About

SSH is a protocol for running commands on a remote computer.

Implementations

  • Dropbear is a lightweight implementation of SSH targeted at embedded devices.
  • OpenSSH, developed by the OpenBSD project, is by far the most common implementation of SSH, both server-side and client-side, in the unix world. If someone mentions SSH in a unix context, assume OpenSSH unless told otherwise.
  • PuTTY is an SSH client mostly found on Windows.

Related programs

  • AutoSSH: Automatically restart SSH sessions and tunnels
  • Corkscrew: tunnel through HTTP proxies
  • SSHFS: mount remote filesystems over SSH

Troubleshooting

If public key authentication doesn't work: make sure that on the server side, your home directory (~), the ~/.ssh directory, and the ~/.ssh/authorized_keys file, are all writable only by their owner. In particular, none of them must be writable by the group (even if the user is alone in the group). chmod 755 or chmod 700 is ok, chmod 770 is not.

What to check when something is wrong:

  • Run ssh -vvv to see a lot of debugging output. If you post a question asking why you can't connect with ssh, include this output (you may want to anonymize host and user names).
  • If you can, check the server logs, typically in /var/log/daemon.log or /var/log/auth.log or similar.
  • If public key authentication isn't working, check the permissions again, especially the group bit (see above).

Further reading

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