Passwordless SSH

I've done this a million times already -- but nevertheless I need to look at a reference every time I do it.

The idea is to be able to ssh from user1@machine1 to user2@machine2, without being asked for a password.

To do it, run the following commands (all of them as user1@machine1):

1. Create a ssa key on machine1:
   ssh-keygen -t rsa
2. Copy the public key over to machine2:
   cat ~/.ssh/id_rsa.pub | ssh user2@machine2 "cat - >> ~/.ssh/authorized_keys"
   (you may need to accept the server's fingerprint (say Yes), and will need to type your password this time).

It's done. Now just ssh to machine2 and it won't ask for a password.

Note: This also works for doing ssh to the same machine, under a different user. Just make sure user1's id_rsa.pub key is added to user2's ~/.ssh/authorized_keys file.

Note2: SSH is very strict with respect to permissions. The .ssh dir MUST have "700" permissions mask, and authorized_keys must be a 600. Anything different (and if you create authorized_keys, it will likely be created with a different access mask) will make this authentication method fail, and move on to the next one (ie. ask for a password).