What is Recursive DNS?

A recursive DNS server is a caching-only DNS (domain name system) server. It is not authoritative for any domain, it obtains all of the DNS information from other DNS servers which are authoritative for the qualified name (composed of multiple domains) that was queried by the client. In the case of website name requests, the browser extracts the name of the server (FQDN) from the URL and asks the DNS for its IP address to be able to connect to it. The recursive DNS server gathers information on records obtained from authoritative DNS servers with regards to the associated IP address. The answers to queries are stored in the cache for other similar requests.

These caching-only DNS servers are required for supporting the authoritative DNS servers to handle the workload created by the enormous number of user requests. By distributing this load, recursive DNS servers make website name requests supportable, as they reduce outgoing DNS traffic and speed up name resolution.

Recursive servers are the very first servers contacted as the result of a request. If the contacted server already has a cached response for the same query (e.g. IP address) from an authoritative server, with a valid DNS TTL (time to live), it is answered directly from the cached data. But if the recursive server does not have the IP address, it starts its check through the hierarchy of authoritative DNS servers. On the other hand, Authoritative DNS servers have their top records (NS) supplied by the internet registrars where website owners have set up their site names.

An example of a caching only name server is your ISP DNS server. For example,  if an end user is trying to locate www.one.org via their web browser, this query will be sent to the ISP DNS server.  Since this is a caching only server, if the answer is not in its cache database with a valid TTL, it will then start the referral process to the Root (.), Top Level Domains (.org) and finally the authoritative DNS server for one.org for the specific record (www). Once an answer is obtained it will be cached in the ISP database and sent back to the end user. At ISP, the cache hit ratio is most of the time between 85% and 95%.

Related Terminology

What is FQDN?
What is DNS TTL?
What is Top Level Domain?
What is DNS & how does it work?