This isn’t necessarily the best way to use rsync… but here is a fairly simple (uneducated) way of getting two-way communication going between a source and backup host.

Steps:

1. Generate keys on source-host: ssh-keygen -t rsa (or ssh-keygen -t dsa)
2. Copy source-host public key to backup-host using ssh-copy-id¹:
   ssh-copy-id -i id_rsa.pub backup-user@backup-host
3. Generate keys on backup-host: ssh-keygen -t rsa
4. Copy backup-host public key to source-host using ssh-copy-id:
   ssh-copy-id -i id_rsa.pub source-user@source-host
5. Make sure ssh-agent is installed and running on backup-host.²
6. Use ssh-add to add key to ssh-agent on backup-host³.
7. Finally, run rsync on backup-host:
   rsync -avz -e ssh source-user@source-host:/path/for/backup/source backup_to_here

Notes:

1. From this article. (I ended up making these keys for the opposite servers… for example: creating public key on source server is actually meant to be put into backup server authorized_keys file… and creating public key on backup server is actually meant to be put into source server authorized_keys file.)
2. From ssh-agent article. (source)
3. If ssh-add doesn’t seem to work, try running this in the shell: eval `ssh-agent`. (source)
4. On Mac OS X, I seemed to have to run rsync in a new session before I got the pop-up dialog box asking if I’d like to remember this ssh passphrase in the Keychain Access utility.

See Also:

• Using Rsync and SSH