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Evading Audible Magic’s Copysense filtering

As I noted on Monday, the Irish branches of several major record companies have brought a case against Eircom, demanding in part that the ISP install Audible Magic’s Copysense anti-filesharing appliances on their network infrastructure.

I thought I’d do a quick bit of research online into how they do their filtering. Here’s what the EFF had to say:

Audible Magic’s technology can easily be defeated by using one-time session key encryption (e.g., SSL) or by modifying the behavior of the network stack to ignore RST packets.

It’s interesting to see that they used RST packets — this is the same mechanism used by the “Great Firewall of China” to censor the internet:

the keyword detection is not actually being done in large routers on the borders of the Chinese networks, but in nearby subsidiary machines. When these machines detect the keyword, they do not actually prevent the packet containing the keyword from passing through the main router (this would be horribly complicated to achieve and still allow the router to run at the necessary speed). Instead, these subsiduary machines generate a series of TCP reset packets, which are sent to each end of the connection. When the resets arrive, the end-points assume they are genuine requests from the other end to close the connection — and obey. Hence the censorship occurs.

But there’s a very easy way to avoid this, according to that blog post:

However, because the original packets are passed through the firewall unscathed, if both of the endpoints were to completely ignore the firewall’s reset packets, then the connection will proceed unhindered! We’ve done some real experiments on this — and it works just fine!! Think of it as the Harry Potter approach to the Great Firewall — just shut your eyes and walk onto Platform 9¾.

Clayton, Murdoch, and Watson’s paper on this technique provides the Linux and FreeBSD firewall commands they used to do this. Here’s Linux:

   iptables -A INPUT -p tcp --tcp-flags RST RST -j DROP

For FreeBSD, the command is:

   ipfw add 1000 drop tcp from any to me tcpflags rst in

So assuming Copysense haven’t changed their approach yet, it’s trivial to block Copysense’s filtering, if both ends are running Linux or BSD. I predict if Copysense becomes widespread, someone will patch Windows TCP to do the same.

I love Audible Magic’s response:

The current appliance happens to use the TCP Reset to accomplish this today. There are many other technical methods of blocking transfers. Again, we have strategies to deal with them should they ever prove necessary. This is why we recommend our customers purchase a software support agreement which provides for these enhancements that keep their purchase up-to-date and protect their investment.

in other words, “hey customers! if you don’t have a support contract, you’re shit out of luck when the p2p guys get around our filters!” Nice. ;)