SSH2 protocol
ssh-keygen -f /etc/ssh/ssh_host_rsa_key -N '' -t rsa ssh-keygen -f /etc/ssh/ssh_host_dsa_key -N '' -t dsa
OpenSSH also support ECDSA key, you may generate a new one using:
ssh-keygen -f /etc/ssh/ssh_host_ecdsa_key -N '' -t ecdsa -b 521