Lets say you want to copy between two hosts host_a and host_b. host_a is the host you run rsync
On host_a, run this command
$ ssh-keygen -t rsa
This will prompt for path for public key and a passphrase. Just press the enter key. It'll then generate an identification (private key) and a public key. The public key by default will save in ~/.ssh/id_rsa.pub
$ ftp host_b
$ ftp > put .ssh/id_rsa.pub
public key will copy to home folder
$ ftp > bye
$ ssh to host_b
$ cat ~/.ssh/id_rsa.pub > ~/.ssh/authorized_keys
$ chmod 700 ~/.ssh/authorized_keys
DONE !!
You can try to rsync, no password will be prompted!!
