DNS resolver

A DNS resolver is software or a service that looks up DNS information in response to a request from an app or device. A recursive resolver usually checks its temporarily saved answers (its cache) first and, if it has no usable answer, queries other DNS servers until it can return an answer or an error.

What happens during a lookup?

When Alex's browser needs the address for example.com, the device's local lookup software, called a stub resolver, sends a query to a recursive resolver, often provided by an internet provider, workplace, or public DNS service. If the answer is not already cached, the recursive resolver can ask DNS servers in sequence, starting with the root at the top of DNS and following directions toward the servers that publish the domain's official records. It then returns the result and may cache it for the time allowed by DNS.

The recursive resolver is different from an authoritative name server. A recursive resolver finds answers on behalf of clients; an authoritative server publishes records for the parts of DNS, called zones, that it manages. One system can perform both roles, but they are distinct functions.

Resolvers can apply filtering or other policies, so two resolvers may return different results for the same name. A result can also depend on cached data, DNSSEC validation, and the network used to reach the resolver.

Related terms

• DNS — system being queried
• DNS cache — stores previous answers
• Nameserver — serves authoritative or other DNS responses
• DNSSEC — resolver validation of signed DNS data