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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
Setting Up rsyslogd
rsyslogdOpen
/etc/rsyslogd.conf and append the following line:1
*.* @127.0.0.1:5140
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Then restart the
rsyslogd service:1
$ sudo systemctl restart syslog
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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
syslog MessagesFirst, let's configure Fluentd to listen to syslog messages.
Open
/etc/td-agent/td-agent.conf and put the following configuration:1
<source>
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@type syslog
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port 5140
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tag system
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</source>
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​
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<match system.**>
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@type stdout
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</match>
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This is the most basic setup: it listens to all syslog messages and outputs them to the standard output.
Now please restart
td-agent:1
$ sudo systemctl restart td-agent
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Let's confirm data is coming in:
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$ less /var/log/td-agent/td-agent.log
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Step 2: Extract syslog Messages from sudo
syslog Messages from sudoNow, let's look at a
sudo message like this one:1
2018-09-27 16:00:01.000000000 +0900 system.authpriv.info: {"host":"localhost",
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"ident":"sudo","message":"pam_unix(sudo:session): session opened for user root
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by admin(uid=0)"}
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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:1
<source>
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@type syslog
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port 42185
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tag system
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</source>
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​
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<filter system.**>
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@type grep
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<regexp>
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key ident
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pattern /^sudo$/
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</regexp>
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<regexp>
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key message
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pattern /COMMAND/
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</regexp>
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</filter>
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​
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<match system.**>
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@type stdout
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</match>
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Step 3: Extract Information from Messages
Now let's extract some information from
syslog messages. For this purpose, we use another plugin called filter-parser. With this plugin, you can parse the content of a field using a regular expression.Here is the final configuration:
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<source>
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@type syslog
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port 5140
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tag system
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</source>
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​
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<filter system.**>
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@type grep
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<regexp>
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key ident
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pattern /^sudo$/
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</regexp>
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<regexp>
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key message
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pattern /COMMAND/
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</regexp>
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</filter>
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​
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<filter system.**>
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@type parser
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key_name message
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<parse>
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@type regexp
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expression /USER=(?<sudoer>[^ ]+) ; COMMAND=(?<command>.*)$/
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</parse>
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</filter>
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​
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<match system.**>
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@type stdout
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</match>
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Then restart
td-agent:1
$ sudo systemctl restart td-agent
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Let's execute some comment with
sudo:1
$ sudo cat /var/log/auth.log
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Now, you should have a line like this in
/var/log/td-agent/td-agent.log:1
2018-09-27 16:00:01.000000000 +0900 system.authpriv.notice: {"sudoer":"root","command":"/bin/cat"}
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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.
Last modified 11mo ago