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Within the last few weeks, there have been a number of reports by Comcast customers claiming that their BitTorrent downloads and uploads have been capped or worse, blocked. Torrent Freak recently reported that Comcast, a major US cable company, is using an application from Sandvine to throttle such connections.

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According to Torrent Freak, Comcast is not doing this. They are instead sending a reset (or RST) packet to the Comcast customer, pretending to be from the host at the end of the BitTorrent connection. This RST packet is the TCP equivalent of stating “I don’t want to talk to you anymore, please terminate the connection”. It is extremely important to note that when Comcast creates and sends this packet, they do not identify themselves as the the source of packet, but instead impersonate one of the parties involved in the BitTorrent connection. This is where things get rather shady.

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Many states make it illegal to impersonate others. New York, a state notorious for its aggressive pro-consumer office of the Attorney General, makes it a crime for someone to “[impersonate] another and [do] an act in such assumed character with intent to obtain a benefit or to injure or defraud another.” (See: NY Sec. 190.25: Criminal impersonation in the second degree). I do not believe that it would be too difficult to prove that Comcast obtains a benefit by impersonating others to eliminate or reduce BitTorrent traffic. Less torrent data flowing over their network will lead to an overall reduction in their bandwidth bill, and thus a huge cost savings.

Another solution:

iptables -A INPUT -p tcp -dport $TORRENT_CLIENT_PORT -tcp-flags RST RST -j DROP