Can I automatically add a new host to known_hosts?

Set the StrictHostKeyChecking option to no, either in the config file or via -o :

ssh -o StrictHostKeyChecking=no [email protected]

IMO, the best way to do this is the following:

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

That will make sure there are no duplicate entries, that you are covered for both the hostname and IP address, and will also hash the output, an extra security measure.

For the lazy ones:

ssh-keyscan -H <host> >> ~/.ssh/known_hosts

-H hashes the hostname / IP address