Authenticating Remote Server Hosts
Remote Secure Shell servers are authenticated by a public-key procedure.
The user checks the fingerprint of the remote server's public key. When
the user has approved the public key, it is stored in the user's
$HOME/.ssh2/hostkeys directory and will be used automatically thereafter.
The verification step normally requires user interaction, so even for
users that are set up to run client programs unattended, the first
connection must be done by a person who logs in as the user, accesses
the remote server, and goes through the fingerprint check dialog.
The same steps must be repeated if the remote host's key is changed.
Caution: When ssh-keydist2 is run with the -a
or -N options, it accepts the received host keys automatically
without prompting the user. You should verify the validity of keys after
receiving them or you risk being subject to a man-in-the-middle attack.
To be able to verify the keys, you should use the plain host
key storage format. See below.
When the host key is received during the first connection to a remote
host (or when the host key has changed) and you choose to save the key,
its filename is by default stored in hashed format, keys_hhh...,
where hhh is a hash of the host port and name. The saved file
contains a hash of the host's public key. A salt is included in the hash
calculations. The value of the salt is stored in the file salt
in the same directory as the host keys ($HOME/.ssh2/hostkeys).
The hashed host key format is a security feature to make address
harvesting on the hosts difficult.
In the plain (traditional) format, the name of a host key file includes
the hosts's name and port, as in key_22_host.example.com.pub,
and the file contains the host's public key in plaintext format.
The storage format can be controlled with the HostKeyFormat
option of the ssh2_config configuration file. The argument must
be plain or hashed (default).
You can display the fingerprint of a received host key by running the
following command:
> ssh-keygen2 -F $HOME/.ssh2/hostkeys/key_<port>_<host>.pub
