What is DNS?

We explain what DNS is, how it works and how outages can be avoided

Domain Name System (DNS) is a hierarchical naming system that's applied to every entity connecting to the internet or a private network, such as a device or service.

The technology acts as a mediator between the user and the web browser, as the former typically works with memorable domain name templates while the latter uses IP addresses to communicate with other services across the internet. In this sense, the DNS removes the need for users to remember unique IP addresses.

Often referred to as the "phonebook of the internet", DNS allows users to stick to site addresses such as www.itpro.co.uk, instead of a string of ordered numbers, punctuated by dots in the case of IPv4, or colons in the case of IPv6.

How does DNS work?

DNS is deceptively simple on the surface, as it hides a bunch of complex processes that can perhaps be best described as a conversation between different bits of hardware.

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The process begins when a user enters a website into their browser url, which prompts their computer to issue a query over the internet to a target server referred to as a recursive resolver - this is effectively the computer asking what the IP address is of the search query. The server then issues further queries across the internet to attempt to resolve the request.

The recursive resolver first issues a query to what's known as the root server, which acts like an index. From here, the root server is able to direct the recursive resolver to the correct top level domain (TLD) - a server that hosts the last section of the url, such as .com, .co.uk or .fr.

From here, the recursive resolver will then be directed to the authoritative nameserver, which is able to provide the final piece of the puzzle by matching the whole url, i.e. www.itpro.co.uk, with its IP address, provided it has access to it, which is then returned to the original web browser.

This whole process, while quite complex, only takes a matter of seconds to complete.

What is a DNS server?

DNS servers are the infrastructure that makes up the domain name system. As listed above, they are the recursive resolver, root server, the TLD name server and the authoritative server (also known as a domain name server).

There are 13 DNS root servers spread across the world that every recursive resolver knows how to contact. These are overseen by the nonprofit known as the Internet Corporation for Assigned Names and Numbers (ICANN) and decide which TLD name server the recursive resolver should contact based on the TLD of the URL.

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The TLD name server, which is managed by a branch of ICANN known as the Internet Assigned Numbers Authority (IANA), will be one of two types. Either it has information on addresses that end in a generic TLD, such as .com, .org or .net, or it has information on addresses that end in country code TLDs, such as .cn, .za or .uk

Finally, the domain name server/authoritative name server has specific information on the domain name it serves, which is how it resolves the final piece of the DNS query puzzle.

What happens when DNS fails?

DNS failure is, unfortunately, a common occurrence and can be a temporary issue when a domain is transferred to a new hosting provider, for example, or can be caused by a more worrying incident such as a cyber attack or another break in the network where it fails to resolve.

The reason you will suffer from what appears to be an outage is usually the same though - the domain name doesn't match with the IP address or the match between the two can't be found.

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Although this makes a DNS failure sound pretty straightforward and thus easy to solve, this may not be the case. It can also have a pretty severe impact on a business. For example, for any organisation hosting apps or services on the internet, a DNS failure can have significant productivity and financial impact, making the service unavailable to customers.

Although there are ways to fix a DNS failure, it's vital you have some kind of DNS failover implemented so if the DNS des suffer an outage, it can easily be switched over to another DNS server so the end user won't even know there's a problem.

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Another option for keeping your systems and services up and running is to install some kind of DNS monitoring to make sure if there is a problem, you know quickly enough to fix the issue before (hopefully) your customers notice there's a problem.

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