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Why the DNS is broken, in plain language

At ICANN’s meeting in Egypt last week, I had the opportunity to try and explain to various non-technical audiences why the Domain Name System (DNS) is vulnerable to attack, and why that is important, without needing a computer science degree to understand it. Here is the summary.

How does the DNS work?

The DNS can be considered to be a question-and-answer system. When you type in an address like “” into a web browser, your computer needs to turn that into a numeric address of the computer hosting that website. To do this, it sends a question over the Internet to a DNS server “Where is” The DNS server sends back an answer, “The address is”.

Typical DNS transaction

How do you attack the DNS?

Let’s say I want to execute an attack on this question and answer exchange, in order to convince the computer to go to the wrong address. When the computer I wish to attack asks a question, my goal is to provide a fake response back to that computer quicker than the real server comes back with the response. By getting my forged answer back faster, the computer will proceed using my answer, rather than the official one.

Execution of a spoofing attack

So, if I send back a fake address to a computer, I can get that computer to go to a different address than the one intended. For example, on that address I might have set up a fake website intended to take someone’s sensitive data, like a replica of a bank’s website.

If I manage to attack one computer why is that a big deal?

A successful attack on one computer in isolation can be problematic for the user of that computer, but it is not that interesting to an attacker to succeed against only one person. Unlucky for us, just one successful attack can very easily have wider consequences. Let me explain.

The DNS is made much more efficient by the use of “caching” name servers. These name servers sit at ISPs, or on corporate networks, and perform DNS lookups on behalf of customers. It then stores the answers it receives in a cache, so for future lookups for the same domain it does not repeat the lookup – it just remembers the previous answer.

This means that if you execute an attack and it gets stored in a cache, it can actually impact many people over and over again, because that answer will be redistributed to everyone that uses that same caching server.

This is why this type of attack is usually called a “cache poisoning” attack, because by poisoning the cache with the wrong data, it creates a much more serious problem.

So I just send back an answer quicker, and that’s it?

It is not quite as simple as just sending a quicker answer back to a computer, you also have to guess certain attributes correctly on the answer that match the question. For example, your answer needs to go back to the same computer the question originated. You also need match the question that was being asked in your answer.

Attributes that need to match

It is simple, however, to guess most of the attributes. As you know which computer you are trying to attack, you don’t need to guess that. As you know which domain you are trying to impersonate, that is also a given. Conventionally, there are only two variables. One variable is you need to guess which server the answer is coming from. The average domain on the Internet has around two or three name servers, only one of which will respond to any given query. Therefore you have about a one in three chance of guessing that correctly. The second variable is a unique reference number (formally, a “transaction ID”), that has about 65000 possibilities. Therefore you have about a 1 in 65000 chance of guessing that correctly.

Earlier this year, security researcher Dan Kaminsky found that it is devastatingly simple to exhaust all those possibilities in a very short amount of time by performing an attack in a certain way. How short? Well, British DNS researcher John Dickinson did some tests and found that on average he could successfully attack a server in just 1.3 seconds.

How do you fix the problem?

The sad news is there is no real solution as far as the regular DNS is concerned. It is not like a security hole in a piece of software that can be repaired with an update. This is an architectural flaw in the DNS protocol itself. There are patches for DNS software, but these only attempt to make executing an attack more difficult, they don’t solve the problem.

Some of the short term approaches to make attacks more difficult are as follows:

  1. Randomise the “source port”. One of the attributes in the packet that an attacker needs to guess is called the port number. For architectural reasons, this needs to be port 53 on the way to the server — this is how the server realises it is a DNS query as opposed to a different type of query. However, the port number that a response is sent back to doesn’t need to be port 53. By randomising this, you make it harder for an attacker to guess. Much of the software updates to this problem in mid-2008 related to adding source port randomisation.
  2. Block open recursive name service. If you provide access to a caching name server to the whole Internet, then it is very easy for the whole Internet to execute an attack against your server. If you limit access to just those who need it (i.e. your local network), then you reduce that risk.
  3. Experimentation with capitalisation of domains. Domains in practice are not case sensitive – if you type ICANN.ORG or, it means the same thing. However, inside the DNS protocol itself, the encoded transmission between computers actually is case sensitive. This property can be used to add some more randomness to transmissions. If my computer sends off a question asking about “iCaNn.OrG” and gets back an answer for “”, it can throw it away as untrustworthy. This approach is still experimental and being discussed.

The net effect of these attempts to reduce the risk of attacks primarily involve adding more randomness for the attacker to guess. These approaches approximately double the number of “bits” of randomness. To be clear though, they only make an attack harder, but an attack is still viable. Furthermore, we know that both network speeds and computer speeds get faster and faster each year. These are the two things that slow down an attacker. Therefore, we know that successful attacks will just be easier and easier into the future.

If there is no short term solution, what is the long term solution?

While the DNS itself can’t be properly fixed for the security problem, a new protocol that overlays the DNS called DNSSEC does. DNSSEC uses a system of certification to show that a DNS answer has not been modified. If someone tries to execute an attack, the certificate won’t validate, and the incorrect answer will be thrown away.

DNSSEC is difficult to deploy. It requires upgrades in DNS servers, it changes the way domain name holders manage their name servers, and it adds extra complexity. However, with the knowledge that DNS attacks are so simple to execute without it, there is growing consensus that the pain it will take to deploy is less than the pain of a DNS that you can no longer trust.

More information

You can view the presentation slides used in Cairo.


    Domain Name System
    Internationalized Domain Name ,IDN,"IDNs are domain names that include characters used in the local representation of languages that are not written with the twenty-six letters of the basic Latin alphabet ""a-z"". An IDN can contain Latin letters with diacritical marks, as required by many European languages, or may consist of characters from non-Latin scripts such as Arabic or Chinese. Many languages also use other types of digits than the European ""0-9"". The basic Latin alphabet together with the European-Arabic digits are, for the purpose of domain names, termed ""ASCII characters"" (ASCII = American Standard Code for Information Interchange). These are also included in the broader range of ""Unicode characters"" that provides the basis for IDNs. The ""hostname rule"" requires that all domain names of the type under consideration here are stored in the DNS using only the ASCII characters listed above, with the one further addition of the hyphen ""-"". The Unicode form of an IDN therefore requires special encoding before it is entered into the DNS. The following terminology is used when distinguishing between these forms: A domain name consists of a series of ""labels"" (separated by ""dots""). The ASCII form of an IDN label is termed an ""A-label"". All operations defined in the DNS protocol use A-labels exclusively. The Unicode form, which a user expects to be displayed, is termed a ""U-label"". The difference may be illustrated with the Hindi word for ""test"" — परीका — appearing here as a U-label would (in the Devanagari script). A special form of ""ASCII compatible encoding"" (abbreviated ACE) is applied to this to produce the corresponding A-label: xn--11b5bs1di. A domain name that only includes ASCII letters, digits, and hyphens is termed an ""LDH label"". Although the definitions of A-labels and LDH-labels overlap, a name consisting exclusively of LDH labels, such as"""" is not an IDN."