A Domain Name System (DNS) search is the process involved in obtaining an Internet Protocol (IP) address when a DNS resolution request is made. It can also be the steps taken to resolve an IP address when a reverse DNS lookup is requested. The DNS system is an interconnected network of computer servers arranged in a hierarchy of domains and subdomains. Depending on the nature of the DNS resolution and what DNS information is being cached by DNS servers, a DNS search may travel laterally across the DNS system or be forwarded to ascendant or root servers. The answer to a resolution request will ultimately be returned to the computer or network device which initiated the DNS search.
Network devices, including computers, usually have a local file called the hosts file, which will be looked to first when a DNS search is initiated. This file is held in permanent storage and contains static entries of IP addresses associated with machine or domain names. If the sought name or IP address is not found in this file, a network device will contact the local DNS server, which is statically or dynamically configured into its network adapter configuration file. This is typically a DNS server which belongs to an Internet Service Provider (ISP) or a company network.
If the local DNS server can resolve the DNS resolution request, it will send a DNS resolution response to the network device which initiated the DNS search. Depending on the information cached by a local DNS server, it may forward the request. The local DNS server will eventually receive a response back, cache the information received, and respond to the initiator of the DNS search with the information requested. A lateral request is usually initiated when the local server knows how to contact the DNS server responsible for a requested domain but does not know how to reach a subdomain or particular machine within a domain.
When a local DNS server does not have any information, it will send a request upward through the DNS hierarchy toward a root DNS server. Ascendant DNS servers in the hierarchy typically behave in the same manner as local DNS servers. They will respond or forward requests and cache information as necessary. If a sought domain or machine within a domain does not exist or cannot be contacted, the DNS server responsible for this information will send a reply backward stating that the DNS search could not resolve the resolution request.
DNS searches are often used with network tools, email tools, during a web search, and for network security. A DNS search is an integral part of the concept of user friendliness, helping to ensure that people can easily use the Internet by utilizing names for resources instead of unintelligible IP addresses. DNS searching works in the background, so it also helps ensure ease of use by hiding the technical details of networking.