improving signal to noise ratio from centralized network syslogs

Brian Knight ml at
Mon Feb 5 23:43:02 UTC 2018

On 2018-02-03 15:49, Scott Weeks wrote:
> Then, you can watch your network in real time
> like so (below is all one line):
> tail -f /var/log/router.log /var/log/switch.log
> | egrep -vi 'term1|term2|termN'
> 'egrep -v' takes out all the lines you don't
> want to see while the syslog messages scroll
> across the screen.

Syslog-ng can do regex filtering on messages also.  So instead of doing 
an 'egrep -v' on a huge file after it has been logged, you can put your 
filter right into the syslog-ng configuration, and have those filtered 
messages output to a file (or any other output that syslog-ng supports). 
  The result is a smaller file to search and work with.

We implemented a simple email alerter using this functionality.  In 
syslog-ng, we set up two filters.  One filter does the 'egrep -v':

filter f_email_msg {
         not message("%PKT_INFRA-LINEPROTO-.*[0-9/]+\\.")   # filter out 
subinterface up/downs
         and not message("%PKT_INFRA-LINEPROTO-.*Multilink")
         and not message("%PKT_INFRA-LINEPROTO-.*Serial")
         and not message("%PKT_INFRA-LINEPROTO-.*Tunnel")
         # etc

Another filter applied to the messages filters messages to just our core 

filter f_email_sources {
         or host("")
         or host("")
         or host("")
         or host("")
         or host("")

Then those are tied together in a syslog-ng rule that outputs to a file:

destination d_email_log {
log { source(s_devices); filter(f_email_sources); filter(f_email_msg); 
destination(d_email_log); };

A lightweight Python script that runs as a daemon checks that file once 
every 10 seconds, and if the file length is non-zero, it sends the 
contents of the file in an email to the admins.  A shell script run as a 
cron job would work equally as well.

(Also, for emailed syslogs, there is more incentive for the admin to 
keep her or his message filter up to date, as opposed to a file the 
administrator must manually examine.  Otherwise the admin has a full 
inbox :) )

It's very simple and stable, and has worked better than the commercial 
product we used to use for this purpose.


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