
- goto err;
- }
- }
-
-2. Create session PPPoX data socket
-
- struct sockaddr_pppol2tp sax;
- int fd;
-
- /* Note, the target socket must be bound already, else it will not be ready */
- sax.sa_family = AF_PPPOX;
- sax.sa_protocol = PX_PROTO_OL2TP;
- sax.pppol2tp.fd = tunnel_fd;
- sax.pppol2tp.addr.sin_addr.s_addr = addr->sin_addr.s_addr;
- sax.pppol2tp.addr.sin_port = addr->sin_port;
- sax.pppol2tp.addr.sin_family = AF_INET;
- sax.pppol2tp.s_tunnel = tunnel_id;
- sax.pppol2tp.s_session = session_id;
- sax.pppol2tp.d_tunnel = peer_tunnel_id;
- sax.pppol2tp.d_session = peer_session_id;
-
- /* session_fd is the fd of the session's PPPoL2TP socket.
- * tunnel_fd is the fd of the tunnel UDP socket.
- */
- fd = connect(session_fd, (struct sockaddr *)&sax, sizeof(sax));
- if (fd < 0 ) {
- return -errno;
- }
- return 0;
-
-Internal Implementation
-=======================
-
-The driver keeps a struct l2tp_tunnel context per L2TP tunnel and a
-struct l2tp_session context for each session. The l2tp_tunnel is
-always associated with a UDP or L2TP/IP socket and keeps a list of
-sessions in the tunnel. The l2tp_session context keeps kernel state
-about the session. It has private data which is used for data specific
-to the session type. With L2TPv2, the session always carried PPP
-traffic. With L2TPv3, the session can also carry ethernet frames
-(ethernet pseudowire) or other data types such as ATM, HDLC or Frame
-Relay.
-
-When a tunnel is first opened, the reference count on the socket is
-increased using sock_hold(). This ensures that the kernel socket
-cannot be removed while L2TP's data structures reference it.
-
-Some L2TP sessions also have a socket (PPP pseudowires) while others
-do not (ethernet pseudowires). We can't use the socket reference count
-as the reference count for session contexts. The L2TP implementation
-therefore has its own internal reference counts on the session
-contexts.
-
-To Do
-=====
-
-Add L2TP tunnel switching support. This would route tunneled traffic
-from one L2TP tunnel into another. Specified in
-http://tools.ietf.org/html/draft-ietf-l2tpext-tunnel-switching-08
-
-Add L2TPv3 VLAN pseudowire support.
-
-Add L2TPv3 IP pseudowire support.
-
-Add L2TPv3 ATM pseudowire support.
-
-Miscellaneous
-=============
-
-The L2TP drivers were developed as part of the OpenL2TP project by
-Katalix Systems Ltd. OpenL2TP is a full-featured L2TP client / server,
-designed from the ground up to have the L2TP datapath in the
-kernel. The project also implemented the pppol2tp plugin for pppd
-which allows pppd to use the kernel driver. Details can be found at
-http://www.openl2tp.org.