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In my /etc/rsyslog.conf, I have the following line to log the auth facility into /var/log/auth.log:

auth,authpriv.*           /var/log/auth.log

but the file is flooded with cron logs, such as these:

CRON[18620]:  pam_unix(cron:session): session opened for user root by (uid=0)
CRON[18620]:  pam_unix(cron:session): session closed for user root

I would like to get rid of the cron logs, and only have real "auth" events being logged into that file. By that I mean, I want to see which users have logged into the system, or made su -.

How can I achieve that?

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up vote 4 down vote accepted

I believe this is what you are looking for:

:msg, contains, "pam_unix(cron:session)" ~
auth,authpriv.* /var/log/auth.log

the first line matches cron auth events, and deletes them. The second line then logs as per your rule, minus the previously deleted lines.

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If an attacker can control part of the message logged, they could mask events by filtering on the message contents.

Alternatively, you can filter out messages from the process CRON with:

#Continue logging CRON to syslog
*.*;auth,authpriv.none          -/var/log/syslog
#Filter events from the process CRON out of auth.log
:programname, isequal, "CRON" ~
auth,authpriv.*                 /var/log/auth.log

If you don't want any CRON events logged, you can put the filter line first in your rsyslog.conf.

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