My style of learning and working has gotten me to the point where I have pretty wide knowledge of various computering things, but not deep knowledge. I’ve bounced around between .NET vs Ruby vs Clojure, Windows vs Linux, colo vs cloud. This means that as I debug problems, I often run head-long into various pits that I didn’t even know were there. Linux logging is one of those pits that I just had no idea how it worked…

The problem

We use Postfix at Postmark for our external SMTP interface that customers interact with. It’s rock solid and performs as advertised, pretty well for software that was written nearly 20 years ago. However, while investigating a support issue, I was combing through the logs and didn’t find the evidence that the customer provided. This happening once, I could chalk it up to the fact that the customer provided bad data for me to look for and I looked for the evidence in other places. But, after a few more cases it become obvious that lines that should have been in the logs definitely were not in the logs. Something was wrong.

The resolution

After spelunking into various log files, I finally saw something that stuck out:

$ journalctl -u systemd-journald

systemd-journal[4431]: Suppressed 316 messages from /system.slice/postfix.service
systemd-journal[4431]: Suppressed 387 messages from /system.slice/postfix.service
systemd-journal[4431]: Suppressed 453 messages from /system.slice/postfix.service

journald is the system that handles system logging in Centos 7, replacing syslog. It’s a centralized logging system that all services on the system can use. Because it’s central to the entire server, it has some safeguards such as rate-limiting built into it so that rogue programs can’t bring down the server. However, we use our postfix logs almost as an audit system, we need all entries from the postfix logs. So while rate-limiting is a very smart default, it wasn’t the behavior that we wanted in our system. To remove the journald rate-limiting:

# in /etc/systemd/journald.conf


$ systemctl restart systemd-journald

Also, we had an additional step in our setup to get log messages going to a /var/log/maillog file where postfix logs are traditionally stored. System log messages would follow journald -> rsyslogd -> /var/log/maillog, so we had to adjust the rate-limiting in rsyslogd.

# in /etc/rsyslog.conf

$imjournalRatelimitInterval 0
$imjournalRatelimitBurst 0

$ systemctl restart rsyslog


At the end of the day, I was pretty happy to learn some more about linux logging and deepened my knowledge a little bit. A very helpful blog post was one of the few things that my frantic googling hit on that provided help.