FTP is one of those protocols that you just have to sit back and ask "What the
heck were they thinking?" FTP has many problems that the firewall administrator needs
to deal with. What's worse, the problems the administrator must face are different between
making ftp clients work and making ftp servers work.
Within the FTP protocol, there are two forms of data transfer, called active and
passive. Active transfers are those where the server connects to an open port on the
client to send data. Conversely, passive transfers are those where the client connects to
the server to recieve data.