Aroot name server is aname server for theroot zone of theDomain Name System (DNS) of theInternet. It directly answers requests for records in the root zone and answers other requests by returning a list of theauthoritative name servers for the appropriatetop-level domain (TLD). The root name servers are a critical part of the Internet infrastructure because they are the first step inresolving human-readable host names intoIP addresses that are used in communication betweenInternet hosts.
A combination of limits in the DNS and certain protocols, namely the practical size of unfragmentedUser Datagram Protocol (UDP) packets, resulted in a decision to limit the number of root servers to thirteen server addresses.[1][2] The use ofanycast addressing permits the actual number of root server instances to be much larger, and is 1,733 as of March 4, 2024[update].[3]
The DNS is a hierarchical naming system for computers, services, or any resource participating in theInternet. The top of that hierarchy is theroot domain. The root domain does not have a formal name and its label in the DNS hierarchy is anempty string. Allfully qualified domain names (FQDNs) on the Internet can be regarded as ending with this empty string for the root domain, and therefore ending in afull stop character (the label delimiter), e.g., "www.example.com.". This is generally implied rather than explicit, as modern DNS software does not actually require that the terminating dot be included when attempting to translate a domain name to an IP address.
The root domain contains all top-level domains of the Internet. As of July 2015[update], it contained 1058 TLDs, including 730generic top-level domains (gTLDs) and 301country code top-level domains (ccTLDs) in the root domain.[4] In addition, theARPA domain is used for technicalname spaces in the management of Internet addressing and other resources. ATEST domain is used for testinginternationalized domain names.
When a computer on the Internet needs to resolve a domain name, it usesresolver software to perform the lookup. A resolver breaks the name up into its labels from right to left. The first component (TLD) is queried using a root server to obtain the responsible authoritative server. Queries for each label return more specific name servers until a name server returns the answer of the original query.
In practice, most of this information does not change very often over a period of hours and therefore it iscached by intermediate name servers or by a name cache built into the user's application. DNS lookups to the root name servers may therefore be relatively infrequent. A survey in 2003 reported that only 2% of all queries to the root servers were legitimate. Incorrect or non-existent caching was responsible for 75% of the queries, 12.5% were for unknown TLDs, 7% were for lookups using IP addresses as if they were domain names, etc.[5] Some misconfigured desktop computers even tried to update the root server records for the TLDs. A similar list of observed problems and recommended fixes has been published in RFC 4697.
Although any local implementation of DNS can implement its own private root name servers, the term "root name server" is generally used to describe the thirteen well-known root name servers that implement the root name space domain for the Internet's official global implementation of the Domain Name System. Resolvers use a small 3KBroot.hints file published by Internic[6] to bootstrap this initial list of root server addresses; in other words, root.hints is necessary to break thecircular dependency of needing to know the addresses of a root name server to lookup the same address.
There are 13 logical root name servers specified, with logical names in the formletter.root-servers.net, whereletter ranges from a to m. The choice of thirteen name servers was made because of limitations in the original DNS specification, which specifies a maximum packet size of 512 bytes when using theUser Datagram Protocol (UDP).[7] Technically however, fourteen name servers fit into an IPv4 packet. The addition of IPv6 addresses for the root name servers requires more than 512 bytes, which is facilitated by theEDNS0 extension to the DNS standard.[8]
This does not mean that there are only 13 physical servers; each operator uses redundant computer equipment to provide reliable service even if failure of hardware or software occurs. Additionally, all operate in multiple geographical locations using a routing technique calledanycast addressing, providing increased performance and even more fault tolerance. An informational homepage exists for every logical server (except G-Root) under theRoot Server Technical Operations Association domain with web address in the formhttp://letter.root-servers.org/, whereletter ranges from a to m.
Ten servers were originally in the United States; all are now operated using anycast addressing. Three servers were originally located in Stockholm (I-Root), Amsterdam (K-Root), and Tokyo (M-Root) respectively.Older servers had their own name before the policy of using similar names was established. With anycast, most of the physical root servers are now outside the United States, allowing for high performance worldwide.
| Letter | IPv4 address | IPv6 address | AS-number[9] | Old name | Operator | Operator origin | Location & no. of sites (global/local)[10] | Software |
|---|---|---|---|---|---|---|---|---|
| A | 198.41.0.4 | 2001:503:ba3e::2:30 | AS19836,[9][note 1] AS36619, AS36620, AS36622, AS36625, AS36631, AS64820[note 2][11] | ns.internic.net | Verisign |  United States | Distributed usinganycast 14/2 | NSD and Verisign ATLAS |
| B | 170.247.170.2[12][note 3] | 2801:1b8:10::b[12] | AS394353[17] | ns1.isi.edu | USC-ISI | United States | Distributed usinganycast 6/0 | BIND andKnot DNS[18] |
| C | 192.33.4.12 | 2001:500:2::c | AS2149[9][19] | c.psi.net | Cogent Communications | United States | Distributed usinganycast 10/0 | BIND |
| D | 199.7.91.13[note 4][20] | 2001:500:2d::d | AS10886[note 5][9][21] | terp.umd.edu | University of Maryland | United States | Distributed usinganycast 22/127 | NSD[22] |
| E | 192.203.230.10 | 2001:500:a8::e | AS21556[9][23] | ns.nasa.gov | NASA Ames Research Center | United States | Distributed usinganycast 117/137 | BIND andNSD |
| F | 192.5.5.241 | 2001:500:2f::f | AS3557[9][24] | ns.isc.org | Internet Systems Consortium | United States | Distributed usinganycast 119/119 | BIND[25] and Cloudflare[26] |
| G[note 6] | 192.112.36.4[note 7] | 2001:500:12::d0d[note 7] | AS5927[9][27] | ns.nic.ddn.mil | Defense Information Systems Agency | United States | Distributed usinganycast 6/0 | BIND |
| H | 198.97.190.53[note 8][28] | 2001:500:1::53[note 9][28] | AS1508[28][note 10][29] | aos.arl.army.mil | U.S. Army Research Lab | United States | Distributed usinganycast 8/0 | NSD |
| I | 192.36.148.17 | 2001:7fe::53 | AS29216[9][30] | nic.nordu.net | Netnod |  Sweden | Distributed usinganycast 63/2 | BIND |
| J | 192.58.128.30[note 11] | 2001:503:c27::2:30 | AS26415,[9][31] AS36626, AS36628, AS36632[31] | — | Verisign | United States | Distributed usinganycast 63/55 | NSD and Verisign ATLAS |
| K | 193.0.14.129 | 2001:7fd::1 | AS25152[9][32][33] | — | RIPE NCC |  Netherlands | Distributed usinganycast 70/3 | BIND,NSD andKnot DNS[34] |
| L | 199.7.83.42[note 12][35] | 2001:500:9f::42[note 13][36] | AS20144[9][37][38] | — | ICANN | United States | Distributed usinganycast 165/0 | NSD andKnot DNS[39] |
| M | 202.12.27.33 | 2001:dc3::35 | AS7500[9][40][41] | — | WIDE Project |  Japan | Distributed usinganycast 4/1 | BIND |
There are also several alternative namespace systems with analternative DNS root using their own set of root name servers that exist in parallel to the mainstream name servers. The first,AlterNIC, generated a substantial amount of press.[citation needed]
The function of a root name server may also be implemented locally, or on a provider network. Such servers are synchronized with the official root zone fileas published byICANN, and do not constitute an alternate root.
 | This sectionis missing information about 2010 and 2012 China GFW issues with anycast endpoints. Please expand the section to include this information. Further details may exist on thetalk page.(July 2020) |
As the root name servers are an important part of the Internet, they have comeunder attack several times, although none of the attacks have ever been serious enough to severely affect the performance of the Internet.
The DNS Root Server System Advisory Committee is anICANN committee. ICANN's bylaws[42] say the committee provides advice to ICANN but the committee claims no authority over the servers or server operators.
The root zone file is a small (about 2MB) data set[6] whose publication is the primary purpose of root name servers. This is not to be confused with theroot.hints file used to bootstrap a resolver.
The root zone file is at the apex of a hierarchical distributed database called theDomain Name System (DNS). This database is used by almost all Internet applications to translate worldwide unique names such aswww.wikipedia.org into other identifiers such asIP addresses.
The contents of the root zone file is a list of names and numeric IP addresses of the root domainauthoritative DNS servers for alltop-level domains (TLDs) such as com, org, edu, and thecountry code top-level domains (it also includes that info for root domain, the dot). On 12 December 2004, 773 different authoritative servers for the TLDs were listed. Later the number of TLDs increased greatly. As of July 2020[update], the root zone consisted of 1511 useful TLDs (excluded are: 55 domains that are not assigned, 8 that are retired, and 11 test domains). Other name servers forward queries for which they do not have any information about authoritative servers to a root name server. The root name server, using its root zone file, answers with a referral to the authoritative servers for the appropriate TLD or with an indication that no such TLD exists.[43]
