The Domain Name System (DNS) is a hierarchical distributed naming system for computers, services, or any resource connected to the Internet or a private network. It associates various information with domain names assigned to each of the participating entities. Most importantly, it translates domain names meaningful to humans into the numerical identifiers associated with networking equipment for the purpose of locating and addressing these devices worldwide.
An often-used analogy to explain the Domain Name System is that it serves as the phone book for the Internet by translating human-friendly computer hostnames into IP addresses. For example, the domain name www.example.com translates to the addresses 192.0.32.10 (IPv4) and 2620:0:2d0:200::10 (IPv6).
The Domain Name System makes it possible to assign domain names to groups of Internet resources and users in a meaningful way, independent of each entity's physical location. Because of this, World Wide Web (WWW) hyperlinks and Internet contact information can remain consistent and constant even if the current Internet routing arrangements change or the participant uses a mobile device. Internet domain names are easier to remember than IP addresses such as 208.77.188.166 (IPv4) or 2001:db8:1f70::999:de8:7648:6e8 (IPv6). Users take advantage of this when they recite meaningful Uniform Resource Locators (URLs) and e-mail addresses without having to know how the computer actually locates them.
The Domain Name System distributes the responsibility of assigning domain names and mapping those names to IP addresses by designating authoritative name servers for each domain. Authoritative name servers are assigned to be responsible for their particular domains, and in turn can assign other authoritative name servers for their sub-domains. This mechanism has made the DNS distributed and fault tolerant and has helped avoid the need for a single central register to be continually consulted and updated.
In general, the Domain Name System also stores other types of information, such as the list of mail servers that accept email for a given Internet domain. By providing a worldwide, distributed keyword-based redirection service, the Domain Name System is an essential component of the functionality of the Internet.
Other identifiers such as RFID tags, UPCs, international characters in email addresses and host names, and a variety of other identifiers could all potentially use DNS.
The Domain Name System also specifies the technical functionality of this database service. It defines the DNS protocol, a detailed specification of the data structures and communication exchanges used in DNS, as part of the Internet Protocol Suite.
Overview
The Internet maintains two principal namespaces, the domain name hierarchy and the Internet Protocol (IP) address spaces. The Domain Name System maintains the domain name hierarchy and provides translation services between it and the address spaces. Internet name servers and a communication protocol implement the Domain Name System. A DNS name server is a server that stores the DNS records for a domain name, such as address (A) records, name server (NS) records, and mail exchanger (MX) records; a DNS name server responds with answers to queries against its database.
History
The practice of using a name as a simpler, more memorable abstraction of a host's numerical address on a network dates back to the ARPANET era. Before the DNS was invented in 1982, each computer on the network retrieved a file called HOSTS.TXT from a computer at SRI (now SRI International). The HOSTS.TXT file mapped names to numerical addresses. A hosts file still exists on most modern operating systems by default and generally contains a mapping of the IP address 127.0.0.1 to "localhost". Many operating systems use name resolution logic that allows the administrator to configure selection priorities for available name resolution methods.
The rapid growth of the network made a centrally maintained, hand-crafted HOSTS.TXT file unsustainable; it became necessary to implement a more scalable system capable of automatically disseminating the requisite information.
At the request of Jon Postel, Paul Mockapetris invented the Domain Name System in 1983 and wrote the first implementation. The original specifications were published by the Internet Engineering Task Force in RFC 882 and RFC 883, which were superseded in November 1987 by RFC 1034 and RFC 1035. Several additional Request for Comments have proposed various extensions to the core DNS protocols.
In 1984, four Berkeley students—Douglas Terry, Mark Painter, David Riggle, and Songnian Zhou—wrote the first Unix implementation, called The Berkeley Internet Name Domain (BIND) Server. In 1985, Kevin Dunlap of DEC significantly re-wrote the DNS implementation. Mike Karels, Phil Almquist, and Paul Vixie have maintained BIND since then. BIND was ported to the Windows NT platform in the early 1990s.
BIND was widely distributed, especially on Unix systems, and is the dominant DNS software in use on the Internet. With the heavy use and resulting scrutiny of its open-source code, as well as increasingly more sophisticated attack methods, many security flaws were discovered in BIND. This contributed to the development of a number of alternative name server and resolver programs. BIND version 9 was written from scratch and now has a security record comparable to other modern DNS software.
Structure
Domain name space
The domain name space consists of a tree of domain names. Each node or leaf in the tree has zero or more resource records, which hold information associated with the domain name. The tree sub-divides into zones beginning at the root zone. A DNS zone may consist of only one domain, or may consist of many domains and sub-domains, depending on the administrative authority delegated to the manager.
Administrative responsibility over any zone may be divided by creating additional zones. Authority is said to be delegated for a portion of the old space, usually in the form of sub-domains, to another nameserver and administrative entity. The old zone ceases to be authoritative for the new zone.
The hierarchical Domain Name System, organized into zones, each served by a name server
Domain name syntax
The definitive descriptions of the rules for forming domain names appear in RFC 1035, RFC 1123, and RFC 2181. A domain name consists of one or more parts, technically called labels, that are conventionally concatenated, and delimited by dots, such as example.com.
- The right-most label conveys the top-level domain; for example, the domain name www.example.com belongs to the top-level domain com.
- The hierarchy of domains descends from right to left; each label to the left specifies a subdivision, or subdomain of the domain to the right. For example: the label example specifies a subdomain of the com domain, and www is a sub domain of example.com. This tree of subdivisions may have up to 127 levels.
- Each label may contain up to 63 characters. The full domain name may not exceed a total length of 253 characters in its external dotted-label specification. In the internal binary representation of the DNS the maximum length requires 255 octets of storage. In practice, some domain registries may have shorter limits.
- DNS names may technically consist of any character representable in an octet. However, the allowed formulation of domain names in the DNS root zone, and most other sub domains, uses a preferred format and character set. The characters allowed in a label are a subset of the ASCII character set, and includes the characters a through z, A through Z, digits 0 through 9, and the hyphen. This rule is known as the LDH rule (letters, digits, hyphen). Domain names are interpreted in case-independent manner. Labels may not start or end with a hyphen.
- A hostname is a domain name that has at least one IP address associated. For example, the domain names www.example.com and example.com are also hostnames, whereas the com domain is not.
Internationalized domain names
The permitted character set of the DNS prevented the representation of names and words of many languages in their native alphabets or scripts. ICANN has approved the Internationalizing Domain Names in Applications (IDNA) system, which maps Unicode strings into the valid DNS character set using Punycode. In 2009 ICANN approved the installation of IDN country code top-level domains. In addition, many registries of the existing top level domain names (TLD)s have adopted IDNA.
Name servers
The Domain Name System is maintained by a distributed database system, which uses the client-server model. The nodes of this database are the name servers. Each domain has at least one authoritative DNS server that publishes information about that domain and the name servers of any domains subordinate to it. The top of the hierarchy is served by the root nameservers, the servers to query when looking up (resolving) a TLD.
Authoritative name server
An authoritative name server is a name server that gives answers that have been configured by an original source, for example, the domain administrator or by dynamic DNS methods, in contrast to answers that were obtained via a regular DNS query to another name server. An authoritative-only name server only returns answers to queries about domain names that have been specifically configured by the administrator.
An authoritative name server can either be a master server or a slave server. A master server is a server that stores the original (master) copies of all zone records. A slave server uses an automatic updating mechanism of the DNS protocol in communication with its master to maintain an identical copy of the master records.
Every DNS zone must be assigned a set of authoritative name servers that are installed in NS records in the parent zone.
When domain names are registered with a domain name registrar, their installation at the domain registry of a top level domain requires the assignment of a primary name server and at least one secondary name server. The requirement of multiple name servers aims to make the domain still functional even if one name server becomes inaccessible or inoperable. The designation of a primary name server is solely determined by the priority given to the domain name registrar. For this purpose, generally only the fully qualified domain name of the name server is required, unless the servers are contained in the registered domain, in which case the corresponding IP address is needed as well.
Primary name servers are often master name servers, while secondary name server may be implemented as slave servers.
An authoritative server indicates its status of supplying definitive answers, deemed authoritative, by setting a software flag (a protocol structure bit), called the Authoritative Answer (AA) bit in its responses. This flag is usually reproduced prominently in the output of DNS administration query tools (such as dig) to indicate that the responding name server is an authority for the domain name in question.
Recursive and caching name server
In principle, authoritative name servers are sufficient for the operation of the Internet. However, with only authoritative name servers operating, every DNS query must start with recursive queries at the root zone of the Domain Name System and each user system must implement resolver software capable of recursive operation.
To improve efficiency, reduce DNS traffic across the Internet, and increase performance in end-user applications, the Domain Name System supports DNS cache servers which store DNS query results for a period of time determined in the configuration (time-to-live) of the domain name record in question. Typically, such caching DNS servers, also called DNS caches, also implement the recursive algorithm necessary to resolve a given name starting with the DNS root through to the authoritative name servers of the queried domain. With this function implemented in the name server, user applications gain efficiency in design and operation.
The combination of DNS caching and recursive functions in a name server is not mandatory; the functions can be implemented independently in servers for special purposes.
Internet service providers typically provide recursive and caching name servers for their customers. In addition, many home networking routers implement DNS caches and recursors to improve efficiency in the local network.
DNS resolvers
The client-side of the DNS is called a DNS resolver. It is responsible for initiating and sequencing the queries that ultimately lead to a full resolution (translation) of the resource sought, e.g., translation of a domain name into an IP address.
A DNS query may be either a non-recursive query or a recursive query:
- A non-recursive query is one in which the DNS server provides a record for a domain for which it is authoritative itself, or it provides a partial result without querying other servers.
- A recursive query is one for which the DNS server will fully answer the query (or give an error) by querying other name servers as needed. DNS servers are not required to support recursive queries.
The resolver, or another DNS server acting recursively on behalf of the resolver, negotiates use of recursive service using bits in the query headers.
Resolving usually entails iterating through several name servers to find the needed information. However, some resolvers function more simply by communicating only with a single name server. These simple resolvers (called "stub resolvers") rely on a recursive name server to perform the work of finding information for them.
Operation
Address resolution mechanism
Domain name resolvers determine the appropriate domain name servers responsible for the domain name in question by a sequence of queries starting with the right-most (top-level) domain label.
The process entails:
- A network host is configured with an initial cache (so called hints) of the known addresses of the root nameservers. Such a hint file is updated periodically by an administrator from a reliable source.
- A query to one of the root servers to find the server authoritative for the top-level domain.
- A query to the obtained TLD server for the address of a DNS server authoritative for the second-level domain.
- Repetition of the previous step to process each domain name label in sequence, until the final step which returns the IP address of the host sought.
The diagram illustrates this process for the host www.ictea.com.
The mechanism in this simple form would place a large operating burden on the root servers, with every search for an address starting by querying one of them. Being as critical as they are to the overall function of the system, such heavy use would create an insurmountable bottleneck for trillions of queries placed every day. In practice caching is used in DNS servers to overcome this problem, and as a result, root nameservers actually are involved with very little of the total traffic.
A DNS recursor consults three nameservers to resolve the address www.ictea.com.
