Hacking the Infrastructure – How DNS works – Part 2

Welcome back. In part 1, I discussed the technical details of how DNS works. In this part, I’ll introduce you to some of the more common DNS server packages. In a future post I will cover some of the common problems with DNS as well as proposed solutions. So let’s dive right in.

The most popular DNS server, by far, is BIND, the Berkley Internet Name Domain. BIND has long and storied past. On the one hand, it’s one of the oldest packages for serving DNS, dating back to the early 1980’s, and on the other, it has a reputation for being one of the most insecure. BIND started out as a graduate student project at the University of California at Berkley, and was maintained by the Computer Systems Research Group. In the late 1980’s, the Digital Equipment Corporation helped with development. Shortly after that, Paul Vixie became the primary developer and eventually formed the Internet Systems Consortium which maintains BIND to this day.

Being the most popular DNS software out there, BIND suffers from the same malady that affects Microsoft Windows. It’s the most popular, most widely installed, and, as a result, hackers can gain the most by breaking it. In short, it’s the most targeted of DNS server softwares. Unlike Windows, however, BIND is open source and should benefit from the extra scrutiny that usually entails, but, alas, it appears that BIND is pretty tightly controlled by the ISC. From the ISC site, I do not see any publicly accessible software repository, no open discussion of code changes, and nothing else that really marks a truly open source application. The only open-source bits I see are a users mailing list and source code downloads. Beyond that, it appears that you either need to be a member of the “Bind Forum,” or wait for new releases with little or no input.

Not being an active user of BIND, I cannot comment too much on the current state of BIND other than what I can find publicly available. I do know that BIND supports just about every DNS convention there is out there. That includes standard DNS, DNSSEC, TSIG, and IPv6. The latter three of these are relatively new. In fact, the current major version of BIND, version 9, was written from the ground up specifically for DNSSEC support.

In late 1999, Daniel J. Bernstein, a professor at the University of Illinois, wrote a suite of DNS tools known as djbdns. Bernstein is a mathematician, cryptographer, and a security expert. He used all of these skills to produce a complete DNS server that he claimed had no security holes in it. He went as far as offering a security guarantee, promising to pay $1000 to the first person to identify a verifiable security hole in djbdns. To date, no one has been able to claim that money. As recently as 2004, djbdns was the second most popular DNS server software.

The primary reason for the existence of djbdns is Bernstein’s dissatisfaction with BIND and the numerous security problems therein. Having both security and simplicity in mind, Bernstein was able to make djbdns extremely stable and secure. In fact, djbdns was unaffected by the recent Kaminsky vulnerability, which affected both BIND and Microsoft DNS. Additionally, configuration and maintenance are both simple, straightforward processes.

On the other hand, the simplicity of djbdns may become its eventual downfall. Bernstein is critical of both DNSSEC and IPv6 and has offered no support for either of these. While some semblance of IPv6 support was added via a patch provided by a third party, I am unaware of any third-party DNSSEC support. Let me be clear, however, while the IPv6 patch does add additional support for IPv6, djbdns itself can already handle serving the AAAA records required for IPv6. The difference is that djbdns only talks over IPv4 transport while the patch adds support for IPv6 transport.

Currently, it is unclear at to whether Bernstein will ever release a new version of djbdns with support for any type of “secure” DNS.

The Microsoft DNS server has existed since Windows NT 3.51 was shipped back in 1995. It was included as part of the Microsoft BackOffice, a collection of software intended for use by small businesses. As of 2004, it was the third most popular DNS server software. According to Wikipedia, Microsoft DNS is based on BIND 4.3 with, of course, lots of Microsoft extensions. Microsoft DNS has become more and more important with new releases of Windows Server. Microsoft’s Active Directory relies heavily on Microsoft DNS and the dynamic DNS capabilities included. Active Directory uses a number of special DNS entries to identify services and allow machines to locate them. It’s an acceptable use of DNS, to be sure, but really makes things quite messy and somewhat difficult to understand.

I used Microsoft DNS for a period of time after Windows 2000 was released. At the time, I was managing a small dial-up network and we used Active
Directory and Steel-Belted RADIUS for authentication. Active Directory integration allowed us to easily synchronize data between the two sites we had, or so I thought. Because we were using Active Directory, the easiest thing to do was to use Microsoft DNS for our domain data and as a cache for customers. As we found out, however, Microsoft DNS suffered from some sort of cache problem that caused it to stop answering DNS queries after a while. We suffered with that problem for a short period of time and eventually switched over to djbdns.

There are a number of other DNS servers out there, both good and bad. I have no experience with any of them other than to know some of them by reputation. Depending on what happens in the future with the security of DNS, however, I predict that a lot of the smaller DNS packages will fall by the wayside. And while I have no practical experience with BIND beyond using it as a simple caching nameserver, I can only wonder why such a package claiming to be open source, but so guarded as it is, maintains its dominance. Perhaps I’m mistaken, but thus far I have found nothing that contradicts my current beliefs.

Next time we’ll discuss some of the more prevalent problems with DNS and DNS security. This will lead into a discussion of DNSSEC and how it works (or, perhaps, doesn’t work) and possible alternatives to DNSSEC. If you have questions and/or comments, please feel free to leave them in the comment section.

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