Dave Lien's Profile

A quick summary of the pros and cons of active vs. passive FTP is also in
order:

Active FTP is beneficial to the FTP server admin, but detrimental
to the client side admin. The FTP server attempts to make connections to
random high ports on the client, which would almost certainly be blocked
by a firewall on the client side. Passive FTP is beneficial to the
client, but detrimental to the FTP server admin. The client will make
both connections to the server, but one of them will be to a random high
port, which would almost certainly be blocked by a firewall on the server
side.

Luckily, there is somewhat of a compromise. Since admins running FTP
servers will need to make their servers accessible to the greatest number
of clients, they will almost certainly need to support passive FTP. The
exposure of high level ports on the server can be minimized by specifying
a limited port range for the FTP server to use. Thus, everything except
for this range of ports can be firewalled on the server side. While this
doesn't eliminate all risk to the server, it decreases it tremendously.
See Appendix 1 for more information.