Intro to DNS and DNS Query Message Format

Introduction to DNS

If you ever wondered where DNS came from, this is your chance to find out ! The quick summary on DNS's history will also help you understand why DNS servers are run mostly on Linux and Unix-type systems. We then get to see the layers of the OSI Model on which DNS works and, towards the end of the page, you will find out how the Domains (and DNS servers) are structured on the Internet to ensure uptime and effectiveness.

The History

DNS began in the early days when the Internet was only a small network created by the Department of Defence for research purposes. Host names (simple computer names) of computers were manually entered into a file (called HOSTS) which was located on a central server. Each site/computer that needed to resolve host names had to download this file. But as the number of hosts grew, so did the HOSTS file (Linux, Unix, Windows and NetWare still use such files) until it was far too large for computers to download and it was generating great amounts of traffic ! So they thought ... Stuff this .. let's find a better solution ... and in 1984 the Domain Name System was introduced.

 

The Protocol

The Domain Name System is a 'hierarchically distributed database', which is a fancy way of saying that its layers are arranged in a definite order and that its data is distributed across a wide range of machines (just like the roots of a tree branch out from the main root).

Most companies today have their own little DNS server to ensure the computers can find each other without problems. If you're using Windows 2000 and Active Directory, then you surely are using DNS for the name resolutions of your computers. Microsoft has created its own version of a "DNS" server, called a WINS server, which stands for Windows Internet Name Service, but this is old technology and uses protocols that are nowhere near as efficient as DNS, so it was natural for Microsoft to move away from WINS and towards DNS, after all, the whole Internet works on DNS :)

The DNS protocol works when your computer sends out a DNS query to a name server to resolve a domain. For example, you type "www.firewall.cx" in your web browser, this triggers a DNS request, which your computer sends to a DNS server in order to get the website's IP Address ! There is a detailed example on the pages to follow so I won't get into too much detail for the moment.

The DNS protocol normally uses the UDP protocol as a means of transport because of its small overhead in comparison to TCP; the less overhead a protocol has, the faster it is !

In the case where there are constant errors and the computer trying to request a DNS resolution can't get an error free answer, or any answer at all, it will switch to TCP to ensure the data arrives without errors.

This process, though, depends on the operating system you're using. Some operating systems might not allow DNS to use the TCP protocol, thus limiting it to UDP only. It is rare that you will get so many errors that you can't resolve any hostname or domain name to an IP Address.

The DNS protocol utilises Port 53 for its service. This means that a DNS server listens on Port 53 and expects any client wishing to use the service to use the same port. There are, however, cases where you might need to use a different port, something possible depending on the operating system and DNS server you are running.

In the following pages we'll be looking at the actual DNS packet format, where you are able to see exactly the contents of DNS query, so we won't analyse the packet structure here.

Next we'll take a close look at how the Internet domains and DNS servers are structured to make sure the model works flawlessly and efficiently !

 

The Internet Domain Name Server Hierarchy

This interesting section will help you understand how domain names on the Internet are structured and where DNS servers fit in to the picture. When you think about the millions of domain names registered today, you probably think that you have to be superhuman to manage such a structure of DNS servers !

Well that's not that case. The DNS structure has been designed in such a way that no DNS server needs to know about all possible domains, but only those immediately above and below it.

The picture below shows part of the Internet DNS hierarchical structure:

 

Let's explain how it works :

Internic controls the "root" domain, which includes all the top level domains. These are marked in a green oval for clarity. Within the green oval you have the ROOT DNS servers, which know all about the authoritative DNS servers for the domains immediately below them e.g firewall.cx, cisco.com, microsoft.com etc. These ROOT DNS servers can tell you which DNS server takes care of firewall.cx, cisco.com, microsoft.com and the rest.

Each domain, including the ones we are talking about (cisco, firewall, microsoft), have what we call a "Primary DNS" and "Secondary DNS". The Primary DNS is the one that holds all the information about its domain. The Secondary acts as a backup in case the Primary DNS fails. The process in which a Primary DNS server sends its copy to the Secondary DNS server is called Zone Transfer and is covered in the DNS Database section.

Today there are hundreds of websites at which you are able to register your own domain and, once you've done that, you have the power to manage it yourself. In the example above, Cisco bought the "Cisco.com" domain and then created your resource records. Some examples of resource records for the Cisco domain in our example are: support , www and routers. These will be analysed in depth on the next pages.

So here comes the million dollar question :)

How do you create subdomains and www's (known as resouce records) ?

The answer is pretty simple:

You use a special DNS administration interface (usually web based - provided by the guys with whom you registered your domain) that allows you to create, change and delete the subdomains, www's or whatever resource record you can come up with. When you're making changes to the DNS settings of your domain, you're actually changing the contents of specific files that are located on that server.

These changes then slowly propagate to the authoritative DNS servers, which are responsible for your domain area and then the whole Internet will contact these DNS servers when they need to access any section of your domain.

For example, if you need to resolve ftp.firewall.cx, your computer will locate and contact the DNS Server responsible for the .CX domains, which will let you know the DNS server that's in charge of the Firewall.cx domain. The DNS server of Firewall.cx in turn will let your computer know the IP Address of ftp.firewall.cx because it holds all the information for the firewall.cx domain.

That completes our first section. It's not that hard after all!

Introduction This section will deal with the analysis of the DNS packets. This will allow us to see the way DNS messages are formatted and the options and variables they contain. To understand a protocol, you must understand the information the protocol carries from one host to another. Because the DNS message format can vary, depending on the query and the answer, I've broken this analysis into two parts. Part 1 analyses the DNS format of a query, in other words, it shows how the packet looks when we ask a DNS server to resolve a domain. Part 2 analyses the DNS format of an answer, where the DNS server is responding to our query. I find this method more informative and easy to understand rather than combining the analysis of queries and answers. DNS Analysis - Host Query As mentioned in the previous sections of the DNS Protocol, a DNS query is generated when the client needs to resolve a domain name into an IP Address. This could be the result of entering "www.firewall.cx" in the url field of your web browser, or simply by launching a program that uses the Internet and therefore generates DNS queries in order to successfully communicate with the host or server it needs. Now, I've also included a live example (using my packet analyser), so you can compare theory with practice for a better understanding. After this we will have a look at the meaning of each field in the packet, so let's check out what a packet containing a DNS query would look like on our network: This is the captured packet we are going to deal with. To generate this packet, I typed "ping www.firewall.cx" from my linux prompt. The command generated this packet, which was put on my network with the destination being a name server in Australia. Notice the Port Destination which is set to 53, on which the port DNS works, and the protocol used for the DNS Query, which is UDP.          Ethernet II (Check Ethernet Frames section for more info) is the most common type of frame found on LANs, in fact it probably is the only type you will find on 95% of all networks if you're only running TCP/IP and Windows or Unix-like machines. This particular one contains a DNS section, which could be either a Query or Response. We are assuming a Query, so it can fit nicely in our example. We are going to take the DNS Section above and analyse its contents, which are already shown in the picture above (Right hand side, labeled "Capture") taken from my packet analyser. Here they are again in a cool 3D diagram: From this whole packet, the DNS Query Section is the part we're interested in (analysed shortly), the rest is more or less overhead and information to let the server know a bit more information about our query. The analysis of each 3D block (field) is shown in the left picture below so you can understand the function of each field and the DNS Query Section captured by my packet sniffer on the right:          All fields in the DNS Query section except the DNS Name field (underlined in red in the picture above), have set lengths. The DNS Name field has no set length because it varies depending on the domain name length as we are going to see soon. For example, a query for www.cisco.com will require DNS Name field to be smaller than a query for support.novell.com simply because the second domain is longer.   The DNS Name Field To prove this I captured a few packets that show different lengths for the domain names I just mentioned but, because the DNS section in a packet provides no length field, we need to look one level above, which is the UDP header, in order to calculate the DNS section length. By subtracting the UDP header length (always 8 bytes - check the UDP article for more information) from the bytes in the Length field, we are left with the length of the DNS section:       The two examples clearly show that the Length Field in the UDP header varies depending on the domain we are trying to resolve. The UDP header is 8 bytes in both examples and all fields in the DNS Section, except for the DNS Name field, are always 2 bytes.   The Flags/Parameters Field The Parameter Field (labeled Flags) is one of the most important fields in DNS because it is responsible for letting the server or client know a lot of important information about the DNS packet. For example, it contains information as to whether the DNS packet is a query or response and, in the case of a query, if it should be a recursive or non-recursive type. This is most important because as we've already seen, it determines how the query is handled by the server. Let's have a closer look at the flags and explain the meaning of each one. I've marked the bit numbers with black on the left hand side of each flag parameter so you can see which ones are used during a response. The picture on the right hand side explains the various bits. You won't see all 16 bits used in a query as the rest are used during a response or might be reserved:     As you can see, only bits 1, 2-5, 7, 8 and 12 are used in this query. The rest will be a combination of reserved bits and bits that are used only in responses. When you read the DNS response message format page, you will find a similar packet captured which is a reponse to the above query and the rest of the bits used are analysed. And that just about does it for the DNS Query message format page. Next up is the DNS Response message format page which I'm sure you will find just as interesting!