Minimize sshd log clutter/spam from unauthenticated connections

[David Lang]

modern syslog daemons (including rsyslog, which is default on just about every 
linux system) allow you to filter efficiently on the message contents, not just 
the severity, so you can opt to throw out the messages you don't want.

I advocate for a slightly different way of dealing with it, filter these 
messages from your main logstream, but put them into either a script directly, 
or a separate file and have a script run against it. Have the script report the 
number of these messgaes that you get in a time period (minute, hour, whatever 
you want) and log that count back into your log stream

as Marcus Ranum said in his Artificial Ignorance writeup, the number of times 
that an uninteresting thing happens can be interesting.

If you see a big spike (or drop) is these attempts, it can indicate cause for 
concern.

David Lang

  On Sat, 
18 Mar 2023, Carsten Andrich wrote:

> Date: Sat, 18 Mar 2023 13:15:29 +0100
> From: Carsten Andrich <carsten.andrich at tu-ilmenau.de>
> To: openssh-unix-dev at mindrot.org
> Subject: Minimize sshd log clutter/spam from unauthenticated connections
> 
> Dear OpenSSH developers,
>
> a publicly accessible sshd on port 22 generates a lot of log clutter from 
> unauthenticated connections. For an exemplary host on a university network, 
> sshd accumulates 5~20k log lines on a single day (more than 90% of the total 
> amount of syslog lines). That is despite the host having a restricted 
> configuration (no SSH password authentication, firewall rate limit for new 
> SSH connections on /24 subnets permitting a few connections per hour, however 
> with a shorter timeout). I'd expect even more log messages for a default 
> configuration (password auth enabled and no firewall rate limit).
>
> Would you be open to introducing a new config option to suppress any log 
> messages from yet unauthenticated connections? If such a suggestion has been 
> discussed before, please direct me to it. I haven't found anything in the 
> archives.
>
> Any log messages including successful authentication and afterwards are still 
> desired, so changing the log level to above INFO will not help. Additionally, 
> even unauthenticated connections cause messages with levels ERROR 
> ("kex_exchange_identification: Connection closed by remote host") or even 
> CRITICAL ("Timeout before authentication"). As I periodically scan my hosts' 
> syslogs for messages with level WARNING or above, I currently have to filter 
> these messages to keep my inbox from overflowing.
>
> Thanks and best regards,
> Carsten
>
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