hashing the know_hosts file

August 15, 2008 in , , , , , , with 23 Comments

OpenSSH client keep a fingerprint of servers to which connections (ssh-client) have been made. Such fingerprints are stored in .ssh/know_hosts and are automatically compared with the current server fingerprint on connection acknowledgment.

Hence, the .ssh/known_hosts file is crucial to system security against man-in-the-middle attack in a networked environment. This file is also a very very good vector of attack on system administrator computer and hashing the content of the file is a good practices. Especially with the current wave of big bugs hitting GNU/Linux systems.

The first step is to enable hashing of the new fingerprints:

# cat /etc/ssh/ssh_config
Host *
SendEnv LANG LC_*
HashKnownHosts yes
GSSAPIAuthentication no
GSSAPIDelegateCredentials no
TCPKeepAlive yes
ServerAliveInterval 60

The “HashKnownHosts yes” configuration option is the way to go – it is a general setting affecting all users on your system (Host *). If you don’t have access to the central ssh_config option, don’t forget you have personalized user setting in .ssh/config.

This enable the hashing of future fingerprints. To modify your actual file, use the following ssh-keygen command. Your unmodified know_hosts will be save as know_hosts.old .

# ssh-keygen -H -f .ssh/know_hosts

Have fun, stay safe.

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