Root name server

A root name server is a name server for the root zone of the Domain Name System (DNS) of the Internet. It directly answers requests for records in the root zone and answers other requests by returning a list of the authoritative name servers for the appropriate top-level domain (TLD). The root name servers are a critical part of the Internet infrastructure because they are the first step in translating (resolving) human readable host names into IP addresses that are used in communication between Internet hosts.

A combination of limits in the DNS and certain protocols, namely the practical size of unfragmented User Datagram Protocol (UDP) packets, resulted in a decision to limit the number of root servers to thirteen server addresses. The use of anycast addressing permits the actual number of root server instances to be much larger, and is 632 as of 25 October 2016.

Root server addresses
There are 13 logical root name servers specified, with logical names in the form letter.root-servers.net, where letter 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 the User Datagram Protocol (UDP). 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 the EDNS0 extension to the DNS standard.

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 called anycast addressing, providing increased performance and even more fault tolerance. An informational homepage exists for every logical server (except G-Root) under the Root Server Technical Operations Association domain with web address in the form http://letter.root-servers.org/, where letter ranges from a to m.

Ten servers were originally in the United States; some 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.



There are also several alternative namespace systems with an alternative 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.

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 file as published by ICANN, and do not constitute an alternate root.

As the root name servers are an important part of the Internet, they have come under attack several times, although none of the attacks have ever been serious enough to severely affect the performance of the Internet.