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DNS root zone
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The DNS root zone is the top-level DNS zone in the hierarchical namespace of the Domain Name System (DNS) of the Internet.
Before October 1, 2016, the root zone had been overseen by the Internet Corporation for Assigned Names and Numbers (ICANN) which delegates the management to a subsidiary acting as the Internet Assigned Numbers Authority (IANA).[1] Distribution services are provided by Verisign. Prior to this, ICANN performed management responsibility under oversight of the National Telecommunications and Information Administration (NTIA), an agency of the United States Department of Commerce.[2] Oversight responsibility transitioned to the global stakeholder community represented within ICANN's governance structures.
A combination of limits in the DNS definition and in certain protocols, namely the practical size of unfragmented User Datagram Protocol[2] (UDP) packets, resulted in a practical maximum of 13 root name server addresses that can be accommodated in DNS name query responses. However the root zone is serviced by several hundred servers at over 130 locations in many countries.[3][4]
Initialization of DNS service
[edit]The DNS root zone is served by thirteen root server clusters which are authoritative for queries to the top-level domains of the Internet.[5][6] Thus, every name resolution either starts with a query to a root server or uses information that was once obtained from a root server.
The root servers clusters have the official names a.root-servers.net to m.root-servers.net.[6] To resolve these names into addresses, a DNS resolver must first find an authoritative server for the net zone. To avoid this circular dependency, the address of at least one root server must be known for bootstrapping access to the DNS. For this purpose, operating systems or DNS servers or resolver software packages typically include a file with all addresses of the DNS root servers. Even if the IP addresses of some root servers change, at least one is needed to retrieve the current list of all name servers. This address file is called named.cache in the BIND name server reference implementation. The current official version is distributed by ICANN's InterNIC.[7]
With the address of a single functioning root server, all other DNS information may be discovered recursively, and information about any domain name may be found.
Redundancy and diversity
[edit]The root DNS servers are essential to the function of the Internet, as most Internet services, such as the World Wide Web and email, are based on domain names. The DNS servers are potential points of failure for the entire Internet. For this reason, multiple root servers are distributed worldwide.[8] The DNS packet size of 512 octets limits a DNS response to thirteen addresses, until protocol extensions (see Extension Mechanisms for DNS) lifted this restriction.[9] While it is possible to fit more entries into a packet of this size when using label compression, thirteen was chosen as a reliable limit. Since the introduction of IPv6, the successor Internet Protocol to IPv4, previous practices are being modified and extra space is filled with IPv6 name servers.
The root name servers are hosted in multiple secure sites with high-bandwidth access to accommodate the traffic load. At first, all of these installations were located in the United States; however, the distribution has shifted and this is no longer the case.[10] Usually each DNS server installation at a given site is a cluster of computers with load-balancing routers.[9] A comprehensive list of servers, their locations, and properties is available at https://root-servers.org/. As of 24 June 2023[update], there were 1708 root servers worldwide.[11]
The modern trend is to use anycast addressing and routing to provide resilience and load balancing across a wide geographic area. For example, the j.root-servers.net server, maintained by Verisign, is represented by 104 (as of January 2016[update]) individual server systems located around the world, which can be queried using anycast addressing.[12]
Management
[edit]The content of the Internet root zone file is coordinated by a subsidiary of ICANN which performs the Internet Assigned Numbers Authority (IANA) functions. Verisign generates and distributes the zone file to the various root server operators.
In 1997, when the Internet was transferred from U.S. government control to private hands, NTIA exercised stewardship over the root zone. A 1998 Commerce Department document stated the agency was "committed to a transition that will allow the private sector to take leadership for DNS management" by the year 2000, however, no steps to make the transition happen were taken. In March 2014, NTIA announced it would transition its stewardship to a "global stakeholder community".[5]
According to Assistant Secretary of Commerce for Communications and Information, Lawrence E. Strickling, March 2014 was the right time to start a transition of the role to the global Internet community. The move came after pressure in the fallout of revelations that the United States and its allies had engaged in surveillance. The chairman of the board of ICANN denied the two were connected, however, and said the transition process had been ongoing for a long time. ICANN president Fadi Chehadé called the move historic and said that ICANN would move toward multi-stakeholder control. Various prominent figures in Internet history not affiliated with ICANN also applauded the move.[5]
NTIA's announcement did not immediately affect how ICANN performs its role.[5][13] On March 11, 2016, NTIA announced that it had received a proposed plan to transition its stewardship role over the root zone, and would review it in the next 90 days.[14]
The proposal was adopted, and ICANN's renewed contract to perform the IANA function lapsed on September 30, 2016, resulting in the transition of oversight responsibility to the global stakeholder community represented within ICANN's governance structures. As a component of the transition plan,[15] it created a new subsidiary called Public Technical Identifiers (PTI) to perform the IANA functions which include managing the DNS root zone.
Data protection of the root zone
[edit]Signing of the root zone
[edit]Since July 2010, the root zone has been signed with a DNSSEC signature,[16] providing a single trust anchor for the Domain Name System that can in turn be used to provide a trust anchor for other public key infrastructure (PKI). The root zone DNSKEY section is re-signed periodically with the root zone key signing key performed in a verifiable manner in front of witnesses in a key signing ceremony.[17][18] The KSK2017 with ID 20326 is valid as of 2020.
ZONEMD record
[edit]While the root zone file is signed with DNSSEC, some DNS records, such as NS records, are not covered by DNSSEC signatures. To address this weakness, a new DNS Resource Record, called ZONEMD, was introduced in RFC 8976. ZONEMD doesn't replace DNSSEC. ZONEMD and DNSSEC must be used together to ensure the full protection of the DNS root zone file.[19][20]
The ZONEMD deployment for the DNS root zone was completed on December 6, 2023.[21]
DNS over TLS
[edit]The B-Root DNS servers offer experimental support for DNS over TLS (DoT) on port 853.[22]
See also
[edit]References
[edit]- ^ "Stewardship of IANA Functions Transitions to Global Internet Community as Contract with U.S. Government Ends". October 1, 2016. Retrieved December 25, 2017.
- ^ a b Jerry Brito (March 5, 2011). "ICANN vs. the World". Time.
- ^ "There are not 13 root servers". www.icann.org. Retrieved January 18, 2018.
- ^ "DNS root servers in the world « stupid.domain.name". stupid.domain.name. Archived from the original on February 11, 2021. Retrieved January 18, 2018.
- ^ a b c d Farivar, Cyrus (March 14, 2014). "In sudden announcement, US to give up control of DNS root zone". Ars Technica. Retrieved March 15, 2014.
- ^ a b "Root Servers". IANA. Retrieved January 17, 2020.
- ^ "named.cache". InterNIC. November 17, 2015. Retrieved November 17, 2015.
- ^ "SANS Institute InfoSec Reading Room". SANS. Retrieved March 17, 2014.
- ^ a b Bradley Mitchell (November 19, 2008). "Why There Are Only 13 DNS Root Name Servers". About.com. Archived from the original on March 18, 2014. Retrieved March 17, 2014.
- ^ "DNS Root Servers: The most critical infrastructure on the internet". Slash Root. November 15, 2013.
- ^ "Root Servers Technical Operations Assn". Archived from the original on June 24, 2023. Retrieved June 29, 2023.
- ^ "Root Server Technical Operations Assn".
- ^ "An Update on the IANA Transition". National Telecommunications and Information Administration. August 17, 2015. Retrieved November 17, 2015.
- ^ Strickling, Lawrence. "Reviewing the IANA Transition Proposal". National Telecommunications and Information Administration. United States Department of Congress. Retrieved May 26, 2016.
- ^ "Proposal to Transition the Stewardship of the Internet Assigned Numbers Authority (IANA) Functions from the U.S. Commerce Department's National Telecommunications and Information Administration (NTIA) to the Global Multistakeholder Community" (PDF). March 2016.
- ^ "Root DNSSEC: Information about DNSSEC for the Root Zone". Internet Corporation For Assigned Names and Numbers. Retrieved March 19, 2014.
- ^ "First KSK Ceremony". Internet Corporation For Assigned Names and Numbers. April 18, 2010. Archived from the original on April 14, 2015. Retrieved October 19, 2014.
- ^ "Root KSK Ceremonies". Internet Assigned Numbers Authority. November 12, 2015. Retrieved November 17, 2015.
- ^ Wessels, Duane (April 18, 2023). "Adding ZONEMD Protections to the Root Zone". Verisign Blog.
- ^ D. Wessels; P. Barber; M. Weinberg; W. Kumari; W. Hardaker (February 2021). "RFC 8976 Message Digest for DNS Zones". Retrieved March 10, 2024.
- ^ Wessels, Duane (December 6, 2023). "[dns-operations] Root zone operational announcement: introducing ZONEMD for the root zone". Retrieved March 10, 2024.
- ^ "B-Root Offers Experimental Support for DNS over TLS".
Further reading
[edit]- "NTIA announces intent to transition key internet domain name functions". Office of Public Affairs. National Telecommunications and Information Administration. March 14, 2014. Retrieved March 15, 2014.
{{cite web}}: CS1 maint: others (link)
External links
[edit]- Root Zone File
- root-servers.org
- IANA's Authoritative Database of TLDs on the DNS Root Zone
- ICANN's Root Server System Advisory Committee
- CircleID.com, on DNS Root Servers
- CAIDA.org, paper on root server location problem
- CirlceID.com, More root server instances outside the U.S. than inside
- List of public DNS servers Continuously verified and updated.
DNS root zone
View on GrokipediaOverview
Definition and Fundamental Role
The DNS root zone represents the uppermost echelon of the Domain Name System (DNS) hierarchical namespace, containing the authoritative delegation records for all top-level domains (TLDs), such as generic TLDs including .com and sponsored TLDs like .edu, as well as country-code TLDs like .uk and .jp.[7] These records specify the name servers operated by TLD registries, ensuring a unified global directory of domain authorities.[1] The zone itself is a discrete DNS resource record set, distributed via a root zone file that lists the IP addresses and hostnames of authoritative root name servers to initiate resolution bootstrapping.[8] In its fundamental role, the root zone anchors the entire DNS resolution process by responding to queries for TLDs with referrals to the corresponding TLD name servers, thereby enabling recursive resolvers—such as those in operating systems and applications—to traverse the hierarchy downward toward final authoritative answers.[5] This delegation mechanism maintains the causal chain of trust and lookup efficiency across the Internet, where a single root query per resolution session suffices to access any subdomain, preventing fragmentation of the namespace.[9] Managed through precise change requests and cryptographic validation via DNSSEC, the root zone's stability directly underpins the scalability and reliability of domain-to-IP mapping for billions of daily queries, as disruptions here would cascade failures throughout subordinate zones.[1] Its design as a minimal, high-authority layer reflects first-principles engineering for distributed systems, prioritizing redundancy over centralization to mitigate single points of failure.[5]Contents and Hierarchical Structure
The DNS root zone, situated at the apex of the namespace represented by the empty label ".", contains resource records that define its own authority and delegate to top-level domains (TLDs). It begins with a single Start of Authority (SOA) record, which specifies the primary authoritative name server (typically a.root-servers.net), the responsible party (often nstld.verisign-grs.com), a serial number for versioning (e.g., incrementing daily as 2025102601), and timing parameters such as refresh (1800 seconds), retry (900 seconds), expire (604800 seconds), and minimum TTL (86400 seconds).[10] These parameters govern zone transfers and caching behaviors for secondary root servers.[11] Following the SOA are 13 Name Server (NS) records designating the logical root name servers, from a.root-servers.net to m.root-servers.net, each with a TTL of 518400 seconds. To facilitate resolution, these include glue records: A records for IPv4 addresses (e.g., 198.41.0.4 for a.root-servers.net) and AAAA records for IPv6 (e.g., 2001:503:ba3e::2:30), both with extended TTLs up to 3600000 seconds to minimize query loads on the root.[8] The majority of the zone—over 1,500 entries as of early 2025—consists of delegations to TLDs, encompassing generic TLDs (gTLDs) like .com and country-code TLDs (ccTLDs) like .us. Each TLD features 2 to 13 NS records pointing to its authoritative servers (e.g., a.gtld-servers.net for .com), with glue A and AAAA records for any server hostnames subordinate to the TLD (e.g., ns1.nic.example. resolved via 192.0.2.1) to prevent circular resolution dependencies during initial queries.[12][13] DNSSEC integration adds Delegation Signer (DS) records for secured TLDs (e.g., key tag 59875 for .bf), which anchor the chain of trust from the root's Key Signing Key downward, alongside RRSIG records signing other entries, NSEC or NSEC3 for authenticated denial of existence, DNSKEY for zone keys, and ZONEMD for integrity validation.[14] No other record types, such as MX or TXT, appear in the root zone, as its role is strictly delegative rather than hosting endpoint data. The zone file, distributed by Verisign under IANA oversight, totals several megabytes and is signed as a whole, with changes propagated via NOTIFY and AXFR/IXFR mechanisms to the 13 root server operators.[8] In hierarchical terms, the root zone embodies the inverted tree structure of the DNS namespace, where the root node branches solely to TLD labels (e.g., com.), creating zone cuts via NS records that shift authority to TLD operators. This delegation enables scalable, distributed management: TLD zones then subdivide into second-level domains (e.g., example.com), and so on, without the root retaining data below the TLD level. The absence of subdomains under the root ensures minimal content while supporting global resolution bootstrapping, with redundancy via anycast deployment across hundreds of physical instances for the 13 logical servers.[15] This design, formalized in RFC 1034 and 1035, prioritizes fault tolerance and load distribution, handling billions of queries daily without central bottlenecks.[11]Historical Development
Origins in ARPANET and Pre-DNS Systems
The Advanced Research Projects Agency Network (ARPANET), operational from 1969, initially relied on numeric host identifiers for network addressing, lacking a standardized system for human-readable names.[16] As the network expanded, informal hostname usage emerged among operators, but resolution remained ad hoc, with each host maintaining local mappings or relying on direct knowledge.[17] By the early 1970s, with approximately 45 hosts connected, the need for centralized coordination grew, leading to the establishment of the Network Information Center (NIC) at SRI International under Elizabeth Feinler.[18][19] Hostname resolution in ARPANET transitioned to a centralized HOSTS.TXT file around 1973-1974, maintained manually by the NIC and distributed weekly via FTP from the SRI host or physical tapes to connected sites.[16][17] This plain-text file mapped hostnames to 32-bit ARPANET addresses (later IP addresses post-1983 TCP/IP transition), serving as a flat, authoritative directory for the entire network.[20] Initially sufficient for a small community of under 200 hosts through the 1970s, the system's scalability faltered as ARPANET interconnected with other networks, reaching hundreds of entries by the early 1980s; manual updates introduced delays of up to a week, propagation errors, and administrative bottlenecks under single-point NIC control.[16][18] These limitations of the HOSTS.TXT regime—centralized maintenance, flat namespace, and vulnerability to human error—directly motivated the design of the Domain Name System (DNS) in 1983 by Paul Mockapetris at the University of Southern California's Information Sciences Institute (ISI), with Jon Postel overseeing implementation.[16][18] DNS introduced a hierarchical namespace to distribute authority, supplanting the monolithic file with delegated zones; the DNS root zone emerged as the apex of this structure, holding delegations to top-level domains (TLDs) like .ARPA (for ARPANET hosts during transition) and providing a singular point of coordination akin to the NIC's prior role, but enabled for global, automated resolution.[6] Early root functionality was hosted on servers at ISI, marking the root zone's operational inception as ARPANET evolved into the broader Internet.[6]Standardization via RFCs and Formal Protocols
The Domain Name System (DNS) was initially proposed through RFC 882, titled "Domain Names - Concepts and Facilities," and RFC 883, "Domain Names - Implementation and Specification," both authored by Paul Mockapetris and published in November 1983.[21] These documents outlined the core architecture of a hierarchical, distributed naming system to replace manual host table maintenance, introducing concepts such as domain names, resource records, name servers, and resolvers, with the root serving as the apex of the namespace tree.[21] They specified the initial protocol mechanics, including message formats for queries and responses over UDP and TCP, and defined the root domain's role in delegating authority to top-level domains via NS records. These 1983 RFCs were obsoleted and refined in November 1987 by RFC 1034, "Domain Names - Concepts and Facilities," and RFC 1035, "Domain Names - Implementation and Specification," which established the enduring standards for DNS operation.[22][23] RFC 1034 formalized the namespace as a tree-structured hierarchy rooted at an unnamed node (conventionally denoted by a trailing dot), where the root zone holds authoritative delegations to top-level domains through glue records for their name servers.[22] RFC 1035 detailed the protocol implementation, including the binary wire format for DNS messages (with fixed headers and variable-length question, answer, authority, and additional sections), query types (e.g., A for IPv4 addresses), and error codes, ensuring interoperable resolution starting from root servers.[23] These specifications mandated that root servers maintain a cache or zone file of TLD delegations, enabling recursive resolution without centralized control. The RFCs emphasized decentralization and fault tolerance, requiring multiple root server instances to distribute load and provide redundancy, with protocols for iterative queries where resolvers contact root servers to obtain TLD referrals.[23] Formal error handling, such as NXDOMAIN for non-existent domains and SERVFAIL for server failures, was defined to maintain protocol robustness.[23] Subsequent RFCs built upon this foundation for root-specific operations, such as RFC 7720 (2015), which describes the external protocol interface for root name servers, confirming the 1987 standards' ongoing relevance while addressing deployment requirements like anycast addressing for global reachability.[24] These documents, developed through the Internet Engineering Task Force (IETF) process, transitioned DNS from experimental ARPANET mechanisms to a standardized Internet protocol suite, with the root zone's structure remaining integral to global name resolution stability.[22]Operational Initialization and Early Deployment
The first root name server, designated A.ROOT-SERVERS.NET, was established in 1984 at the University of Southern California's Information Sciences Institute (ISI) by Paul Mockapetris and Jon Postel to test DNS software implementations and facilitate protocol development following the publication of RFC 882 and RFC 883.[6] This single-server setup initially served as the authoritative source for the root zone, with Postel manually maintaining the zone file and distributing updates via email or file transfers to early experimenters.[18] Operational initialization emphasized reliability testing in the ARPANET environment, where DNS queries were routed to this ISI-hosted server to resolve initial domain hierarchies replacing the centralized hosts.txt file.[25] Early deployment accelerated in 1985, as DNS transitioned from experimental status to production use under Postel's guidance at ISI, which functioned as the de facto Internet Assigned Numbers Authority (IANA).[26] The root zone was expanded to include the first generic top-level domains (gTLDs)—.com, .edu, .gov, .mil, .org, and .net—delegated in January 1985, enabling the registration of domains such as symbolics.com on January 1, 1985, marking the initial operational rollout of hierarchical name resolution across ARPANET hosts.[27] Additional root servers were incrementally added, including instances at SRI International and other U.S.-based sites, to provide basic redundancy; by late 1985, the system supported limited global queries with zone updates coordinated manually by Postel.[18] This phase relied on UNIX-based implementations like the Berkeley Internet Name Domain (BIND), first developed in 1984 at UC Berkeley, which handled root zone serving on early hardware.[28] Deployment challenges included ensuring compatibility with legacy ARPANET resolvers and managing load on the nascent infrastructure, with the root zone file remaining small—containing fewer than 10 TLD delegations by mid-1985.[6] Postel's centralized role extended to approving delegations on a case-by-case basis, prioritizing academic, military, and commercial entities, which laid the groundwork for scalable operations without formal governance structures at the time.[29] By 1987, the root server constellation had grown to seven logical instances, primarily U.S.-operated, reflecting cautious expansion to mitigate single points of failure while the Internet user base remained under 10,000 hosts.[30]Technical Architecture
Root Name Servers and Logical/Physical Instances
The DNS root zone is authoritatively served by 13 logical root name servers, identified as A through M, which collectively hold the complete root zone data and respond to queries directing resolvers to top-level domain (TLD) servers.[5] These logical servers share a fixed set of 13 IPv4 addresses and corresponding IPv6 addresses, with hostnames such as a.root-servers.net for the A server.[5] Each logical server is operated by one of 12 independent organizations, ensuring decentralized management and avoiding single points of control; Verisign, Inc. uniquely operates both A and J.[31] The operators include government agencies, academic institutions, non-profits, and commercial entities, reflecting the system's origins in diverse U.S. research networks while incorporating international participants for broader resilience.[5]| Letter | Hostname | Operator |
|---|---|---|
| A | a.root-servers.net | Verisign, Inc. |
| B | b.root-servers.net | University of Southern California, Information Sciences Institute |
| C | c.root-servers.net | Cogent Communications |
| D | d.root-servers.net | University of Maryland |
| E | e.root-servers.net | NASA (Ames Research Center) |
| F | f.root-servers.net | Internet Systems Consortium, Inc. |
| G | g.root-servers.net | U.S. Department of Defense (NIC) |
| H | h.root-servers.net | U.S. Army (Research Lab) |
| I | i.root-servers.net | Netnod |
| J | j.root-servers.net | Verisign, Inc. |
| K | k.root-servers.net | RIPE NCC |
| L | l.root-servers.net | ICANN |
| M | m.root-servers.net | WIDE Project |
