UltraSurf is an enormously popular program used to circumvent Internet censorship in countries like China (as well as schools and workplaces in mostly-free countries like the US, with mixed success). When you run UltraSurf on your computer, it re-routes your outgoing Internet traffic to external IP addresses controlled by UltraSurf, so that it looks to observers (and network censors) as if you are connecting to UltraSurf's IP addresses, rather than a website like YouTube or Facebook that may be banned on your network.
UltraSurf uses a list of thousands of external IP addresses, to make it non-trivial for an adversary to locate all of their IP addresses and block them all. However, using a few steps that would be obvious to many programmers facing the same problem, I did find a way to detect all the IP addresses that UltraSurf connects to, and block all of them so that UltraSurf stopped working. It would not be hard for a government censor operating the filter in a country like China to do the same thing. But this does not mean that UltraSurf's network is likely to collapse any day now; on the contrary, it means that it and similar programs are likely to flourish for years to come, since the censors obviously have other priorities.
Some background information first. Most Internet censorship circumvention tools fall into one of two categories (whose names I have just invented for the purpose of this article):
(1) Self-bootstrapping. If a program is self-bootstrapping, then in a censored country you simply run a copy of the program and it will establish a connection to an IP address outside the country, one of many in a large "cloud" of IP addresses controlled by the software program's publisher. Thereafter, your Internet usage is routed through that connection in order to evade your country's filter. UltraSurf and Tor fall into this category.
(2) Non-self-bootstrapping. To use one of these programs from a censored country, first you have to get a friend in a non-censored country to install the software on their computer (or their webserver, if they have one). Then they give this location (normally in the form of a URL) to their friend in the censored country, and their friend types that URL into their browser to circumvent their country's filtering. Psiphon is the best-known program in this group.
In 2006 I wrote that even though the first category of programs was more convenient to use (not requiring you to rely on a friend in an uncensored country), any program in that category could be blocked by an adversary willing to make only a modest amount of effort: Install the program, see what IP addresses it connects to, block those, see if the program connects to any other backup IP addresses, block those, and so on, until the program runs out of IP addresses to use. There are a few simple countermeasures that designers of a program could take, but they can also be defeated easily.
(For example, if the program randomly chooses an IP address from a large internally stored list, then you just have to run the program over and over until you've found most of the IP address chosen by its random algorithm. A cleverly written program could try to evade this as follows: Pick a set of IP addresses at random from the list, and then "lock in" to that set of IP addresses, so that future runs of the program on that PC will always connect to those IP addresses, ignoring the other ones in the list. This makes it a little bit harder for the censor to pry out all of the IP addresses in the program's internal list. But then you, as the censor, can either (a) run the program repeatedly, but find where the program stores its "locked set" and erase that between each run, so that on future runs the program will keep selecting a different IP address set, or (b) if you can't figure out where the program is storing its "locked set" between each run, then just install the program repeatedly on different machines.)
One way or another, if the program knows what IP addresses to connect to when it bootstraps itself, the attacker can trick the program into revealing all of them. The attacker doesn't even need to reverse-engineer the software to see the set of instructions that it's executing internally; they only need to be able to see the IP addresses that the program is connecting to.
Much later, I was able to reduce this to practice in an experiment on my own machine, using a Perl script, the built-in Windows "netstat" tool to list connections from locally running programs to outside IP addresses, and the "ipseccmd" tool to add new firewall rules blocking those IP addresses. After the script was left running overnight, it had collected and blocked all the IP addresses that UltraSurf apparently used, and on future runs, UltraSurf would display an error message saying that it couldn't find any IPs to connect to.
(Interestingly, netstat also showed that UltraSurf frequently opened connections to www.google.com over SSL -- that is, accessing URLs that would begin with "https://www.google.com/" -- so that traffic between the program and the Google website would be encrypted, and the contents would be invisible to censors in China. When I saw it was doing that, I added an exception to the script so that the Google IP addresses would not be blocked. Perhaps it was submitting search terms to Google in order to find pages that give the location of the latest UltraSurf connection points, or perhaps it was checking a GMail account created by UltraReach that stores messages containing more IP addresses; I didn't reverse-engineer UltraSurf to find out. But even if this was UltraSurf's clever means of obtaining new IP addresses, the system still runs up against the same problem: Any IPs that can be connected to by the UltraSurf client, can also be ascertained by the attacker who watches UltraSurf to see where it connects to, and then blocks those IPs as well.)
Naturally I had mixed feelings about pointing this out publicly, since I agree with UltraReach's goal of providing unfiltered access to users in China and other censored countries. But this idea is sufficiently obvious, that I don't think anything is lost by demonstrating it. There may be programmers interested in creating even more programs to help users in censored countries, and it would be counterproductive for those programmers to believe that existing programs like UltraSurf "magically" evade the censors by using some complex algorithm to hide the IP addresses that they connect to. In fact, the program doesn't conceal the IP addresses that it connects to (how could it?), and it would be straightforward to design and build a new program that did roughly the same thing. We should give UltraReach credit for the right things: they made a tool that provides unfiltered access to millions of people, they made the tool small and easy to use, and they arranged with their partners to subsidize the unfiltered Internet connections at no expense to those end users (although see some caveats, which have been pointed out the Hal Roberts at the Berkman Center, about the price of this "free" access). But the one thing UltraReach did not do is find a way to get around the problem of an attacker installing the problem to see what IP addresses it connects to. That's not a criticism of UltraReach; this is presumably an impossible problem to solve.
(Side note about counter- and counter-counter-measures: If UltraReach does think that censoring countries might try harder to block UltraSurf at some point in the future, they should start releasing different versions of the product every month that use different sets of IP addresses. Release one version for September 2009 that uses one set of IP addresses, then another version in October 2009 that uses another set, and so on. Then if the censors decide in December 2009 to start seriously trying to block all UltraSurf IP addresses, they'll be able to find and block all the IP addresses used by the Dec09 version, just by installing a copy of the program and observing it. But, users who downloaded previous months' versions of the program will be able to continue using their copies. If the Chinese censors wanted to find and block the IP addresses used by preivous months' copies of UltraSurf, they would have to either (a) figure out how to distinguish UltraSurf traffic from other Internet traffic, not an easy thing since UltraSurf uses encrypted traffic on port 443, the same port used for encrypted Web traffic, or (b) obtain copies of the program that users had downloaded in previous months, which is no longer as trivial as simply observing the current version of the program. The more often UltraReach swaps out a new version of UltraSurf that connects to a new set of IP addresses, the harder it will be for the Chinese censors to find all the sets of IPs used by previously released versions. However, once the Chinese censors start trying seriously to block UltraSurf, even though the trick just described will allow previous downloaders of the program to continue surfing freely, all new users who download the program after that point, can be easily blocked -- because the Chinese censors can just watch how often a new version of UltraSurf is made available for download, and block the IPs used by that copy.)
But I think the fact that the Chinese have not done this reveals something usually overlooked about the nature of the anti-censorship arms race. The situation is frequently cast as a battle between the evil geniuses who run the government filters and the good geniuses who write the software to get around the filters, while the grateful citizens of the censored country are the beneficiaries. But if the government censors haven't even done some simple experiments like this in order to block UltraSurf, they must not think it's a high priority to stop the program from working. This in turn suggests that the number of people using UltraSurf in a country like China, while large in absolute numbers, don't constitute a large enough proportion of the population to worry the government. Presumably either the ideas leaking in through an unfiltered Internet are not reaching a large enough proportion of the population, or the ideas are not expected to take hold in enough people's minds to reach a tipping point that causes a problem for the ruling party.
It's not that the Chinese censors don't care about controlling the Internet and the effect that it has on their citizens' thinking. The Chinese have reported fielded a droid army of about 50,000 cubicle drones to help fight Internet propaganda battles, such as drowning out anti-government posts on public forums. Why would they spend such enormous efforts to generate forum posts, but not make the effort to find and block all UltraSurf IP addresses? Because the battlefront is about defaults. If the user tries to access a site and it's blocked, then only a tiny proportion will make a significant effort to circumvent the block. (The exception would be when an extremely popular site like YouTube is blocked; operators of Web proxy sites report that during these periods, they get so much traffic from Chinese users trying to view YouTube videos, that the servers often crash.) Similarly, if users see that 90% of the posts on a given forum are on one side of the issue, then they're more likely to think that's the majority viewpoint (whether they agree with it or not). Hence the usefulness of the army of 50,000 to invade forum threads. Defaults matter; would Internet Explorer have ever displaced Netscape's browser (kids, ask your parents) if it hadn't been the default browser in all versions of Windows?
So the moral for any would-be designers of new anti-Internet-censorship tools, is not to worry too much about whether there's a theoretical way (or even a practical way) that the censors could shut the tool down. UltraSurf became enormously popular without solving that problem, and perhaps another tool could as well.