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A DNS root zone is the top level of the Domain Name System (DNS) hierarchy for a given DNS system. The term, when not otherwise qualified, is generally used to refer to the root zone of the largest global DNS system deployed on the Internet. This "official" DNS system is by far the largest deployment of DNS in the world. The zone is managed by the Internet Assigned Numbers Authority, a role performed by ICANN. A combination of limits in the DNS and IP protocols, namely the practical size of unfragmented User Datagram Protocol (UDP) packets, resulted in a limited number of root server addresses that can be accommodated in DNS name query responses. This limit has determined the number of name server installation at currently 13 clusters, serving the needs of the entire public Internet world-wide.
[edit] Parent zonesAny DNS zone can become a parent zone if it delegates a portion of its name space to at least one subordinate zone, or sub-domain. The DNS root zone is the parent zone of all top-level domains of the Internet. A parent zone is responsible for defining the points of delegation to the sub-zones and ensuring the availability of a server to respond with the appropriate referrals (name server records) to those sub-zones, including updating the referral information upon request from the sub-zones in a timely fashion. [edit] Technical details of root server lookupThere are thirteen root servers that are authoritative for queries to the global DNS root zone. The root servers hold the list of addresses for the authoritative servers for the top-level domains. Every name lookup must either start with an access to a root server, or use information that was once obtained from a root server. The root servers have the official names a.root-servers.net to m.root-servers.net. However, to look up the IP address of a root server from these names, you must first be able to look up a root server, to find the address of an authoritative server for the .net DNS zone. Clearly this creates a paradox, so the address of at least one root server needs to be known by a host in order to bootstrap access to the DNS system. This is usually done by shipping the addresses of all known root DNS servers as a file with the computer operating system: the IP addresses of some root servers will change over the years, but only one correct address is needed for the lookup process to complete. This file is called named.cache in the BIND nameserver reference implementation and a current version is officially distrubuted by ICANN's InterNIC website.[1] Once the address of a single functioning root server is known, the rest of the DNS information can be discovered recursively, and the address of any machine on the Internet can be looked up. An additional level of redundancy is provided by the fact that a single root server name, and its corresponding IP address, may correspond to many physical servers around the world, using a method called anycast addressing. [edit] Redundancy and diversityThe root DNS servers are essential to the function of the Internet, as most Internet services, such the World-Wide Web and electronic mail, are based on domain names, either directly or indirectly. They are potential points of failure for the entire Internet. For this reason, there are multiple named root servers worldwide. The number has been limited to 13, because a single IP packet can only be guaranteed to be unfragmented to a limit of 576 bytes (in IPv4). While it is possible to fit 15 entries into a packet of this size, 13 was chosen as a reliable limit. Since the advent of IPv6, the next generation IP address structure, 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. Initially all of these installations were based in the United States, however the distribution has dramatically shifted and this is no longer the case. Usually each DNS server in a given site is actually a cluster of servers behind a load-balancing set of routers. A comprehensive list of servers, their locations and properties, can be found at [1]. 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 root server, maintained by Verisign, is represented by 41 (as of July 2008) individual server systems located around the world using anycast addressing. [edit] See also[edit] References
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