Regenerating SSH host keys

For a Linux VPS, all you have to do to generate new hostkeys on the server is use ssh-keygen (included with the standard OpenSSH package) to perform the following commands for both SSH protocols (ultimately you must perform all three commands). Make sure that the -N is followed by two single quotation marks and that all three file names are different.

SSH1 protocol

For the SSH1 protocol you only require an RSA1 key:

ssh-keygen -f /etc/ssh/ssh_host_key -N '' -t rsa1
SSH2 protocol

For the SSH2 protocol you need two keys, for rsa and dsa:

ssh-keygen -f /etc/ssh/ssh_host_rsa_key -N '' -t rsa
ssh-keygen -f /etc/ssh/ssh_host_dsa_key -N '' -t dsa
The authenticity of host xxx can't be established

If you have logged into the VPS via SSH in the past, after you regenerate the hostkeys for a new SSH connection you should get a message stating that the authenticity of the server cannot be established. This is true, because the server's fingerprint is stored in a local cache on your system:

On Linux this is stored in ~/.ssh/known_hosts
On Windows it is stored in the registry under HKEY_CURRENT_USER\Software\SimonTatham\PuTTY\SshHostKeys
Every time a connection is established, a comparison of the fingerprints (which look something like xx:xx:xx:xx:xx:xx:xx:xx:xx:xx:xx:xx:xx:xx:xx:xx (for rsa)) is made between the local database and the server you are contacting.

Removing the offending key



You can remove the locally saved cache for the server and re-establish the connection, in which case you will be prompted to save the fingerprint again. On a Linux terminal, you will see the following warning:

Offending key in /home/USER/.ssh/known_hosts:15
where /home/USER/.ssh/known_hosts is the file location and 15 is the line number. This enables you to use vi to jump directly to the right line so you can delete it:

vi /home/USER/.ssh/known_hosts +15
For Windows, you use regedit to find the correct registry entry and delete the offending key:

HKEY_CURRENT_USER\Software\SimonTatham\PuTTY\SshHostKeys

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