rsyslog

This time, we want to parse log messages received from a Cisco ASA for specific parts. The log messages will be parsed by liblognorm and then they will be written into a specific file resembling the sent code.

This guide has been tested with v7.3.4 of rsyslog.

Things to think about

We basically need a receiver, the normalizer module mmnormalize, some templates and several actions. To keep the logs seperated from other log messages, we also will need a filter. But that is just valid for the rsyslog configuration.

We also need liblognorm installed and a rulebase to work with. The rulebase is needed for liblognorm to know what to do with the messages, so it knows, which part should be fed into a custom property and which to ignore.

Config Statements

We now have basically two parts to configure. The first part should be the rulebase which holds format representations of the expected log messages. For this example we will work with the following log line:

2012-11-23T10:47:42+01:00 0.0.0.0 : %ASA-3-313001: Denied ICMP type=8, code=0 from 77.2.2.2 on interface outside

The rulebase entry for this log line looks like this:

rule=: %date:word% %host:ipv4% : \x25ASA-%ddd:char-to:-%-%eee:number%: Denied ICMP type=%number2:number%, code=%number3:number% from %origin:ipv4% on interface outside

And here is the rsyslog configuration:

module (load="immark")
module (load="imuxsock")
module (load="imklog")
module (load="mmnormalize")
module (load="imudp")

input(type="imudp" port="514")

template(name="filename" type="string" string="/var/log/%$!ddd%-%$!eee%")

if $msg contains '%ASA' then{
 action(type="mmnormalize" userawmsg="off" rulebase="/path/to/rulebase.rb")
 action(type="omfile" DynaFile="filename")
}

How it works

The rulebase holds currently one rule. The rule is a representation of the above log message. For this case, we are only interested in the ASA code. The code identifies a specific message, thus, the message itself is not interesting. Though, we have to reflect complete message and put temporary variables, where the same message of a kind still has different values. These would be the timestamp, source, type and code the the IP which triggered the message. The problem is here, that we really only need two parts to be filled into a property, but other parts can be variable. The message format can be the same, but i.e. the IP from the source can vary. If you fill in a concrete value into the rule where such parts are, the message will only be parsed correctly if the message originates from the exact same IP.

The configuration itself isn’t very complicated. It loads several standard modules, which are mainly used to ensure local logging, as well as the UDP input module and the precious mmnormalize. The mmnormalize module will be later used as a output module. It handles the loading of the rulebase, as well as implements the normalization features of liblognorm.

After loading the modules, we have our single UDP input configured, which is necessary to receive the log messages from the Cisco ASA. Nothing fancy  here, but a standard syslog receiver for UDP.

The next step is to create a template. The template is not for a message output format, but a file name format. It will help writing the messages into log files that reflect the ASA number in the filename.

Finally we will filter for “%ASA” in the message. That will ensure, that only ASA messages are processed by the following actions. The first action runs the message through mmnormalize, while using our rulebase. The messages will be parsed by liblognorm. If the messages do fit the rules in the rulebase, the parts we need will be put into properties. These properties come into play in the second action where we simply write the messages via Dynafile to disk. The template and our new properties will be used to define the filename.

Important

This configuration works or fails with the rulebase. All messages, that should be run through the normalizer must fit to a rule. If you are in doubt, that the rules are correct, follow the linked guide. It will show you how to create a output format that shows if a message has been parsed correctly or if and where the parsing broke.