To enable passwordless SSH login to a *nix server, first you will need a pair of keys. If you don‘t already have a keypair generated you’ll first of all need to create one. To generate a new keypair you run the following command:
your-user@host1:~$ ssh-keygen -t rsa
This will prompt you for a location to save the keys, and a pass-phrase:
Generating public/private rsa key pair. Enter file in which to save the key (/home/skx/.ssh/id_rsa): Enter passphrase (empty for no passphrase): Enter same passphrase again: Your identification has been saved in /home/skx/.ssh/id_rsa. Your public key has been saved in /home/skx/.ssh/id_rsa.pub.
Then, assuming that you want to enable passwordless SSH login to the host2 server from host1 with the id_rsa and id_rsa.pub files you've just generated you should run the following command:
your-user@host1:~$ ssh-copy-id -i ~/.ssh/id_rsa.pub username@host2
If host1 doesn't have ssh-copy-id installed, you can either install ssh-copy-id or manually copy the id_rsa.pub key to the host2 by issuing the following commands:
host1# cat ~/.ssh/id_rsa.pub | ssh user@host2 'cat >> .ssh/authorized_keys'
or if you need to make a .ssh directory on host2
host1#cat ~/.ssh/id_rsa.pub | ssh user@host2 'mkdir .ssh; chmod 700 .ssh; cat >> .ssh/authorized_keys; chmod 644 .ssh/authorized_keys'