keyed login

About

With Linux and similar systems, it is possible to set up access from a client to a server in such a way that it is unnecessary to manually enter a password. This technique actually makes the server more secure, since you are using public key encryption which is kind of like using a much longer password but never actually transmitting it (and it's all handled automatically).

Once this has been set up, a number of commands including ssh and rsync can connect without a password. This is especially useful for scripts, since otherwise you would have to have the password in plaintext within the script or in some other plaintext file.

How To

• On the client: Generate a public/private key pair using ssh-keygen
• Add the public key file (~/.ssh/id_rsa.pub) from the client to ~/.ssh/authorized_keys on the host.
  • If the latter file doesn't already exist, then it can just be a direct copy of id_rsa.pub.
  • If the latter file does already exist, then id_rsa.pub can just be appended to authorized_keys.
  • The following command should work for either case:

    • cat .ssh/id_rsa.pub | ssh user@host 'cat >> .ssh/authorized_keys'
      

• It may be necessary to manually log on once to the client from the host (using the same usernames as above) in order to add the client to the host's known_hosts file; I'm not sure about this part. Need to test.
• Manually log on from the client to the host to make sure that the host is added to the client's known_hosts file, otherwise the verification prompt may interrupt any processes trying to log on automatically.
• If you disable logins now (e.g. via Webmin's "No login allowed" option), you will still be able to log in from a client with an authorized key.

Specific commands for doing this may be found here.