Can I automatically add a new host to known_hosts?

Question

Here's my situation; I'm setting up a test harness that will, from a central client, launch a number of virtual machine instances and then execute commands on them via SSH. The virtual machines will have previously unused hostnames and IP addresses, so they won't be in the ~/.ssh/known_hosts file on the central client.

The problem I'm having is that the first SSH command run against a new virtual instance always comes up with an interactive prompt:

The authenticity of host '[hostname] ([IP address])' can't be established.
RSA key fingerprint is [key fingerprint].
Are you sure you want to continue connecting (yes/no)?

Is there a way that I can bypass this and get the new host to be already known to the client machine, maybe by using a public key that's already baked into the virtual machine image ? I'd really like to avoid having to use Expect or whatever to answer the interactive prompt if I can.

Answer 1

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

Answer 2

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.

Answer 3

You could use ssh-keyscan to grab the public key and append that to your known_hosts

Answer 4

As mentioned, using key-scan would be the right & unobtrusive way to do it.

ssh-keyscan -t rsa,dsa HOST 2>&1 | sort -u - ~/.ssh/known_hosts > ~/.ssh/tmp_hosts
cat ~/.ssh/tmp_hosts > ~/.ssh/known_hosts

The above will do the trick to add a host, ONLY if it has not yet been added.