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.- A basic understanding of Fluentd
- A running instance of
rsyslogd
Open
/etc/rsyslogd.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.In this section, we will evolve our Fluentd configuration step-by-step.
First, let's configure Fluentd to listen to syslog messages.
Open
/etc/td-agent/td-agent.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
td-agent:$ sudo systemctl restart td-agent
Let's confirm data is coming in:
$ less /var/log/td-agent/td-agent.log
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$/
</regexp>
<regexp>
key message
pattern /COMMAND/
</regexp>
</filter>
<match system.**>
@type stdout
</match>
Now let's extract some information from
syslog messages. For this purpose, we use another plugin called filter-parser. With this plugin, you can parse the content of a field using a regular expression.Here is the final configuration:
<source>
@type syslog
port 5140
tag system
</source>
<filter system.**>
@type grep
<regexp>
key ident
pattern /^sudo$/
</regexp>
<regexp>
key message
pattern /COMMAND/
</regexp>
</filter>
<filter system.**>
@type parser
key_name message
<parse>
@type regexp
expression /USER=(?<sudoer>[^ ]+) ; COMMAND=(?<command>.*)$/
</parse>
</filter>
<match system.**>
@type stdout
</match>
Then restart
td-agent:$ sudo systemctl restart td-agent
Let's execute some comment with
sudo:$ sudo cat /var/log/auth.log
Now, you should have a line like this in
/var/log/td-agent/td-agent.log:2018-09-27 16:00:01.000000000 +0900 system.authpriv.notice: {"sudoer":"root","command":"/bin/cat"}
There it is, as you can see in the line!
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.
Last modified 2yr ago