Block MSN file transfer: impossible mission?
It is convenient to transfer files via messengers like msn/live, yahoo, icq… But it is also necessary for organizations to block unauthorized file transfers to keep their networks safe.
However, messenger software uses several ways to avoid being blocked. They use dynamic ports, encrypted connections, variety connection type to bypass network firewall.
Let me take msn as an example. By our test, there have four type of msn file transfer as described below:
1. For two buddies, if one of them is connected to internet directly, direct connection will be established to transfer files. This is the quickest way. There has three type of direct connections with dynamic ports which is negotiated by two sides.
1.1) Direct TCP connection.
1.2) Direct TCP connection use TLS encryption.
1.3) Direct UDP transmission.
2. If direct connection can not be established, msn servers can act as a relay server to transfer files. The file transfer packets will be among with normal msn messages.
As you can see from above, there is no way to block msn file transfer simply by blocking some ports in the firewall. The firewall should be smart enough to recognize msn file transfer direct connections, and it shall be able to pick up file transfer packets from normal msn messages.