I wrote Monday about pharming attacks, in which a villain corrupts the DNS system, which translates textual names (like “www.freedom-to-tinker.com”) into the IP addresses (like “216.157.129.231”) that are used to route traffic on the Internet. By doing this, the villain can impersonate an Internet site convincingly. Today I want to talk about how to address this problem.

The best approach would be to secure the DNS system. We know how to do this. Solutions involve having authoritative DNS servers put some kind of digital signature on the information they give out, so that a computer receiving DNS translation information can verify that the information is endorsed by an authoritative server. Such a system, if universally deployed, would put the pharmers out of business. Unfortunately, secure DNS is not widely deployed.

A partial solution, for web access at least, is to access websites via secure (HTTPS) connections. The user, on seeing a valid site, would notice the lock icon on his browser, and would know that his machine was connected to the legitimate owner of the URL that his browser was displaying. A pharmer could make accesses to “www.citibank.com” go to his evil site, but he couldn’t fool the secure-connection mechanism, so he could not make the lock icon on the user’s browser light up.

This approach works fine, as long as users notice the lock icon and refuse to deal with sites that don’t use secure connections. Will users be so vigilant? Probably not. In practice, many sites fail to use secure connections, and browsers give subtle indications of whether a connection is secure but don’t scream about insecure connections. (How could they, when insecure connections are so common?)

One drawback of relying on secure web connections is that it doesn’t protect other communication services, such as email and instant messaging. Pharmers might try to attract a user’s email or IM connections to hostile servers. We know how to secure email, by assigning encryption keys to individuals and having them encrypt and digitally sign their email. Standard email programs even know how to handle encryption and signing. But, again, few people use these facilities.

You may have noticed a common pattern here: each of these mechanisms would be effective if widely adopted, but aren’t used much in practice. In each case, we have a collective action problem. If nearly everybody adopted one of these technologies, then the holdouts would have an incentive to adopt it too; but until a critical mass of adoption is reached, there is little incentive for others to join.

Consider secure web connections. If nearly every website used secure connections, then insecure connections would be rare enough that browsers could issue prominent warnings whenever they saw an insecure connection. This would give legitimate websites a strong incentive to use secure connections, in order to avoid scaring users away. Today, insecure connections are so common that they don’t attract any suspicion. (An online banking site that used insecure connections would be odd, and might arouse suspicion from alert users; but we’re far from the point when browsers expect secure connections from everybody.)

A similar problem holds for secure email. I could digitally sign my outgoing email, but this wouldn’t do much to prevent forged messages in practice. A forged message would of course be unsigned, but unless unsigned messages were rare, nobody would be taken aback on seeing one. But if almost all messages were digitally signed, than an unsigned message would be rare enough to arouse suspicion, and might trigger a prominent warning from the user’s email program.

In all of these cases, there is a tipping point, where the authentication technology is used so widely that failing to use it attracts suspicion. Once the tipping point is reached, the remaining holdouts will switch to using the technology. Assuming we agree that it would be good to adopt one of these technologies, how can we get to the tipping point?

Fred Cohen is predicting that VoIP will bring with it a flood of unsolicited commercial phone calls. (VoIP, or “Voice over Internet Protocol,” systems deliver telephone-like service, making connections via the Internet rather than using the wires of the plain old telephone system.) Cohen argues that VoIP will drive down the cost of international calling to nearly zero, thereby making international telemarketing calls very cheap. He also argues that small overseas call centers will violate the U.S. Do Not Call List with impunity.

This comes on top of concerns about SPIT, or Spam over Internet Telephony. SPIT sends machine-generated voice calls to the phones or voicemail boxes of VoIP users; Cohen worries about VoIP-mediated calls from live people. In a previous article about SPIT, VoIP vendors argue, unconvincingly, that they can handle the SPIT problem.

The root cause of this problem is the same as for email spam. Whenever a communication technology (1) allows anybody to communicate with anybody else, (2) at very low cost, unsolicited and unwanted communication will be a problem. We saw it with spam, and now we’ll see it with SPIT and VoIP telemarketing.

End-users can try to protect themselves from VoIP annoyances by using some of the same methods used against email spam. Whitelists (lists of trusted people), blacklists (lists of suspected spammers), challenge-response, and ultimately even automatic classification and filtering of voice messages, all seem likely to be tried at some point in the future. But as with email spam, don’t expect them to solve the problem, but only to reduce the annoyance level somewhat.

An even more interesting question is whether service providers can address the problem, perhaps by ejecting bad actors from their networks. This depends on how a particular network is structured. Some networks are closely controlled; these will have some chance of ejecting villains. Some networks rely on open protocols, so that nobody is in a position of control – the villains will just connect to the network as they please, and perhaps reconnect periodically under new names. Things get more challenging when different networks connect to each other, so that their legitimate clients can talk to each other. If a closed network connects to an open one, villains on the open network may be able to reach customers of the closed network, despite the best efforts of the closed network’s administrator.

Can’t we just use closed networks instead of open ones? If only it were so simple. Open networks have important advantages over closed ones; and many people will choose open networks because of these advantages, and in spite of the possibly heavier spam load on open networks. They may well be right to make that choice.

Because all of this calling will be done on the Internet, an open and tremendously flexible network, there are many creative attacks on these problems. For example, an open authentication infrastructure might provide a kind of CallerID service for VoIP, or even a certification of non-spammerness. Expect the technological battle to go on for years.

Internet spoofing attacks have been getting more and more sophisticated. The latest evil trick is “Pharming,” which relies on DNS poisoning (explanation below) to trick users about which site they are viewing. Today I’ll explain what pharming is. I’ll talk about fixes later in the week.

Spoofing attacks, in general, try to get a user to think he is viewing one site (say, Citibank’s home banking site) when he is really viewing a bogus site created by a villain. The villain makes his site look just like Citibank’s site, so that the user will trust the site and enter information, such as his Citibank account number and password, into it. The villain then exploits this information to do harm.

Today most spoofing attacks use “phishing.” The villain sends the victim an email, which is forged to look like it came from the target site. (Forging email is very easy – the source and content of email messages are not verified at all.) The forged email may claim to be a customer service message asking the victim to do something on the legitimate site. The email typically contains a hyperlink purporting to go to the legitimate site but really going to the villain’s fake site. If the victim clicks the hyperlink, he sees the fake site.

The best defense against phishing is to distrust email messages, especially ones that ask you to enter sensitive information into a website, and to distrust hyperlinks in email messages. Another defense is to have your browser tell you the name of the site you are really visiting. (The browser’s Address line tries to do this, so in theory you could just look there, but various technical tricks may make this harder than you think.) Tools like SpoofStick display “You’re on freedom-to-tinker.com” in big letters at the top of your browser window, so that you’re not fooled about which site you’re viewing. The key idea in these defenses is that your browser knows which domain (e.g. “citibank.com” or “freedom-to-tinker.com”) the displayed page is coming from.

“Pharming” tries to fool your computer about where the data is coming from. It does this by attacking DNS (Domain Name Service), the service that interprets names like “freedom-to-tinker.com” for you.

The Internet uses two types of addresses to designate machines. IP addresses are numbers like 128.112.68.1. Every data packet that travels across the Internet is labeled with source and destination IP addresses, which are used to route the packet from the packet’s source to its destination.

DNS addresses are text-strings like www.citibank.com. The Internet’s routing infrastructure doesn’t know anything about DNS addresses. Instead, a DNS address must be translated into an IP address before data can be routed to it. Your browser translated the DNS address “www.freedom-to-tinker.com” into the IP address “216.157.129.231” in the process of fetching this page. To do this, your browser probably consulted one or more servers out on the Internet, to get information about proper translations.

“Pharming” attacks the translation process, to trick your computer somehow into accepting a false translation. If your computer accepts a false translation for “citibank.com,” then when you communicate with “citibank.com” your packets will go to the villain’s IP address, and not to the IP address of Citibank. I’ll omit the details of how a villain might do this, as this post is already pretty long. But here’s the scary part: if a pharming attack is successful, there is no information on your computer to indicate that anything is wrong. As far as your computer (and the software on it) is concerned, everything is working fine, and you really are talking to “citibank.com”. Worse yet, the attack can redirect all of your Citibank-bound traffic – email, online banking, and so on – to the villain’s computer.

What can be done about this problem? That’s a topic for another day.