What Is A DNS Resolver?

Date First Published: 1st June 2022

Topic: Computer Networking

Subtopic: Network Services

Article Type: Computer Terms & Definitions

Difficulty Level: 8/10

Learn more about what a DNS resolver is in this article.

A DNS resolver is a server that handles requests and translates domain name to their corresponding IP address. For example, when visiting a website by its domain name, such as computerhelp4all.com, your web browser contacts the DNS resolver to get the IP address of that domain name. DNS resolvers may be maintained by ISPs, mobile carriers, or other third parties.

DNS Resolution Process

DNS resolvers work by receiving the queries made by web browsers and other applications, receiving a domain name, and then finding the IP address of that domain name. The queries can be recursive, non-recursive, or iterative.

Note:

At first, the resolver starts by taking a look in its local cache or the cache stored on the user’s operating system. Domain names are resolved straight away if they can be found in the cache. The period of time that this cached information is kept before it expires depends on the operator of the DNS resolver and the settings of the user’s computer.

The DNS resolution process is:

1. The resolver receives the query made by the web browser, passes the query to the server that handles that TLD, and provides details for the root TLD nameserver, all of which handle a specific TLD, such as .com, .org, .net, .uk. There are 13 DNS root servers around, maintained by organisations, such as Cogent, VeriSign, the University of Maryland, and the US Army Research Lab.
2. The TLD nameserver contains the nameservers for a specific domain name. The query is then forwarded to the server associated with that domain name.
3. The authoritative nameserver index takes the server IP address information and then returns the correct IP address to the DNS resolver.

Types Of DNS Resolvers

Recursive Resolver

A recursive resolver, also known as a DNS recursor acts as an intermediary between the client and the DNS nameserver. After it has received a DNS query from a client, it will send a request to a root nameserver or respond with cached data.

Afterwards, it will make another request to a TLD server and then a final one to an authoritative nameserver. The recursive resolver sends a response to the client after a response has been received from the authoritative nameserver.

DNS Root Nameservers

Root nameservers are responsible for the functionality of DNS and are the first step in the DNS resolution of a domain name. The root zone is signed using DNSSEC and the root zone file contains resource records for the authoritative servers of all the TLDs.

There are 13 DNS root nameservers around. Root nameservers accept queries that come from a recursive resolver and then respond by directing the recursive resolver to a TLD nameserver. These TLD nameservers handle a specific TLD, such as .com, .org, .net, .uk. ICANN operates the root nameservers for one of the 13 root nameservers in the root zone. The other 12 root nameservers are maintained by other organisations, such as VeriSign, the University of Maryland, Netnod, and WIDE Project.

A list of DNS root nameservers are:

TLD Nameservers

TLD nameservers contain information about domain name extensions, such as .com, .net, and .org. For example, a .net nameserver will contain information about each domain name that ends in .net. If ‘computerhelp4all.com‘ was typed in a web browser, after receiving the response from the root nameservers, a query would be sent by the recursive resolver to the TLD nameserver. It would respond by pointing it to the authoritative nameserver, which contains information about that domain name, such as its IP address.

Authoritative Nameservers

Authoritative nameservers are the last step in DNS resolution. Every time a recursive nameserver receives a response from a TLD nameserver, it will then be directed to the authoritative nameserver, which contains information about that domain name, such as the IP address. It can provide the recursive resolver with the IP address, found in a DNS A record, which is used for mapping a domain name to an IPv4 address. If the correct IP address is returned to the DNS resolver, the DNS lookup is successful.