Before starting the SSH server, run as root "ssh-keygen -A" 
and then back up the /etc/ssh directory to save the generated 
key files.  Or use the specific commands to generate the desired keys:

ssh-keygen -A   (will create dsa, ecdsa, rsa, and ed25519 keys)
ssh-keygen -t dsa
ssh-keygen -t ecdsa
ssh-keygen -t ed25519
ssh-keygen -t rsa
ssh-keygen -t rsa1


The ssh server is started with the below command -

/etc/init.d/ssh start