The ssh-keygen command is used to create SSH keys, which are used to authenticate an SSH client to an SSH server. To create a host key file, you can use the ssh-keygen command with the -t option to specify the type of key you want to generate and the -f option to specify the output file name.
Here is an example of how to create an RSA host key file:
ssh-keygen -t rsa -f /etc/ssh/ssh_host_rsa_key
This will create a file named /etc/ssh/ssh_host_rsa_key that contains the RSA host key. You can specify a different file name or path if desired.
By default, the ssh-keygen command generates keys with a length of 2048 bits, but you can use the -b option to specify a different key length if desired.
After you have generated the host key file, you will need to configure your SSH server to use it. This typically involves adding a line to the /etc/ssh/sshd_config file to specify the path to the host key file.