Lot of people have said that the file-transfer using MSN happens via port range 6891-6900.  But when I experimented, I found that MSN is using same 1863 port.  Linux machines are able to identify this port as “msnp”.  I used “tcpdump” to verify this.  Whenever MSN attempts to send a file across the Internet, I creates several smaller packets (typically in the range of 536-1350 bytes) and sends them one after the other.  During the file transfer process, if there are messages (text or IM) to be transmitted, the message data block is piggy-backed on the file-transfer packets and sent across.

I was desperately looking for filtering the MSN based file-transfer for some official purposes and wrote some iptables rules based on the Internet literature that said file transfers happen via 6891-6900 by TCP.  Then, to validate the rules, I used tcpdump again on the gateway machine to monitor the packets that are originated from my machine. 

To my surprise, the file transfer was still happening also the ports are blocked.  Then I made an “iptables” rule to drop all the packets other than 1863 and repeated the experiment.  Still the file-transfer was happening.  I could see using tcpdump that the packet transfer is happening via port 1863.

So, it became apparant that the file-transfer and the text messaging are all happening via port 1863 instead of the port range 6891-6900.  I then decided to write an iptables rule to filter the packets using the packet size constraint.  A rule was written to drop packets that are more then 600 bytes assuming that the IM packets shall never reach the limit.

I did see that the packet size is around 1350 bytes when files are transferred and that’s why I chose 600 bytes as the limit.  When I monitoring using the “tcpdump” command as before, I was surprise to see that the protocol adjusted the packet size automatically to 560 bytes to continue the transmission.  It tried with 1350 bytes for 3 times and as the acknowledgments were not received, it’s flow control mechanism reduced the packet size to 560 bytes and finished the transfer.  So, I had to redo the iptables rule:-

The happiness did not last long.  When people logout and tried logging in, they were not able to login at all.  When I investigated the cause of this problem, I could see that the packets that are exchanged during the login process is more than 512 bytes (typically 1350 bytes).  So, I had to relax the rule for a brief time to let people login to MSN messenger service.  I enabled the file-transfer restriction after everybody logged in by enabling the iptables rule.

NOTE: iptables based filtering shall work only for packets that travel across the network.  If the MSN file-transfer happens inside the LAN, MSN cleverly does the file-transfer using P2P where the gateway is not involved at all.