What is DNS?

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

DNS, or Domain Name System to give it its full name, is the technology that allows us to access websites quickly and easily.

Each website (and indeed each computer or other connected device) has its own IP address a string of seemingly random numbers punctuated by dots in the case of IPv4 or colons in the case of IPv6. Committing these addresses to memory isn't exactly practical, however, which is where DNS comes in.

Sometimes described as the "phonebook of the internet", DNS enables users to instead type in a website address (www.itpro.co.uk for example) and have the appropriate page appear in a matter of seconds.

DNS is also a comprehensive tool: Within a DNS database different types of record may be stored in addition to IP addresses, including SMTP mail exchangers, name servers, and domain name aliases among others.

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It can also scale without affecting latency, meaning it can keep pace with the ever expanding internet.

How does DNS work?

DNS is, in effect, a conversation between different pieces of hardware carried out in several parts.

When the user enters a website address into their browser, their computer sends out a query effectively "what is the IP address of www.itpro.co.uk?" over the internet to a server known as a recursive resolver. It's this server that will contact others within the internet to solve the initial query.

The recursive resolver then contacts the first in a series of DNS servers the root server. These servers hold DNS information on top level domains (TLDs), such as .com, .fr, or in our case, .co.uk.

The root server then directs the recursive resolver to a TLD name server, which stores address information for second level domains within the given TLD. For us, this is itpro.co.uk.

The TLD name server provides the recursive resolver with the IP address for the domain name server of the website the user wants to reach. This server tells the recursive resolver with the full IP address of the full domain (www.itpro.co.uk), which is then returned to the user's browser as the answer to its original query, allowing the site to appear.

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While this may seem like a very laborious process, in most cases it will take just 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 domain name server (also known as an authoritative 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.

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

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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.

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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.

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.

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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