What is DNS, and why does it fail?

DNS is the internet’s phone book. It turns the names people type into the numeric addresses that computers use. It is invisible when it works and baffling when it does not, and a surprising share of “the internet is down” moments are really DNS problems.

How a lookup works

When you visit a site, your device asks a recursive resolver, normally run by your internet provider or a public service, for the address. If the resolver does not already know, it works down a chain. It asks one of the root servers who handles the last part of the name, such as .com. It then asks that registry who is responsible for the domain, and finally asks the domain’s own authoritative name servers for the answer. The resolver passes the answer back to you and remembers it for a while.

Caching and TTL

Every DNS answer carries a time to live, or TTL, measured in seconds. It tells resolvers how long they may reuse the answer before asking again. A short TTL means changes spread quickly and a long TTL means fewer lookups but slower changes. Caching exists at several levels: your browser, your operating system, your router and your provider’s resolver. That is why two people can see different results for the same name at the same moment, and why a fix sometimes seems to work for some people before others. See what DNS propagation really means.

The common record types

  • A and AAAA point a name at an IPv4 or IPv6 address.
  • CNAME says one name is an alias for another. Many services you use are reached this way.
  • MX says which servers handle email for the domain. Our MX record lookup shows them.
  • TXT holds free text, used for things like email authentication and ownership checks.
  • NS says which name servers are responsible for the domain.

You can inspect any of these with our DNS lookup tool.

Ways DNS fails

  • The domain does not exist, or has expired. The resolver answers “no such name”, which browsers show as NXDOMAIN. Expired domains are more common than people expect. See what happens when a domain expires.
  • The name servers are unreachable. The records may be perfect, but if every authoritative server is down or unreachable, nobody can look them up. This is the classic cause of big, sudden outages that affect everything a company runs at once.
  • A bad change. A wrong or missing record is published, such as a typo in an address or a deleted record. Everything looks healthy on the servers, but the name now leads to the wrong place or nowhere.
  • Your resolver is the problem. Your provider’s resolver may be slow, overloaded or misbehaving, so some names fail while others work.
  • Stale cache. Your device or router remembers an old answer after a site has moved. A DNS flush clears it.
  • Blocking and filtering. Some networks, parental controls and security tools refuse to resolve certain names on purpose.

Telling DNS from a down site

The quickest test is to try the site’s numeric address, if you know it, or to check the same name from somewhere else. If a site is reachable from other places but not for you, switch your device to a public resolver and try again. If the name now loads, your original resolver or a stale cache was the problem. If it still fails everywhere, the site’s own DNS is broken, and only its owner can fix that.

Our checker does this automatically. Each run reports whether the name resolved, how long the lookup took, and what answer came back. A name that resolves but then times out is a different story from a name that does not resolve at all.

For site owners

  • Use at least two name servers, ideally with different providers or networks.
  • Lower the TTL a day or two before a planned change, then raise it afterwards.
  • Turn on auto-renew for the domain, and keep contact details current so renewal warnings reach you.
  • Keep a copy of your records somewhere outside your DNS provider so you can rebuild quickly.
  • After a change, check it from several places with our DNS propagation checker rather than only from your own computer.

The short version

DNS is a chain of helpers, each remembering answers for a limited time. Most problems are one of three things: a name that has genuinely stopped existing, name servers that cannot be reached, or an old answer stuck in a cache. Working out which one it is takes a couple of minutes and saves a lot of guessing.

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Last updated 2 November 2026. Written by the siteupdown.com team. Spotted a mistake?

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