Linux / Unix ssh-keygen: Create A Host Key File

To create a host key file using ssh-keygen on Linux or Unix, follow these steps:

1. Open a terminal and enter the following command:

ssh-keygen -t rsa -b 4096 -f /etc/ssh/ssh_host_rsa_key

This will generate a 4096-bit RSA host key and save it to the file /etc/ssh/ssh_host_rsa_key. The -t option specifies the type of key to generate (RSA in this case), and the -b option specifies the number of bits in the key.

2. Enter a passphrase when prompted. This passphrase will be used to encrypt the host key file, and will be required every time the key is used.

3. Repeat this process for other key types, such as DSA and ECDSA, using the appropriate options. For example:

ssh-keygen -t dsa -f /etc/ssh/ssh_host_dsa_key
ssh-keygen -t ecdsa -f /etc/ssh/ssh_host_ecdsa_key

4. Restart the SSH server to apply the new host keys.

Once the host keys have been generated and installed, they will be used by the SSH server to identify the host to connecting clients. The client will verify the host key by checking it against a known host key database, and will alert the user if the host key has changed since the last time they connected. This helps to prevent man-in-the-middle attacks and other types of SSH spoofing.