Domains

The domain name system or domain name server (DNS) is a system that stores, domain name system (domain name server) associates many types of information with domain names, but most importantly, it translates the domain name (computer hostnames) to IP addresses. It also lists mail exchange servers accepting e-mail for each domain. In providing a worldwide keyword-based redirection service, DNS is an essential part of contemporary Internet use.

DNS is useful for several reasons. Most well known, the DNS makes it possible to attach easy-to-remember domain names to hard-to-remember IP addresses. People take advantage of this when they recite URLs and e-mail addresses. Less recognized, the domain name system makes it possible for people to assign authoritative names, without needing to talk with a central registrar each time.

A brief history of the DNS

The practice of using a name as a more human-legible abstraction of a machine's numerical address on the network predates even TCP/IP, all the way back to the ARPAnet era. Originally, each computer on the network retrieved a file called HOSTS.TXT from SRI which mapped an address to a name. The Hosts file still exists on most modern operating systems either by default or through configuration and allows users to specify an IP Address to use for a hostname without checking the DNS. This file is now used primarily for troubleshooting DNS errors or mapping local addresses to more organic names (the Hosts file can also be used for ad blocking, or it can be used by spyware to hijack a computer). Such a system had inherent limitations, because of the obvious requirement that every time a given computer's address changed, every computer that wanted to talk with it would need an update to its Hosts file.

The growth of networking called for a more scalable system: one which recorded a change in a host's address in one place only. Other hosts would learn about the change dynamically through a notification system, therefore finishing a globally accessible network of all hosts' names and their associated IP Addresses. Enter the DNS.

DNS in the real world

DNS resolving from program to OS resolver to ISP resolver to greater system.

Users generally do not talk directly with a DNS resolver. Instead DNS resolution is handled transparently by way of client applications such as web browsers (Internet Explorer, Mozilla Firefox, Safari, Opera, and so on), mail clients (Outlook Express, Mozilla Thunderbird, and so forth), and other Internet applications. When a request is made which requires a DNS lookup, such programs send a resolution request to the local DNS resolver in the operating system which in turn handles the communications needed.

The DNS resolver will almost invariably have a cache (see above) containing recent lookups. If the cache can provide the answer to the request, the resolver will return the value in the cache to the program that made the request. If the cache does not contain the answer, the resolver will send the request to a named DNS server or servers. For most home users, the Internet service provider to which the machine connects will usually give this DNS server: such a user will either configure that server's address manually or allow DHCP to set it; still, where systems administrators have configured systems to use their own DNS servers, their DNS resolvers will generally point to their own nameservers. This name server will then follow the process outlined above in DNS in theory, until it either successfully finds a result, or does not. It then returns its results to the DNS resolver; assuming it has found a result, the resolver caches that result for future use, and hands the result back to the software which began the request.

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