Security always requires a multi-layered scheme. SSH is a good example of this. Methods range from simple sshd configuration through the use of PAM to specify who can use SSH, to application of port-knocking techniques, or to hide the fact that SSH access even exists. Applying these techniques can make life much harder for possible intruders, who will have to go past three unusual barriers."Learn 3 ways of hardening SSH access to your system to block would-be crackers"