The Domain Name System (DNS) is the distributed, hierarchical system that lets people and applications use domain names to request information associated with those names, including IP addresses. Rather than being a single directory or a simple one-step name-to-number converter, DNS uses resource records held across name servers; resolvers find the requested information by consulting caches and, when needed, following referrals through the hierarchy.
What does DNS stand for?
DNS stands for Domain Name System. It is the system of names, data, name servers, and resolver queries that makes information about internet domain names available to users and applications. People often use DNS to find the IP address associated with a host name, but DNS can also provide other kinds of information, such as mail-related records.
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A domain name is a readable name in the DNS namespace; DNS is the system that organizes names and answers queries about them. A domain registrar or DNS hosting provider may help someone register or manage a name, but those commercial services are not DNS itself.
How is the DNS name space organized?
DNS names are arranged as a tree, with the root at the top and successive labels below it. For example, in whois.icann.org, org is the top-level domain, icann is the second-level label, and whois is the third-level label, as ICANN explains.
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The information is distributed rather than stored in one global file. DNS data is grouped into zones, and name servers hold information for particular parts of the namespace. A resource record associates data with a name; the type of query determines what information is requested. The foundational descriptions of this structure and query model appear in RFC 1034 and RFC 1035.
What happens during a DNS lookup?
When an application needs information for a domain name, a resolver obtains the answer on its behalf. If it does not already have a usable cached answer, it can query DNS servers and follow referrals toward servers that are authoritative for the relevant zone. A simplified lookup for a host address proceeds through the hierarchy:
- Start with the root. The resolver asks a root server where to find information for the requested top-level domain. Root service directs the query toward the appropriate TLD; see ICANN’s explanation of the root.
- Ask the TLD servers. The resolver follows the referral to a server for the top-level domain, such as
org, and obtains information pointing toward the domain’s authoritative name servers. - Ask an authoritative server. The resolver queries a server authoritative for the domain’s zone for the requested record, such as a host address.
- Return and potentially cache the answer. The resolver gives the result to the requesting application and may cache it for a period, allowing later requests to be answered more quickly. ICANN describes this resolver role and caching in its resolver glossary entry.
This is a simplified account, not a guarantee that every lookup contacts each level. Existing cache entries, resolver configuration, the query type, and protocol extensions can change the path or the details of an exchange.
What are the main DNS components?
- Domain name space: The hierarchical collection of names and the associated resource information.
- Resource record: A data item associated with a name. The query type specifies the kind of information being requested.
- Name server: A server program that holds DNS information and can answer from its local data or refer a query onward.
- Authoritative name server: A name server that provides authoritative information for a zone it serves.
- Zone: A portion of the DNS namespace for which authoritative information is organized and distributed to servers.
- Resolver: The role that obtains DNS information for a user or application, using queries, referrals, and often caching. ICANN describes a resolver as a DNS server that resolves host names for users and applications.
- Root: The top of the public DNS hierarchy, from which resolution is directed toward top-level domains.
Is DNS just a domain-name-to-IP-address converter?
No. Finding a host address is a common use, but DNS queries can request different resource-record types and therefore different information associated with a name. Resolution is also not necessarily a direct lookup: a resolver may already have a cached result, or it may need to follow referrals to reach the authority for the requested zone. The query and response mechanisms are specified in RFC 1035, while the hierarchy and resolver roles are described in RFC 1034.
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