Defining Server Host Key
The key pair used for server authentication is defined on the server in the
file with the following parameters:
During the setup process, one RSA key pair (with the file names
) is generated and stored in the
directory. By default this key pair is used for server authentication. Make sure that only the user running
sshd2 has access to the private key.
In SSH Tectia Server for IBM z/OS, each server daemon can have only one host key pair. This is different from SSH Tectia Server on other platforms.
By default, the server uses a public key with the filename of the private key plus the extension
PublicHostKeyFile keyword has to be defined only if the public-key file is stored with a different filename.
Generating the Server Host Key Pair
The host public-key pair (1536-bit RSA) is generated during the setup of SSH Tectia Server (Section Running the Setup Script). You only need to regenerate it if you want to change your host key pair.
SSH Tectia Server for IBM z/OS includes a program that generates a key pair,
ssh-keygen2, which is located in
Generate the key pair for the server in such a way that the private key has no passphrase (option
-P). The server will then start up without any operator interaction to enter a passphrase. Protect the key with file system access rules. The private key (
/etc/ssh2/hostkey) must be accessible only by the
To (re)generate the host key, perform the following tasks:
su to switch to a UID 0 user (if you are not already logged in as one).
ssh-keygen2 to generate the host key, for example:
> /usr/lpp/ssh2/bin/ssh-keygen2 -t rsa -P /etc/ssh2/hostkey
This will generate a 2048-bit RSA key pair without a passphrase and store it under
/etc/ssh2. For more information on the key generation options, see the
ssh-keygen2 man page (Appendix ssh-keygen2).
- Restart the server as instructed in Section Restarting sshd2.
Using an OpenSSH Server Host Key
SSH Tectia Server for IBM z/OS can use a key created with OpenSSH as the server host key. The key must be configured with the
HostKeyFile option in
sshd2_config or have the default file names,
Both RSA and DSA keys with key lengths from 512 (OpenSSH requires at least 768 for DSA keys) to 4096 bits or more are supported.
Notifying the Users of the Host Key Change
Administrators that have other users connecting to their server should notify the users of the host key change. If you do not, the users will receive a warning the next time they connect because the host key the users have saved on their disk for your server does not match the host key now being actually provided by your server. The users may not know how to respond to this error.
You can run the following to display a fingerprint of your new public host key which you can provide to your users via some unalterable method (for example, by a digitally signed e-mail or by displaying the fingerprint on secured bulletin board):
> /usr/lpp/ssh2/bin/ssh-keygen2 -F hostkey.pub
When the users connect and receive the error message about the host key having changed, they can compare the fingerprint of the new key with the fingerprint you have provided in your e-mail, and ensure that they are connecting to the correct
sshd2 daemon. Inform your users to notify you if the fingerprints do not match, or if they receive a message about a host key change and do not receive a corresponding message from you notifying them of the change.
This procedure can help ensure that you do not become a victim of a man-in-the-middle attack, as your users will notify you if the host key fingerprints do not match. You will also be aware if the users encounter host key change messages when you have not regenerated your host key pair.
If you want to avoid the risk associated with the first connection, you can do one of the following:
- As an administrator of both the client and server machines, you can copy the server public key in advance to the
/etc/ssh2/hostkeys directory on the client computer as
<hostname> is the hostname the client uses when it connects to the server).
In this case, manual fingerprint check is not needed, and you can also set the
keyword in the
file on the client to
yes. After this,
ssh2 will refuse to connect if the server's public key is not in the
- The server administrator can also send the public host key to the users via an unalterable method. The users can save the key in their
$HOME/.ssh2/hostkeys directory as
key_22_<hostname>.pub. If all remote host keys are received in this manner, the
StrictHostKeyChecking option can be enabled on the client.