how to avoid ssh asking permission?

Update December 2019:

As Chris Adams pointed out below, there has been a fairly significant change to Openssh in the 6.5 years since this answer was written, and there is a new option that is much safer than the original advice below:

* ssh(1): expand the StrictHostKeyChecking option with two new
   settings. The first "accept-new" will automatically accept
   hitherto-unseen keys but will refuse connections for changed or
   invalid hostkeys. This is a safer subset of the current behaviour
   of StrictHostKeyChecking=no. The second setting "off", is a synonym
   for the current behaviour of StrictHostKeyChecking=no: accept new
   host keys, and continue connection for hosts with incorrect
   hostkeys. A future release will change the meaning of
   StrictHostKeyChecking=no to the behaviour of "accept-new". bz#2400

So instead of setting StrictHostKeyChecking no in your ssh_config file, set StrictHostKeyChecking accept-new.

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Set StrictHostKeyChecking no in your /etc/ssh/ssh_config file, where it will be a global option used by every user on the server. Or set it in your ~/.ssh/config file, where it will be the default for only the current user. Or you can use it on the command line:

ssh -o StrictHostKeyChecking=no -l "$user" "$host"

Here's an explanation of how this works from man ssh_config (or see this more current version):

StrictHostKeyChecking

If this flag is set to “yes”, ssh will never automatically add host keys to the $HOME/.ssh/known_hosts file, and refuses to connect to hosts whose host key has changed.  This provides maximum protection against trojan horse attacks, however, can be annoying when the /etc/ssh/ssh_known_hosts file is poorly maintained, or connections to new hosts are frequently made.  This option forces the user to manually add all new hosts.  If this flag is set to “no”, ssh will automatically add new host keys to the user known hosts files.  If this flag is set to “ask”, new host keys will be added to the user known host files only after the user has confirmed that is what they really want to do, and ssh will refuse to connect to hosts whose host key has changed.  The host keys of known hosts will be verified automatically in all cases.  The argument must be “yes”, “no” or “ask”.  The default is “ask”.

ssh-keyscan - Gather ssh public keys

If you already know the list of hosts you will connect to, you can just issue:

ssh-keyscan host1 host2 host3 host4

You can give the -H option to have it hash the results like ssh defaults to now

Also you can give -t keytype were keytype is dsa, rsa, or ecdsa if you have a preference as to which type of key to grab instead of the default.

Once you have run ssh-keyscan it will have pre-populated your known-hosts file and you won't have ssh asking you for permission to add a new key.

Ignore Host

Ignore the HostKeyChecking. For this I use e.g.:

ssh -oStrictHostKeyChecking=no -oUserKnownHostsFile=/dev/null [email protected]

Add Host

Add the host's/server's fingerprint to .ssh/known_hosts prior to your first connect. This is the safer way.