Re: Minimize sshd log clutter/spam from unauthenticated connections

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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@xxxxxxxxxxxxx>
To: openssh-unix-dev@xxxxxxxxxxx
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,

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