There are some protocols and applications that it’s very difficult to manage without and FTP surely comes into this category. The File Transfer Protocol is more normally thought of as an application than a protocol but it’s something that most of us couldn’t do without. It’s correct definition is probably the simplest – a TCP/IP service used primarily for copying and transmitting files. Although it takes many different forms, all of them are basically built around the client/server model.
FTP uses two different ports when it’s configured on a server, the standard command one is port 21. However FTP uses a second port for actually transferring data, keeping it separate from the initial connection. This actual data port also varies depending on whether you’re using active or passive FTP.
Active FTP is named because it is the server which opens up the data connection to the client. Although both passive and active FTP use the command port 21, in active FTP the second port is 20 which is used for the data transmission from client to server. This data port is used for all transmission of files and directory listings requested though the command port. The port used on the client is usually and ephemeral one in active FTP transmissions which is necessary as FTP uses a three way handshake for initial communications.
In active FTP the data connection is initiated from the server to the client which can make it difficult to use in most corporate networks. This is because you have two issues, an outside server being required open access to client and the facility to choose an ephemeral port to connect to. So if you wanted to transfer some files from a server stored on http://bbciplayerabroad.co.uk/, you’d have to allow an inbound data connection made to the client on your network. Neither of these would normally be allowed in most networks simply because of the intrinsic security risk in allowing it.
There are very few well configured networks which use active FTP unless under very specific situations. It’s simply too difficult to allow outside connections to be made into the core internal network from random FTP servers. The solution is something called passive FTP which allows the client and not the server to make the data connection.
This bypasses the main security risk of active FTP although the functionality is not really affected. It’s normally quite easy to distinguish between these two modes through analysing the traffic simply because of the direction of the data transmission handshake. Although it’s worth checking though because the initial client data connection is sometimes difficult to pick up, ensure it’s come from a internal client and not from some random french ip address which is being used to spoof connections into your network.