OpenSSH is a common tool for most of network and system administrators. It is used daily to open remote sessions on hosts to perform administrative tasks. But, it is also used to automate tasks between trusted hosts. Based on public/private key pairs, hosts can exchange data or execute commands via a safe (encrypted) pipe. When you ssh to a remote server, your ssh client records the hostname, IP address and public key of the remote server in a flat file called “known_hosts“. The next time you start a ssh session, the ssh client compares the server information with the one saved in the “known_hosts” file. If they differ, an error message is displayed. The primary goal of this mechanism is to block MITM (“Man-In-The-Middle“) attacks.
More information :
Bruteforcing SSH Known_Hosts Files « /dev/random
"If you aim the gun at your foot and pull the trigger, it's
UNIX's job to ensure reliable delivery of the bullet to
where you aimed the gun (in this case, Mr. Foot)."