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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
<html><head><title>TLS-protected syslog: recommended scenario</title>
</head>
<body>
<h1>Encrypting Syslog Traffic with TLS (SSL)</h1>
<p><small><i>Written by <a href="http://www.adiscon.com/en/people/rainer-gerhards.php">Rainer
Gerhards</a> (2008-06-06)</i></small></p>
<h2>Introduction</h2>
<p>This document describes a secure way to set up rsyslog TLS. A secure logging
environment requires more than just encrypting the transmission channel. This document
provides one possible way to create such a secure system.
<p>Rsyslog's TLS authentication can be used very flexible and thus supports a
wide range of security policies. This section tries to give some advise on a
scenario that works well for many environments. However, it may not be suitable
for you - please assess you security needs before using the recommendations
below. Do not blame us if it doesn't provide what you need ;)</p>
<p>Our policy offers these security benefits:</p>
<ul>
<li>syslog messages are encrypted while traveling on the wire</li>
<li>the syslog sender authenticates to the syslog receiver; thus, the
receiver knows who is talking to it</li>
<li>the syslog receiver authenticates to the syslog sender; thus, the sender
can check if it indeed is sending to the expected receiver</li>
<li>the mutual authentication prevents man-in-the-middle attacks</li>
</ul>
<p>Our secrity goals are achived via public/private key security. As such, it is
vital that private keys are well protected and not accessible to third parties.
I private keys have become known to third parties, the system does not provide
any security at all. Also, our solution bases on X.509 certificates and a (very
limited) chain of trust. We have one instance (the CA) that issues all machine
certificates. The machine certificate indentifies a particular machine. hile in
theory (and practice), there could be several "sub-CA" that issues machine
certificates for a specific adminitrative domain, we do not include this in our
"simple yet secure" setup. If you intend to use this, rsyslog supports it, but
then you need to dig a bit more into the documentation (or use the forum to ask).
In general, if you depart from our simple model, you should have good reasons
for doing so and know quite well what you are doing - otherwise you may
compromise your system security.</p>
<p>Please note that security never comes without effort. In the scenario
described here, we have limited the effort as much as possible. What remains is
some setup work for the central CA, the certificate setup for each machine as
well as a few configuration commands that need to be applied to all of them.
Proably the most important limiting factor in our setup is that all senders and
receivers must support IETF's syslog-transport-tls standard (which is not
finalized yet). We use mandatory-to-implement technology, yet you may have
trouble finding all required features in some implementations. More often,
unfortunately, you will find that an implementation does not support the
upcoming IETF standard at all - especially in the "early days" (starting May
2008) when rsyslog is the only implementation of said standard.</p>
<p>Fortunately, rsyslog supports allmost every protocol that is out there in the
syslog world. So in cases where transport-tls is not available on a sender, we
recommend to use rsyslog as the initial relay. In that mode, the not-capabe
sender sends to rsyslog via another protocol, which then relays the message via
transport-tls to either another interim relay or the final destination (which,
of course, must by transport-tls capable). In such a scenario, it is best to try
see what the sender support. Maybe it is possible to use industry-standard plain
tcp syslog with it. Often you can even combine it with stunnel, which then, too,
enables a secure delivery to the first rsyslog relay. If all of that is not
possible, you can (and often must...) resort to UDP. Even though this is now
lossy and insecure, this is better than not having the ability to listen to that
device at all. It may even be reasonale secure if the uncapable sender and the
first rsyslog relay communicate via a private channel, e.g. a dedicated network
link.</p>
<p>One final word of caution: transport-tls protects the connection between the
sender and the receiver. It does not necessarily protect against attacks that
are present in the message itself. Especially in a relay environment, the
message may have been originated from a malicious system, which placed invalid
hostnames and/or other content into it. If there is no provisioning against such
things, these records may show up in the receivers' repository. -transport-tls
does not protect against this (but it may help, properly used). Keep in mind
that syslog-transport-tls provides hop-by-hop security. It does not provide
end-to-end security and it does not authenticate the message itself (just the
last sender).</p>
<h3>Sample Szenario</h3>
<p> We have a quite simple scenario. There is one central syslog server,
named central.example.net. These server is being reported to by two Linux
machines with name zuse.example.net and turing.example.net. Also, there is a
third client - ada.example.net - which send both its own messages to the central
server but also forwards messages receive from an UDP-only capable router. We
hav decided to use ada.example.net because it is in the same local network
segment as the router and so we enjoy TLS' security benefits for forwarding the
router messages inside the corporate network.</p>
<h3>Setting up the CA</h3>
<p>The first step is to set up a certificate authoroty (CA). It must be
maintained by a trustworthy person (or group) and approves the indentities of
all machines. It does so by issuing there certificates. In a small setup, the
administrator can provide the CA function. What is important is the the CA's
private key is well-protocted and machine certificates are only issued if it is
know they are valid (in a single-admin case that means the admin should not
issue certificates to anyone else except himself).</p>
<p>The CA creates a so-called self-signed certificate. That is, it approves its
own authenticy. This sounds useless, but the key point to understand is that
every machine will be provided a copy of the CA's certificate. Accepting this
certificate is a matter of trust. So by configuring the CA certificate, the
administrator tells rsyslog which certificates to trust. This is the root of all
trust under this model. That is why the CA's private key is so important -
everyone getting hold of it is trusted by our rsyslog instances.</p>
<p>In our example, we will use the name "example.net" for our network. You may
use any domain name of your liking.
<p>To create a self-signed certificate, use the following commands with GnuTLS (which
is currently the only supported TLS library, what may change in the future): </p>
<ol>
<li>generate the private key:
<pre>certtool --generate-privkey --outfile ca-key.pem</pre>
<br>
This takes a short while. Be sure to do some work on your workstation,
it waits for radom input. Switching between windows is sufficient ;)
</li>
<li>now create the (self-signed) CA certificate itself:<br>
<pre>certtool --generate-self-signed --load-privkey ca-key.pem --outfile ca.pem</pre>
This generates the CA certificate. This command queries you for a
number of things. Use appropriate responses. When it comes to
certificate validity, keep in mind that you need to recreate all
certificates when this one expires. So it may be a good idea to use a
long period, eg. 3650 days (roughly 10 years). You need to specify that
the certificates belongs to an authority. The certificate is used to
sign other certificates.<br>
</li>
</ol>
<h3>Feedback requested</h3>
<p>I would appreciate feedback on this tutorial. If you have
additional ideas, comments or find bugs (I *do* bugs - no way... ;)),
please
<a href="mailto:rgerhards@adiscon.com">let me know</a>.</p>
<h2>Revision History</h2>
<ul>
<li>2008-06-06 * <a href="http://www.gerhards.net/rainer">Rainer
Gerhards</a> * Initial Version created</li>
</ul>
<h2>Copyright</h2>
<p>Copyright (c) 2008 <a href="http://www.adiscon.com/en/people/rainer-gerhards.php">Rainer
Gerhards</a> and
<a href="http://www.adiscon.com/en/">Adiscon</a>.</p>
<p> Permission is granted to copy, distribute and/or modify this
document under the terms of the GNU Free Documentation License, Version
1.2 or any later version published by the Free Software Foundation;
with no Invariant Sections, no Front-Cover Texts, and no Back-Cover
Texts. A copy of the license can be viewed at
<a href="http://www.gnu.org/copyleft/fdl.html">http://www.gnu.org/copyleft/fdl.html</a>.</p>
</body></html>
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