GELF forwarding in rsyslog#

Written by Florian Riedl

Situation#

The current setup has a system with rsyslog as the central syslog server and a system with Graylog for storage and analyzing the log messages. Graylog expects the log messages to arrive in GELF (Graylog Extended Log Format).

Changing the default log format to GELF#

To make rsyslog send GELF we basically need to create a custom template. This template will define the format in which the log messages will get sent to Graylog.

template(name="gelf" type="list") {
    constant(value="{\"version\":\"1.1\",")
    constant(value="\"host\":\"")
    property(name="hostname")
    constant(value="\",\"short_message\":\"")
    property(name="msg" format="json")
    constant(value="\",\"timestamp\":")
    property(name="timegenerated" dateformat="unixtimestamp")
    constant(value=",\"level\":\"")
    property(name="syslogseverity")
    constant(value="\"}")
}

This is a typical representation in the list format with all the necessary fields and format definitions that Graylog expects.

Applying the template to a syslog action#

The next step is applying the template to our output action. Since we are forwarding log messages to Graylog, this is usually a syslog sending action.

# syslog forwarder via UDP
action(type="omfwd" target="graylogserver" port="12201" protocol="udp" template="gelf")

We now have a syslog forwarding action. This uses the omfwd module. Please note that the case above only works for UDP transport. When using TCP, Graylog expects a Nullbyte as message delimiter. So, to use TCP, you need to change delimiter via TCP_FrameDelimiter option.

# syslog forwarder via TCP
action(type="omfwd" target="graylogserver" port="12201" protocol="tcp" template="gelf" TCP_FrameDelimiter="0" KeepAlive="on")

Conclusion#

With this quick and easy setup you can feed Graylog with the correct log message format so it can do its work. This case can be applied to a lot of different scenarios as well, but with different templates.

This documentation is part of the rsyslog project.

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