GELF forwarding in rsyslog#

Forward rsyslog messages to Graylog by rendering a GELF JSON template and using omfwd with UDP or TCP framing.

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" option.jsonf="on") {
    constant(outname="version" value="1.1" format="jsonf")
    property(outname="host" name="hostname" format="jsonf")
    property(outname="short_message" name="msg" format="jsonf")
    property(outname="timestamp" name="timegenerated"
             dateformat="unixtimestamp" datatype="number" format="jsonf")
    property(outname="level" name="syslogseverity"
             datatype="number" format="jsonf")
    property(outname="_app_name" name="app-name" format="jsonf")
    property(outname="_syslog_facility" name="syslogfacility-text"
             format="jsonf")
}

This list template uses option.jsonf="on" and format="jsonf" so rsyslog adds JSON braces, commas, field names, and string escaping. The timestamp and level fields are emitted as JSON numbers, which avoids Graylog warnings about string-typed GELF fields. The _app_name and _syslog_facility fields are optional GELF additional fields.

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 null byte as message delimiter. To use TCP, configure the delimiter via the TCP_FrameDelimiter parameter of the omfwd module.

# 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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