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Chapter 2 Connecting to Remote Computers 33
Copy the resultant public file, which contains the local computer’s public key to the
user’s home folder in .ssh/ on the remote computer. The next time you log in to the
remote computer from the local computer you won’t need to enter a password.
Note: If you are using an Open Directory user account and have already logged in
using the account, you do not have to supply a pasword for SSH login. On Mac OS X
Server computers, SSH uses Kerberos for single sign-on authentication with any user
account that has an Open Directory password (Kerberos must be running on the Open
Directory server). See the Open Directory administration guide for more information.
Updating SSH Key Fingerprints
The first time you connect to a remote computer using SSH, the local computer
prompts for permission to add the remote computer’s fingerprint (or encrypted public
key) to a list of known remote computers. You might see a message like this:
The authenticity of host "server1.example.com" can’t be established.
RSA key fingerprint is a8:0d:27:63:74:f1:ad:bd:6a:e4:0d:a3:47:a8:f7.
Are you sure you want to continue connecting (yes/no)?
The first time you connect, you have no way of knowing whether this is the correct
host key. Most people respond “yes.” The host key is then inserted into the ~/.ssh/
known_hosts file so it can be compared against in later sessions. Be sure this is the
correct key before accepting it. If at all possible, provide your users with the encryption
key either through FTP, email, or a download from the web, so they can be sure of the
identity of the server.
If you later see a warning message about a man-in-the-middle attack when you try to
connect, it might be because the key on the remote computer no longer matches the
key stored on the local computer. This can happen if you:
 Change your SSH configuration on either the local or remote computer.
 Perform a clean installation of the server software on the computer you are
attempting to log in to using SSH.
 Start up from a Mac OS X Server CD on the computer you are attempting to log in to
using SSH.
 Are attempting to SSH in to a computer that has the same IP address as a computer
that you previously used SSH with on another network.
To connect again, delete the entries corresponding to the remote computer (which can
be stored by both name and IP address) in the file ~/.ssh/known_hosts.