ssh_config and sshd_config

ssh_config  &  sshd_config ::  Examples and notes on Configuration settings
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To import a target's ssh-pubkey
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ssh-keygen -R [hostname]
ssh-keygen -R [ip_address]
ssh-keygen -R [hostname],[ip_address]
ssh-keyscan -H [hostname],[ip_address] >> ~/.ssh/known_hosts
ssh-keyscan -H [ip_address] >> ~/.ssh/known_hosts
ssh-keyscan -H [hostname] >> ~/.ssh/known_hosts


To remove (#6, line 6) from 'known_hosts' file (using `sed`)
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#>  sed -i '6d' ~/.ssh/known_hosts


To remove (#6, line 6) from 'known_hosts' file (using `perl`)
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#>  perl -pi -e 's/\Q$_// if ($. == 6);' ~/.ssh/known_hosts



The below will do the trick to add a host, ONLY if it has not yet been added. It is also not concurrency safe; you must not execute the snippet on the same origin machine more than once at the same time, as the tmp_hosts file can get clobbered, ultimately leading to the known_hosts file becoming bloated...  [[http://serverfault.com/questions/132970/can-i-automatically-add-a-new-host-to-known-hosts]]
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$>  ssh-keyscan -t rsa,dsa HOST 2>&1 | sort -u - ~/.ssh/known_hosts > ~/.ssh/tmp_hosts
mv ~/.ssh/tmp_hosts ~/.ssh/known_hosts