Graylog is able to accept and parse RFC 5424 and RFC 3164 compliant syslog messages and supports TCP transport with both the octet counting or termination character methods. UDP is also supported and the recommended way to send log messages in most architectures.

Many devices, especially routers and firewalls, do not send RFC compliant syslog messages. This might result in wrong or completely failed parsing. In that case, you might have to go with a combination of raw/plaintext message inputs that do not attempt to do any parsing and Extractors.

Rule of thumb is that messages forwarded by rsyslog or syslog-ng are usually parsed flawlessly.

Sending Syslog from Linux Hosts

rsyslog

Sending syslog data from Linux hosts with  rsyslog  is done by defining an output Action that uses the RFC 5424 format.

The output action can be placed at the end of the  /etc/rsyslog.conf or as an included file such as /etc/rsyslog.d/graylog.conf. 

These examples will send all syslog selectors to the example host  yourgraylog.example.com  (change this to the hostname or IP that resolves in the installed environment) on port 514 using the predefined format of  RSYSLOG_SyslogProtocol23Format.

UDP (single @):

*.*@yourgraylog.example.org:514;RSYSLOG_SyslogProtocol23Format

TCP (double @@):

*.*@@yourgraylog.example.org:514;RSYSLOG_SyslogProtocol23Format

This can be improved by configuring rsyslog to use TLS.

An opinionated example configuration using a local queue, TCP with TLS, recycling connections, and using the rsyslog advanced  format:

*.*  action(
   Action.resumeInterval="10"
   RebindInterval="10000"            # cycling TCP connections allows for load balancing
   Queue.Size="100000"
   Queue.DiscardMark="97500"
   Queue.HighWaterMark="80000"
   Queue.Type="LinkedList"
   Queue.FileName="rsyslogqueue"
   Queue.CheckpointInterval="100"
   Queue.MaxDiskSpace="2g"
   Action.ResumeRetryCount="-1"
   Queue.SaveOnShutdown="on"
   Queue.TimeoutEnqueue="10"
   Queue.DiscardSeverity="0"
   type="omfwd"
   target="yourgraylog.example.org"
   protocol="tcp"
   port="514"
   template="RSYSLOG_SyslogProtocol23Format"
   StreamDriver="gtls"
   StreamDriverMode="1"               # run driver in TLS-only mode
   StreamDriverAuthMode="x509/name"   # host TLS cert CN will be used for authentication
   StreamDriverPermittedPeers="yourgraylog.example.org" # only allowed hosts
)

For extremely old (pre-5.10 from 2010) versions of rsyslog that do no have the RSYSLOG_SyslogProtocol23Format built-in, a template must be defined:

$template GRAYLOG_SyslogProtocol23Format,"<%PRI%>%PROTOCOL-VERSION% %TIMESTAMP:::date-rfc3339% %HOSTNAME% %APP-NAME% %PROCID% %MSGID% %STRUCTURED-DATA% %msg%\n"

# Then referenced in the output action
*.* @graylog.example.org:514;GRAYLOG_SyslogProtocol23Format

syslog-ng

Use the syslog function in syslog-ng to send RFC 5424 formatted messages via TCP to a Graylog host:

# Define TCP syslog destination.
destination d_net {
    syslog("graylog.example.org" port(514));
};
# Send from the default source s_src to the d_net destination configured above.
log {
    source(s_src);
    destination(d_net);
};

Sending Syslog from MacOS X Hosts

Sending log messages from MacOS X syslog daemons is easy. Just define a graylog-server instance as UDP log target by adding this line in your /etc/syslog.conf: 

*.* @graylog.example.org:514

Now restart syslogd: 

$ sudo launchctl unload /System/Library/LaunchDaemons/com.apple.syslogd.plist
$ sudo launchctl load /System/Library/LaunchDaemons/com.apple.syslogd.plist

$ ps aux | grep syslog
lennart         58775   0.0  0.0  2432768    592 s004  S+    6:10PM   0:00.00 grep syslog
root            58759   0.0  0.0  2478772   1020   ??  Ss    6:09PM   0:00.01 /usr/sbin/syslogd

That’s it! Your MacOS X syslog messages should now appear in your Graylog system.