Syslogd MARK option
Syslog can be run with a -m " option that just writes "- MARK --" at whatever intervals you specify. The purpose is to provide reassurance that syslogd is listening and doing its job even if it has nothing else to do right now.
I've had people worried by their seeing MARK in logs and not knowing why it was there, but until this thread I'd never seen anyone wondering why it WASN'T there. The reason is that syslogd doesn't bother to write a mark if something else cause it to write a log entry during the interval you specified. After all, the point of -m is to know that syslogd is alive and working, so why write a special mark if it just did real work? That's apparently the attitude of whoever wrote it, and it isn't entirely unreasonable, though as someone mentioned in this thread, it probably should be noted in the documentation.
So - if you are seeing "MARK, that's normal. Most systems are set up to do that. It lets you know that syslog has been doing its job, it isn't dead, there's just nothing to do right now. If you are NOT seeing it, but are seeing regular activity from other sources, that's normal too - don't worry about it.
Other Syslog issues
Another syslog issue that comes up now and then is syslog NOT writing to a log after you've told it to do so in /etc/syslog.conf. That's usually because you forgot to create the file; "man syslog" specifically says "For security reasons, syslogd will not append to log files that do not exist; therefore, they must be created manually before running syslogd.".
"ttloop: peer died" could mean someone is scanning for open ports.
Entries complaining about passwords mean someone has mistyped a password or is trying to guess one. Back in the days of dumb terminals, you'd see this when someone left a book lying on a keyboard.
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