Encrypting Syslog Traffic with TLS (SSL)
Written by Rainer
Gerhards (2008-06-18)
Setting up the Central Server
In this step, we configure the central server. We assume it accepts messages only
via TLS protected plain tcp based syslog from those peers that are explicitely permitted
to send to it. The picture below show our configuration. This step configures
the server central.example.net.
Important: Keep in mind that the order of configuration directives
is very important in rsyslog. As such, the samples given below do only work if the given
order is preserved. Re-ordering the directives can break configurations and has broken them
in practice. If you intend to re-order them, please be sure that you fully understand how
the configuration language works and, most importantly, which statements form a block together.
Please also note that we understand the the current configuration file format is
ugly. However, there has been more important work in the way of enhancing it. If you would like
to contribute some time to improve the config file language, please let us know. Any help
is appreciated (be it doc or coding work!).
Steps to do:
- make sure you have a functional CA (Setting up the CA)
- generate a machine certificate for central.example.net (follow instructions in
Generating Machine Certificates)
- make sure you copy over ca.pem, machine-key.pem ad machine-cert.pem to the central server.
Ensure that no user except root can access them (even read permissions are really bad).
- configure the server so that it accepts messages from all machines in the
example.net domain that have certificates from your CA. Alternatively, you may also
precisely define from which machine names messages are accepted. See sample rsyslog.conf
below.
In this setup, we use wildcards to ease adding new systems. We permit the server to accept
messages from systems whos names match *.example.net.
$InputTCPServerStreamDriverPermittedPeer *.example.net
This will match zuse.example.net and
turing.example.net, but NOT pascal.otherdepartment.example.net. If the later would be desired,
you can (and need) to include additional permitted peer config statments:
$InputTCPServerStreamDriverPermittedPeer *.example.net
$InputTCPServerStreamDriverPermittedPeer *.otherdepartment.example.net
$InputTCPServerStreamDriverPermittedPeer *.example.com
As can be seen with example.com, the different permitted peers need NOT to be in a single
domain tree. Also, individual machines can be configured. For example, if only zuse, turing
and ada should be able to talk to the server, you can achive this by:
$InputTCPServerStreamDriverPermittedPeer zuse.example.net
$InputTCPServerStreamDriverPermittedPeer turing.example.net
$InputTCPServerStreamDriverPermittedPeer ada.example.net
As an extension to the (upcoming) IETF syslog/tls standard, you can specify some text
together with a domain component wildcard. So "*server.example.net", "server*.example.net"
are valid permitted peers. However "server*Fix.example.net" is NOT a valid wildcard. The
IETF standard permits no text along the wildcards.
The reason we use wildcards in the default setup is that it makes it easy to add systems
without the need to change the central server's configuration. It is important to understand that
the central server will accept names only (no exception) if the client certificate was
signed by the CA we set up. So if someone tries to create a malicious certificate with
a name "zuse.example.net", the server will not accept it. So a wildcard is safe
as long as you ensure CA security is not breached. Actually, you authorize a client by issuing
the certificate to it.
At this point, please be reminded once again that your security needs may be quite different from
what we assume in this tutorial. Evaluate your options based on your security needs.
Sample syslog.conf
Keep in mind that this rsyslog.conf accepts messages via TCP, only. The only other
source accepted is messages from the server itself.
$ModLoad imuxsock # local messages
$ModLoad imtcp # TCP listener
# make gtls driver the default
$DefaultNetstreamDriver gtls
# certificate files
$DefaultNetstreamDriverCAFile /rsyslog/protected/ca.pem
$DefaultNetstreamDriverCertFile /rsyslog/protected/machine-cert.pem
$DefaultNetstreamDriverKeyFile /rsyslog/protected/machine-key.pem
$InputTCPServerStreamDriverAuthMode x509/name
$InputTCPServerStreamDriverPermittedPeer *.example.net
$InputTCPServerStreamDriverMode 1 # run driver in TLS-only mode
$InputTCPServerRun 10514 # start up listener at port 10514
Be sure to safeguard at least the private key (machine-key.pem)!
If some third party obtains it, you security is broken!
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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