DNS Query Process

A DNS query is the process of a computer or networking device making an inquiry to get an IP address for a DNS name such as w3.org

The client computer will send a DNS query to one of their internet service provider's DNS servers. The DNS server looks in it's DNS database to tell whether it can answer the query authoritatively. If the DNS server can answer authoritatively, the DNS server answers the query and the DNS query process is complete.

If the server cannot answer the query authoritatively it will look in its DNS cache of previous queries. If the DNS server finds a matching entry in its cache, it will answer the query with a non-authoritative answer based on the information in its cache and the DNS query process is complete.

If the ISP DNS server did not have the DNS information in its DNS database or its DNS cache the DNS query process will use recursion to complete the DNS query. The ISP DNS server will use its root hints file to find information to contact other DNS servers. The root hints file specified DNS servers that are authoritative for the DNS domain root and top level domains in the DNS system. This includes the .com, .org, .net, .gov and other domain types. If the query is for www.w3.org the ISP DNS server would contact an authorititative server for the top level "org" domain and send an iterative query to the org DNS server asking for information about the authoritative server for w3.org. The org domain DNS server responds with the the nameserver information including IP address of the nameserver for w3.org. Then the ISP DNS server sends a query to the w3.org DNS server asking for the IP address of www.w3c.org. The w3.org DNS server sends an authoritative answer back to the ISP DNS server which is cached in the ISP DNS server cache and also sent to the client computer.

If another client computer later does a request for information about www.w3.org the ISP DNS server has the information in its cache and will not neet to ask other DNS servers for additional information.