Encrypting Syslog Traffic with TLS (SSL)

Written by Rainer Gerhards (2008-05-06)

Abstract

In this paper, I describe how to encrypt syslog messages on the network. Encryption is vital to keep the confidiental content of syslog messages secure. I describe the overall approach and provide an HOWTO do it with rsyslog's TLS features. 

Please note that TLS is the more secure successor of SSL. While people often talk about "SSL encryption" they actually mean "TLS encryption". So don't look any further if you look for how to SSL-encrypt syslog. You have found the right spot.

This is a quick guide. There is a more elaborate guide currently under construction which provides a much more secure environment. It is highly recommended to at least have a look at it.

Background

Traditional syslog is a clear-text protocol. That means anyone with a sniffer can have a peek at your data. In some environments, this is no problem at all. In others, it is a huge setback, probably even preventing deployment of syslog solutions. Thankfully, there are easy ways to encrypt syslog communication. 

The traditional approach involves running a wrapper like stunnel around the syslog session. This works quite well and is in widespread use. However, it is not thightly coupled with the main syslogd and some, even severe, problems can result from this (follow a mailing list thread that describes total loss of syslog messages due to stunnel mode and the unreliability of TCP syslog).

Rsyslog supports syslog via GSSAPI since long to overcome these limitatinos. However, syslog via GSSAPI is a rsyslog-exclusive transfer mode and it requires a proper Kerberos environment. As such, it isn't a really universal solution. The IETF has begun standardizing syslog over plain tcp over TLS for a while now. While I am not fully satisfied with the results so far, this obviously has the  potential to become the long-term solution. The Internet Draft in question, syslog-transport-tls has been dormant for some time but is now (May of 2008) again being worked on. I expect it to turn into a RFC within the next 12 month (but don't take this for granted ;)). I didn't want to wait for it, because there obviously is need for TLS syslog right now (and, honestly, I have waited long enough...). Consequently, I have implemented the current draft, with some interpretations I made (there will be a compliance doc soon). So in essence, a TLS-protected syslog transfer mode is available right now. As a side-note, Rsyslog is the world's first implementation of syslog-transport-tls.

Please note that in theory it should be compatible with other, non IETF syslog-transport-tls implementations. If you would like to run it with something else, please let us know so that we can create a compatibility list (and implement compatbility where it doesn't yet exist). 

Overall System Setup

Encryption requires a reliable stream. So It will not work over UDP syslog. In rsyslog, network transports utilize a so-called "network stream layer" (netstream for short). This layer provides a unified view of the transport to the application layer. The plain TCP syslog sender and receiver are the upper layer. The driver layer currently consists of the "ptcp" and "gtls" library plugins. "ptcp" stands for "plain tcp" and is used for unencrypted message transfer. It is also used internally by the gtls driver, so it must always be present on a system. The "gtls" driver is for GnutTLS, a TLS library. It is used for encrypted message transfer. In the future, additional drivers will become available (most importantly, we would like to include a driver for NSS).

What you need to do to build an encrypted syslog channel is to simply use the proper netstream drivers on both the client and the server. Client, in the sense of this document, is the rsyslog system that is sending syslog messages to a remote (central) loghost, which is called the server. In short, the setup is as follows:

Client

  • forwards messages via plain tcp syslog using gtls netstream driver to central sever on port 10514

Server

  • accept incoming messages via plain tcp syslog using gtls netstream driver on port 10514

Setting up the system

Server Setup

At the server, you need to have a digital certificate. That certificate enables SSL operation, as it provides the necessary crypto keys being used to secure the connection. There is a set of default certificates in ./contrib/gnutls. These are key.pem and cert.pem. These are good for testing. If you use it in production, it is very easy to break into your secure channel as everybody is able to get hold of your private key. So it is a good idea to generate the key and certificate yourself.

You also need a root CA certificate. Again, there is a sample CA certificate in ./contrib/gnutls, named ca.cert. It is suggested to generate your own.

To configure the server, you need to tell it where are its certificate files, to use the gtls driver and start up a listener. This is done as follows:

# make gtls driver the default
$DefaultNetstreamDriver gtls

# certificate files
$DefaultNetstreamDriverCAFile /path/to/contrib/gnutls/ca.pem
$DefaultNetstreamDriverCertFile /path/to/contrib/gnutls/cert.pem
$DefaultNetstreamDriverKeyFile /path/to/contrib/gnutls/key.pem

$ModLoad imtcp # load TCP listener

$InputTCPServerStreamDriverMode 1 # run driver in TLS-only mode
$InputTCPServerStreamDriverAuthMode anon # client is NOT authenticated
$InputTCPServerRun 10514 # start up listener at port 10514
This is all you need to do. You can use the rest of your rsyslog.conf together with this configuration. The way messages are received does not interfer with any other option, so you are able to do anything else you like without any restrictions.

Restart rsyslogd. The server should now be fully operational.

Client Setup

The client setup is equally simple. You need less certificates, just the CA cert. 

# certificate files - just CA for a client
$DefaultNetstreamDriverCAFile /path/to/contrib/gnutls/ca.pem

# set up the action
$DefaultNetstreamDriver gtls # use gtls netstream driver
$ActionSendStreamDriverMode 1 # require TLS for the connection
$ActionSendStreamDriverAuthMode anon # server is NOT authenticated
*.* @@(o)server.example.net:10514 # send (all) messages

Note that we use the regular TCP forwarding syntax (@@) here. There is nothing special, because the encryption is handled by the netstream driver. So I have just forwarded every message (*.*) for simplicity - you can use any of rsyslog's filtering capabilities (like epxression-based filters or regular expressions). Note that the "(o)" part is not strictly necessary. It selects octet-based framing, which provides compatiblity to IETF's syslog-transport-tls draft. Besides compatibility, this is also a more reliable transfer mode, so I suggest to always use it.

Done

After following these steps, you should have a working secure syslog forwarding system. To verify, you can type "logger test" or a similar "smart" command on the client. It should show up in the respective server log file. If you dig out your sniffer, you should see that the traffic on the wire is actually protected.

Limitations

The RELP transport can currently not be protected by TLS. A work-around is to use stunnel. TLS support for RELP will be added once plain TCP syslog has sufficiently matured and there either is some time left to do this or we find a sponsor ;).

Certificates

In order to be really secure, certificates are needed. This is a short summary on how to generate the necessary certificates with GnuTLS' certtool. You can also generate certificates via other tools, but as we currently support GnuTLS as the only TLS library, we thought it is a good idea to use their tools.

Note that this section aims at people who are not involved with PKI at all. The main goal is to get them going in a reasonable secure way. 

CA Certificate

This is used to sign all of your other certificates. The CA cert must be trusted by all clients and servers. The private key must be well-protected and not given to any third parties. The certificate itself can (and must) be distributed. To generate it, do the following:

  1. generate the private key:
    certtool --generate-privkey --outfile ca-key.pem

    This takes a short while. Be sure to do some work on your workstation, it waits for radom input. Switching between windows is sufficient ;)
  2. now create the (self-signed) CA certificate itself:
    certtool --generate-self-signed --load-privkey ca-key.pem --outfile ca.pem
    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 authrity. The certificate is used to sign other certificates.
  3. You need to distribute this certificate to all peers and you need to point to it via the $DefaultNetstreamDriverCAFile config directive. All other certificates will be issued by this CA.
    Important: do only distribute the ca.pem, NOT ca-key.pem (the private key). Distributing the CA private key would totally breach security as everybody could issue new certificates on the behalf of this CA.

Individual Peer Certificate

Each peer (be it client, server or both), needs a certificate that conveys its identity. Access control is based on these certificates. You can, for example, configure a server to accept connections only from configured clients. The client ID is taken from the client instances certificate. So as a general rule of thumb, you need to create a certificate for each instance of rsyslogd that you run. That instance also needs the private key, so that it can properly decrypt the traffic. Safeguard the peer's private key file. If somebody gets hold of it, it can malicously pretend to be the compromised host. If such happens, regenerate the certificate and make sure you use a different name instead of the compromised one (if you use name-based authentication). 

These are the steps to generate the indivudual certificates (repeat: you need to do this for every instance, do NOT share the certificates created in this step):

  1. generate a private key (do NOT mistake this with the CA's private key - this one is different):
    certtool --generate-privkey --outfile key.pem
    Again, this takes a short while.
  2. generate a certificate request:
    certtool --generate-request --load-privkey key.pem --outfile request.pem
    If you do not have the CA's private key (because you are not authorized for this), you can send the certificate request to the responsible person. If you do this, you can skip the remaining steps, as the CA will provide you with the final certificate. If you submit the request to the CA, you need to tell the CA the answers that you would normally provide in step 3 below.
  3. Sign (validate, authorize) the certificate request and generate the instances certificate. You need to have the CA's certificate and private key for this:
    certtool --generate-certificate --load-request request.pem --outfile cert.pem \
    --load-ca-certificate ca.pem --load-ca-privkey ca-key.pem
    Answer questions as follows: Cert does not belogn to an authority; it is a TLS web server and client certificate; the dnsName MUST be the name of the peer in question (e.g. centralserver.example.net) - this is the name used for authenticating the peers. Please note that you may use an IP address in dnsName. This is a good idea if you would like to use default server authentication and you use selector lines with IP addresses (e.g. "*.* @@192.168.0.1") - in that case you need to select a dnsName of 192.168.0.1. But, of course, changing the server IP then requires generating a new certificate.
After you have generated the certificate, you need to place it onto the local machine running rsyslogd. Specify the certificate and key via the $DefaultNetstreamDriverCertFile /path/to/cert.pem and $DefaultNetstreamDriverKeyFile /path/to/key.pem configuration directives. Make sure that nobody has access to key.pem, as that would breach security. And, once again: do NOT use these files on more than one instance. Doing so would prevent you from distinguising between the instances and thus would disable useful authentication.

Troubleshooting Certificates

If you experience trouble with your certificate setup, it may be useful to get some information on what is contained in a specific certificate (file). To obtain that information, do 

$ certtool --certificate-info --infile cert.pem

where "cert.pem" can be replaced by the various certificate pem files (but it does not work with the key files).

Conclusion

With minumal effort, you can set up a secure logging infrastructure employing TLS encrypted syslog message transmission.

Feedback requested

I would appreciate feedback on this tutorial. If you have additional ideas, comments or find bugs (I *do* bugs - no way... ;)), please let me know.

Revision History

Copyright

Copyright (c) 2008 Rainer Gerhards and Adiscon.

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 http://www.gnu.org/copyleft/fdl.html.

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Copyright © 2008 by Rainer Gerhards and Adiscon. Released under the GNU GPL version 2 or higher.