Trends Post #6 on Why Spam Filters Suck “trickle blog” series By Desmond Liao | 4 minute read Blocking Spam In 2008 Like a shepherd, the duty of a bot herder (botnet operator) is to keep his/her botnet army intact. Bot herders make money by amassing a botnet, then contracting out the botnet services to spammers. That’s right, spammers employ bot herders to do the dirty work for them! Bot herders only get paid by the spammer when a message is actually delivered to the receiving email server. For those readers familiar with SMTP protocol, this means that the bot herder only gets paid once the server has sent 250 Ok after the DATA phase. In order to make a lot of money, bot herders have to send as much as possible in the shortest possible time. If a zombie is being blocked, the bot herder doesn’t make any money. The bot herder only makes money when a message is actually received by the receiving email server. Spamming software is impatient. In programming terms, spamming software has a very low timeout. The SMTP RFC recommends that email servers wait at least three minutes for each chunk of data they send to be received by the receiving server and acknowledged via a TCP acknowledgement packet. Furthermore, the RFC recommends that senders wait at least ten minutes for the final message delivery acknowledgement. These long timeouts were established because in the early days of the Internet, the infrastructure was slow and unreliable, and the machines were easily overloaded, leading to frequent message delivery delays. Today, email servers and our networks are much faster at processing incoming messages in a matter of seconds. Delays still occur, but thetimeouts defined in the RFC are vastly higher than what is required in today’s world. Because bot herders don’t get paid until they receive the 250 Ok, their software earns a higher profit by disconnecting after a few seconds and seeking out new victims whose servers respond more quickly. Bots can’t afford to wait for a slow connection to go through, and they can’t risk being discovered and put on a blacklist. A few years ago, the MIT Spam Conference was a very interesting place. Each year, bright-eyed graduate students and intrepid industry types would present new filtering techniques that pushed the accuracy of spam filters to new levels. For the past three years, improvements in spam filter effectiveness has plateaued. A great result is a paper that shows the accuracy improvement of half a percent. Spam filtering has essentially become maxed out as a technology, and there isn’t much more we can do but tweak rules to avoid falling behind the spammer’s arms race. Similarly, reputation systems which identify suspicious IP addresses have become asymptotic in their effectiveness. The spread of botnets has led to a virtually inexhaustible supply of new IP addresses, that spam us a few times and then disappear forever. Most of the large anti-spam companies now have comprehensive blacklists that are updated every minute. In other words, anti-spam systems worldwide are blocking everything they possibly can. And yet spam continues to grow as a problem — it’s unbelievable. So what can we do? Bill Gates was right in 2004. He boldly posited that the way to solve the spam problem was to introduce a cost barrier that caused spamming to be no longer profitable. Unfortunately, spammers created botnets, which have rendered to them more computing power than most governments. One way to think of the problem is that the spammers have millions of computers. You only have a handful. And you have to pay for yours. Who’s going to win? While we can’t win the spam war with better filters or better blacklists, there are alternatives. To deter spamming we must undermine spammers, not simply block messages. You can make botnets unprofitable by slowing down SMTP traffic from spammers. This not only gives the receiver control of each email connection, but it also consumes sender resources to reduce the spammer’s sending rate significantly. Imagine the chaos at an airport without air traffic controllers and you begin to see why mail servers need email traffic control. NEXT: Post #7 Slowing Things DownPREVIOUS: Post #5 Why Are Botnets So Difficult To Stop?