In general, your passwordless SSH needs fall into three categories:

Interactive Administrator Access
An administrator may want to be able to 'ssh destination' and log in without a password between the many machines she maintains.

Triggered Commands
Some commands may be triggered by other events. For example you may set up your CVS system to run a 'sync_website' command whenever the tree is committed which will update a local copy and run 'wmk -a' and then scp the html files to the actual webserver. Or perhaps you want your /etc/tinydns/root/data file to be automatically rsync'd when you make changes to your DJBDNS files.

Cronjobs/etc
Say you want to have certain directories copied to a remote system as poor-man's backup system using scp or rsync. Or perhaps you want to push /etc/passwd and /etc/shadow to all your machines from the master server, rather than run NIS, LDAP, or other real-time password syncing options.

There are many different ways you may implement passwordless access via SSH.

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