How to Parse Syslog Messages

Syslog is a popular protocol that virtually runs on every server. It is used to collect all kinds of logs. The problem with syslog is that services have a wide range of log formats, and no single parser can parse all syslog messages effectively.

In this tutorial, we will show how to use Fluentd to filter and parse different syslog messages robustly.

Prerequisites

• A basic understanding of Fluentd

• A running instance of rsyslogd

In this guide, we assume you are running fluent-package on Ubuntu.

Setting Up rsyslogd

Create /etc/rsyslogd.d/90-fluentd.conf and append the following line:

*.* @127.0.0.1:5140

Then restart the rsyslogd service:

$ sudo systemctl restart syslog

This tells rsyslogd to forward logs to port 5140 to which Fluentd will listen.

Setting up Fluentd

In this section, we will evolve our Fluentd configuration step-by-step.

Step 1: Listening to syslog Messages

First, let's configure Fluentd to listen to syslog messages.

Open /etc/fluent/fluentd.conf and put the following configuration:

<source>
  @type syslog
  port 5140
  tag system
</source>

<match system.**>
  @type stdout
</match>

This is the most basic setup: it listens to all syslog messages and outputs them to the standard output.

Now please restart fluentd:

$ sudo systemctl restart fluentd

Let's confirm data is coming in:

$ less /var/log/fluent/fluentd.log

Step 2: Extract syslog Messages from sudo

Now, let's look at a sudo message like this one:

2018-09-27 16:00:01.000000000 +0900 system.authpriv.info: {"host":"localhost",
"ident":"sudo","message":"pam_unix(sudo:session): session opened for user root
by admin(uid=0)"}

For security reasons, it is worth knowing which user performed what using sudo. In order to do so, we need to parse the message field. In other words, we need to extract syslog messages from sudo and handle them differently.

For this purpose, we can use the grep filter plugin. It examines the fields of events, and filter them based on regular expression patterns. In the following example, Fluentd filters out events that come from sudo and contain command data:

<source>
  @type syslog
  port 42185
  tag system
</source>

<filter system.**>
  @type grep
  <regexp>
    key ident
    pattern /^sudo$/
  </regexp>
  <regexp>
    key message
    pattern /COMMAND/
  </regexp>
</filter>

<match system.**>
  @type stdout
</match>

Step 3: Extract Information from Messages

Now let's extract some information from syslog messages. For this purpose, we use another plugin called filter-parser. With this plugin, you can parse the content of a field using a regular expression.

Here is the final configuration:

<source>
  @type syslog
  port 5140
  tag system
</source>

<filter system.**>
  @type grep
  <regexp>
    key ident
    pattern /^sudo$/
  </regexp>
  <regexp>
    key message
    pattern /COMMAND/
  </regexp>
</filter>

<filter system.**>
  @type parser
  key_name message
  <parse>
    @type regexp
    expression /USER=(?<sudoer>[^ ]+) ; COMMAND=(?<command>.*)$/
  </parse>
</filter>

<match system.**>
  @type stdout
</match>

Then restart fluentd:

$ sudo systemctl restart fluentd

Let's execute some comment with sudo:

$ sudo cat /var/log/auth.log

Now, you should have a line like this in /var/log/fluent/fluentd.log:

2025-01-21 08:51:04.625222474 +0000 system.authpriv.notice: {"sudoer":"root","command":"/usr/bin/cat /var/log/auth.log"}

There it is, as you can see in the line!

Conclusion

Fluentd makes it easy to ingest syslog events. You can immediately send data to the output systems like MongoDB and Elasticsearch, but also you can do filtering and further parsing inside Fluentd before passing the processed data onto the output destinations.

If this article is incorrect or outdated, or omits critical information, please let us know. Fluentd is an open-source project under Cloud Native Computing Foundation (CNCF). All components are available under the Apache 2 License.