Root nameserver

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The AMS-IX mirror of the K root-server.

A root name server is a DNS server that answers requests for the DNS root zone, and redirects requests for a particular top-level domain (TLD) to that TLD's nameservers. 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 namespace domain for the Internet's official global implementation of the Domain Name System.

All domain names on the Internet can be regarded as ending in a full stop character e.g. "". This final dot is generally implied rather than explicit, as modern DNS software does not actually require that the final dot be included when attempting to translate a domain name to an IP address. The empty string after the final dot is called the root domain, and all other domains (.com, .org, .net, and so on) are contained within the root domain.

When a computer on the Internet wants to resolve a domain name, it works from right to left, asking each name server in turn about the element to its left. The root name servers (which have responsibility for the . domain) know which servers are responsible for the top-level domains. Each top-level domain (such as .com) has its own set of servers, which in turn delegate to the name servers responsible for individual domain names (such as, which in turn answer queries for IP addresses of subdomains or hosts (such as www).

In practice, most of this information does not change very often and gets cached, and necessary DNS lookups to the root nameservers are relatively rare. A survey from 2003 [1] found 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. Some misconfigured desktop computers even tried to update the root server records for the TLDs, which is incorrect. A similar list of observed problems and recommended fixes can be found in RFC 4697.


[edit] Root server addresses

There are currently 13 root name servers specified, with names in the form, where letter ranges from A to M. (Seven of these are not actual single servers, but represent several physical servers each in multiple geographical locations; cf. below.):

Letter IPv4 address IPv6 address Old name Operator Location Software
A 2001:503:BA3E::2:30 VeriSign Dulles, Virginia, U.S. BIND
B 2001:478:65::53 USC-ISI Marina Del Rey, California, U.S. BIND
C Cogent Communications distributed using anycast BIND
D University of Maryland College Park, Maryland, U.S. BIND
E NASA Mountain View, California, U.S. BIND
F 2001:500:2f::f ISC distributed using anycast BIND
G Defense Information Systems Agency Columbus, Ohio, U.S. BIND
H 2001:500:1::803f:235 U.S. Army Research Lab Aberdeen Proving Ground, Maryland, U.S. NSD
I 2001:7fe::53 (testing) Autonomica distributed using anycast BIND
J 2001:503:C27::2:30 VeriSign distributed using anycast BIND
K 2001:7fd::1 RIPE NCC distributed using anycast NSD
L (since November 2007; was[2] 2001:500:3::42 ICANN distributed using anycast NSD
M 2001:dc3::35 WIDE Project distributed using anycast BIND

Older servers had their own name before the policy of using similar names was established.

The choice of 13 nameservers was made because of limitations in the original DNS specification, which specifies a maximum packet size of 512 bytes using the User Datagram Protocol (UDP).[3] This restriction existed because the minimum IP packet size that was required to be transmitted without fragmentation was 576 bytes. The addition of IPv6 addresses for the root nameservers requires more than 512 bytes, which is facilitated by the EDNS0 extension to the DNS standard.[4] While only 13 names are used for the root namesevers, there are many more physical servers; C, F, I, J, K, L and M servers now exist in multiple locations on different continents, using anycast address announcements to provide decentralized service. As a result most of the physical root servers are now outside the United States, allowing for high performance worldwide.

At the end of 2006 there was a total of 123 root nameservers, including Anycast servers.

There are also quite a few alternative namespace systems with their own set of root nameservers that exist in opposition to the mainstream nameservers. The first, AlterNIC, generated a substantial amount of press. See Alternative DNS root for more information.

Root name servers may also be run locally, on provider or other types of networks, synchronized with the US Department of Commerce delegated root zone file as published by ICANN. Such a server is not an alternative root, but a local implementation of A through M.

As the root nameservers function as 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 hamper the performance of the Internet.

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