[syslog-ng] snmptrap handler parses SNMP traps correctly, but the file it reads from is not emptying

Allen Pouratian Allen.Pouratian at AlticeUSA.com
Tue May 21 15:41:43 UTC 2019

Balázs –

What guarantee is there that I won’t lose traps?

  1.  Does logrotate make sure snmptrapd has finished writing to "/var/log/snmptrapd.log"? Is the write by snmptrapd 5.7.3+ always atomic for one or more traps?
  2.  Does syslog-ng 3.20.1 make sure it instantly reads every trap written by snmptrapd to "/var/log/snmptrapd.log" before it’s switched out by logrotate?

I apologize if this is basic knowledge I’m not aware of.

Best Regards,


From: syslog-ng <syslog-ng-bounces at lists.balabit.hu> On Behalf Of Scheidler, Balázs
Sent: Monday, May 20, 2019 11:55 AM
To: Syslog-ng users' and developers' mailing list <syslog-ng at lists.balabit.hu>
Subject: Re: [syslog-ng] snmptrap handler parses SNMP traps correctly, but the file it reads from is not emptying

[External Email]
Just set up logrotate on that file, syslog-ng will read till the end and start the new one if the rotation happens.

So instead of truncating, rename it to a new name, and let snmptrapd write to a new file.

On Mon, May 20, 2019, 15:55 Allen Pouratian <Allen.Pouratian at alticeusa.com<mailto:Allen.Pouratian at alticeusa.com>> wrote:
Hello –

I set up snmptrapd to write to /var/log/snmptrapd.log and syslog-ng is configured to read from it like this …

source {

… but /var/log/snmptrapd.log is not getting emptied, so it needs to be truncated every so often to avoid filling up the filesystem.

But the problem with truncating /var/log/snmptrapd.log is that we’re going to lose traps, since we get a lot of them.

When I was testing this syslog-ng snmptrap facility with a few traps a minute, I saw /var/log/snmptrapd.log empty, but with hundreds of traps incoming per second, it does not empty any more.

I have what looks like a work-around to the syslog-ng snmptrap handler/parser where …

  1.  1) Snmptrapd 5.7.3 writes to syslog with -Lsd
  2.  2) Syslog-ng 3.20 reads from syslog with system-journal()
  3.  3) Rewrite the trap $MESSAGE with a series of “substitutions” (subst) into space separated key=value pairs
  4.  4) point kv-parser() at $MESSAGE and specify “ “ as a separator
  5.  5) delete the original $MESSAGE block

… but perhaps I didn’t have to do that, since perhaps I’m mis-using the snmptrap facility built into syslog-ng, and thus causing /var/log/snmptrapd.log to not empty.

Your comments and insights would be appreciated.

- Allen

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