Occasionally a host key changes (like when the machine is reinstalled), and users need to be able to remove the corresonding line from known_hosts. Now that the hostnames are hashed, that's difficult. OpenSSH supposedly comes with utilities remove-knownhost and ssh-showkey for dealing with this. Perhaps the next version of the Debian package should include these. Until then, I'll just turn off HashKnownHosts in ~/.ssh/config. AMC