Glossary · ENCheck a domain

DNS

The Domain Name System (DNS) is a system spread across many servers that publishes information about domain names. It lets a browser look up network addresses for a name such as example.com, and it can also provide information used by email and other internet services.

Why does the internet use DNS?

People can remember names more easily than numeric IP addresses. When someone opens https://example.com, the browser needs address information to send its web request to the right service. DNS lets the site owner publish that information under a name people can use. DNS is not the website itself: it does not contain the site’s pages or images.

DNS also stores different kinds of information, called DNS records. For example, one record can give address information for a website, while another can name the servers that receive email for the domain. Alex can keep her website at one company and email at another by publishing the appropriate information for each service. The domain name is the shared name; DNS records direct different kinds of requests to the services that handle them.

How does a DNS lookup work?

When Alex types example.com into her browser, the computer first checks whether it already has a recent answer saved. If it does not, it asks a service that looks up DNS answers, called a recursive resolver, usually provided by her internet provider, workplace, or a public DNS service. The resolver looks for the requested record and returns an answer or an error.

If the resolver has no answer saved in its temporary store, or cache, it can follow directions from servers at successive levels of DNS. A root name server points it toward the server responsible for .com. The .com server points it toward the authoritative name servers for example.com. Those servers hold the domain’s published DNS information and can answer for it. The resolver returns the answer to Alex’s computer, and the browser can then make its web request. Many lookups skip some of these steps because the resolver already has useful information cached.

The resolver and the authoritative name server have different jobs. The resolver searches for an answer on behalf of Alex’s computer and may save it for later. The authoritative server publishes the current DNS information for a particular part of the name system. A company may provide both services, but a resolver used by a visitor is not necessarily the server that publishes the website owner’s records.

What can DNS records tell a service?

An address record can associate a name with an IPv4 or IPv6 address. An MX record tells sending mail systems which mail servers accept email for a domain. Other record types carry different information, such as aliases or text used by a service. A record answers a particular kind of question; having an address record does not mean the domain also has working email, and an MX record does not create a mailbox.

For example, Alex may publish website address information for example.com and separate mail-routing information for example.com. Her browser uses the website information to reach the site. A sending mail service checks the mail information to find where to deliver a message to alex@example.com. Both lookups use DNS, but they ask for different records and may lead to different providers.

Why can old DNS information keep appearing?

Resolvers can save DNS answers temporarily to avoid repeating the same lookup. Each record has a time-to-live (TTL), which tells a resolver how long it may keep that answer in its cache. If Alex changes the address for her website, a resolver that still has the earlier answer may use it until the cached copy expires. Other resolvers may already have the new answer. This is why a DNS change can appear at different times for different visitors; it is not one update traveling through every device in order.

How is DNS different from a domain name?

A domain name, such as example.com, is a name in the DNS hierarchy. DNS is the system and protocol that lets computers request information associated with names. Registering a domain does not automatically publish the records needed for a website or email; those records must be supplied by a DNS service and point to services that can handle the requests.

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