FTP (File Transfer Protocol) Question:

FTP bounce attack is an exploit of the FTP protocol whereby an attacker is able to use the PORT command to request access to ports indirectly through the use of the victim machine as a middle man for the request.

This technique can be used to port scan hosts discreetly, and to access specific ports that the attacker cannot access through a direct connection.

nmap is a port scanner that can utilize an FTP bounce attack to scan other servers.