known_hosts

Idempotently adding an SSH key for a host to known_hosts file with bash

I noticed on one of the CI servers I'm running that the .ssh/known_hosts file had ballooned up to over 1,000,000 lines!

Looking into the root cause (I tailed the file until I could track down a few jobs that ran every minute), I found that there was the following line in a setup script:

ssh-keyscan -t rsa github.com >> /var/lib/jenkins/.ssh/known_hosts

"This can't be good!" I told myself, and I decided to add a condition to make it idempotent (that is, able to be run once or one million times but only affecting change the first time it's run—basically, a way to change something only if the change is required):

if ! grep -q "^github.com" /var/lib/jenkins/.ssh/known_hosts; then
  ssh-keyscan -t rsa github.com >> /var/lib/jenkins/.ssh/known_hosts
fi

Now the host key for github.com is only scanned once the first time that script runs, and it is only stored in known_hosts one time for the host github.com... instead of millions of times!

Fastest way to reset a host key when rebuilding servers on the same IP or hostname frequently

I build and rebuild servers quite often, and when I want to jump into the server to check a config setting (when I'm not using Ansible, that is...), I need to log in via SSH. It's best practice to let SSH verify the host key every time you connect to make sure you're not getting MITMed or anything else is going on.

However, any time you rebuild a server from a new image/OS install, the host key should be new, and this will result in the following message the next time you try to log in:

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