This article is about the Domain Name System (DNS) and explains how the “Internet’s telephone book” works. Alongside the fundamental details of name resolution on the Internet, we also look at special topics like administering DNS records in dynamic environments and debugging DNS setups during operation. We look at it within the technical framework of Univention Corporate Server, because it contains not only a dyed-in-the-wool DNS server, but also diverse tools to make managing DNS records significantly easier.
DNS: A basic introduction
The Domain Name System, DNS for short, is an absolutely elementary component of the Internet. While many users and most admins have some idea of the terms “DNS server” or “name server”, the actual structure of the DNS system and its basic functionality is often unclear. Therefore, it is not a bad idea to look at the central aspects of the topic DNS, before looking as specific aspects of DNS in detail.
Numbers and figures
The phenomenon given the simple name “the Internet” is actually a global network of very many individual computers. Naturally, direct connections do not exist from each computer to every other computer. Instead, the computers comprising the Internet are partitioned into individual network segments. Within their network segments, the systems communicate directly with one another.
If a server from one segment wants to connect to a server from a different segment, it must use a router. But how does a server know how to reach another local server, or another server in a different segment of the network?
Here is where IP addresses come into play. An IP address is a sequence of numbers in 32 bits (for IPv4) or 128 bits (for IPv6) “IP” stands for “Internet Protocol”, which highlights this principle’s general validity. The IP address of the computer containing this blog, for example, is
220.127.116.11 (IPv4) and
2001:8d8:100f:f000::219 (IPv6) (see Fig. 1).
You can see that when a reader wants to access this blog via its IP address, they need to remember a complicated sequence of numbers. This is difficult for IPv4, but for IPv6 it is practically impossible – and this IPv6 address is not as complicated as it theoretically could be! Just imagine that you had to remember all the sequences of numbers just for the websites one uses regularly. It’s impossible!
The founding fathers of the Internet already realized that this would be a problem. They thought up a system which translated the complicated IP addresses into understandable texts (“hosts” or “host names”). For example, the IP addresses named above translate to blog.univention.de. And this can be remembered much easier than the actual IP addresses.
However, this system brings up a new question: how does the web browser know that it needs the server with IP address
18.104.22.168 when the user types https://blog.univention.de/ into the address bar? This is where the Domain Name System servers, DNS servers for short, come into play, because they function like a sort of telephone book and know which IP addresses belong to which names. It works the other way too, of course: individual IP addresses can have names defined in text form.