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The Domain Name System or Domain Name Server ('''DNS''') is a system that stores information associated with ''' Domain Name s''' in a Distributed Database on networks, such as the Internet . The domain name system (Domain Name Server) associates many types of information with domain names, but most importantly, it provides the IP Address associated with the domain name. It also lists Mail Exchange Server s accepting E-mail for each domain. In providing a worldwide keyword-based redirection service, DNS is an essential component of contemporary Internet use. DNS is useful for several reasons. Most well known, the DNS makes it possible to attach hard-to-remember IP addresses (such as 207.142.131.206) to easy-to-remember domain names (such as "wikipedia.org"). Humans take advantage of this when they recite URL s and E-mail Address es. Less recognized, the domain name system makes it possible for people to assign authoritative names, without needing to communicate with a central registrar each time. A BRIEF HISTORY OF THE DNS The growth of networking called for a more scalable system: one which recorded a change in a host's address in one place only. Other hosts would learn about the change dynamically through a notification system, thus completing a globally accessible network of all hosts' names and their associated IP Addresses. Enter the DNS. Paul Mockapetris invented the DNS in 1983 ; the original specifications appear in RFC 882 and 883. In 1987 , the publication of RFC 1034 and RFC 1035 updated the DNS specification and made RFC 882 and RFC 883 obsolete. Several more recent RFCs have proposed various extensions to the core DNS protocols. HOW THE DNS WORKS IN THEORY The domain name space is a Tree of domain names. Each node or leaf in the tree is associated with resource records, which hold the information associated with the domain name. The tree is divided into '''zones'''. A zone is a collection of connected nodes that are authoritatively served by an '''authoritative DNS nameserver'''. (Note that a single nameserver can host several zones.) When a system administrator wants to let another administrator control a part of the domain name space within his or her zone of authority, he or she can delegate control to the other administrator. This splits a part of the old zone off into a new zone, which is served by the second administrator's nameservers. The old zone is no longer authoritative for what is under the authority of the new zone. The information associated with nodes is looked up by a resolver. A resolver knows how to communicate with name servers by sending DNS requests, and heeding DNS responses. Resolving usually entails '''recursing''' through several name servers to find the needed information. Some resolvers are simple, and can only communicate with a single name server. These simple resolvers rely on a recursing name server to perform the work of finding information for them. Understanding the parts of a domain name A Domain Name usually consists of two or more parts (technically ''labels''), separated by dots. For example wikipedia.org.
The DNS consists of a hierarchical set of DNS servers. Each domain or subdomain has one or more '''authoritative DNS servers''' that publish information about that domain and the name servers of any domains "beneath" it. The hierarchy of authoritative DNS servers matches the hierarchy of domains. At the top of the hierarchy stand the ''' Root Servers ''': the servers to query when looking up ('''resolving''') a top-level domain name. An example of theoretical DNS recursion An example may clarify this process. Suppose an application needs to find the IP address of www.wikipedia.org. It puts this question to a local DNS recursor.
This process utilizes Recursive Searching . Understanding domain registration and glue records Reading the example above, one might reasonably wonder how DNS server 204.74.112.1 knows what IP address to give out for the authoritative DNS server for the wikipedia.org domain. In the first step of the process, we noted that a DNS recursor has the IP addresses of the root servers more-or-less hard coded. Equally, the name servers that are authoritative for the Top-Level Domains change very infrequently. However, the name servers that provide authoritative answers for common domain names may change relatively often. As part of the process of registering a domain name (and at any time thereafter), a registrant provides the registry with the name servers that will be authoritative for that domain name; therefore, when registering wikipedia.org, that domain is associated with the name servers gunther.bomis.com and zwinger.wikipedia.org at the .org registry. Consequently, in the example above, when the server identified by 204.74.112.1 receives a request, the DNS server scans its list of domains, locates wikipedia.org, and returns the name servers associated with that domain. Name servers in delegations are listed by name, rather than by IP address. This means that a resolving name server must issue another DNS request to find out the IP address of the server to which it has been referred. Since this can introduce a Bootstrapping problem when the name of the nameserver is in the domain about which nothing is yet known, it is occasionally necessary for the nameserver providing the delegation to also provide the IP address of the next nameserver. This record is called a glue record. DNS IN PRACTICE When an application (such as a Web Browser ) tries to find the IP address of a domain name, it doesn't necessarily follow all of the steps outlined in the ''Theory'' section above. We will first look at the concept of caching, then outline the operation of DNS in "the real world". Caching and time to live Because of the huge volume of requests generated by a system like the DNS, the designers wished to provide a mechanism to reduce the load on individual DNS servers. The mechanism devised provided that when a DNS resolver (i.e. client) received a DNS response, it would Cache that response for a given period of time. A value (set by the administrator of the DNS server handing out the response) called the '''time to live''', or TTL defines that period of time. Once a response goes into cache, the resolver will consult its cached (stored) answer; only when the TTL expires (or when an administrator manually flushes the response from the resolver's memory) will the resolver contact the DNS server for the same information. Generally, the time to live is specified in the Start of Authority (SOA) record. SOA parameters are:
(Newer versions of ''named'' will accept 'M','H','D' & 'W' suffixes indicating that the time interval is respectively in Minutes, Hours, Days and Weeks). Caching time A noteworthy consequence of this distributed and caching architecture is that changes to the DNS are not always immediately effective globally. This is best explained with an example: If an administrator has set a expires. This essentially leads to an important logistical consideration when making changes to the DNS: ''not everyone is necessarily seeing the same thing you're seeing''. RFC1537 helps to convey basic rules for how to set the TTL. Note that the term "propagation", although very widely used, is a poor term to describe the effects of caching. Specifically, it implies that when you make a DNS change, it somehow spreads to all other DNS servers (instead, other DNS servers check in with yours as needed), and [2 that you do not have control over the amount of time the record is cached (you have complete control for all DNS records on your domain, except your NS records and any authoritative DNS servers that use your domain name). Many people incorrectly refer to a mysterious 48 hour or 72 hour propagation time when you make a DNS change. When you change the NS records for your domain or the IP addresses for hostnames of authoritative DNS servers using your domain (if any), there can be a lengthy period of time before all DNS servers use the new information. This is because those records are handled by the zone parent DNS servers (for example, the .com DNS servers if your domain is example.com), which typically cache those records for 48 hours. However, those DNS changes will be immediately available for any DNS servers that do not have them cached. And, any DNS changes on your domain other than the NS records and authoritative DNS server names can be nearly instantaneous, if you choose for them to be (by lowering the TTL once or twice ahead of time, and waiting until the old TTL expires before making the change). DNS in the real world Users generally do not communicate directly with a DNS resolver. Instead DNS resolution is handled transparently via client applications such as web browsers ( Internet Explorer , Mozilla Firefox , Safari , Opera , etc), mail clients ( Outlook Express , Mozilla Thunderbird , etc), and other Internet applications. When a request is made which necessitates a DNS lookup, such programs send a resolution request to the local DNS resolver in the operating system which in turn handles the communications required. The DNS resolver will almost invariably have a cache (see above) containing recent lookups. If the cache can provide the answer to the request, the resolver will return the value in the cache to the program that made the request. If the cache does not contain the answer, the resolver will send the request to a designated DNS server or servers. In the case of most home users, the to set it; however, where systems administrators have configured systems to use their own DNS servers, their DNS resolvers will generally point to their own nameservers. This name server will then follow the process outlined above in ''DNS in theory'', until it either successfully finds a result, or does not. It then returns its results to the DNS resolver; assuming it has found a result, the resolver duly caches that result for future use, and hands the result back to the software which initiated the request. Broken resolvers An additional level of complexity is introduced when resolvers violate the rules of the DNS protocol. A number of large ISPs have configured their DNS servers to violate rules (presumably to allow them to run on less-expensive hardware than a fully-compliant resolver), such as disobey TTLs, or indicate a domain name does not exist just because one of its name servers does not respond. As a final level of complexity, some applications such as Web browsers also have their own DNS cache, in order to reduce use of the DNS resolver library itself, which can add extra difficulty to DNS debugging, as it obscures which data is fresh, or lies in which cache. These caches typically have very short caching times of the order of 1 minute. A notable exception is Internet Explorer ; recent versions cache DNS records for 30 minutes.1 Other DNS applications The system outlined above provides a somewhat simplified scenario. The DNS includes several other functions:
The DNS uses TCP and UDP on Port 53 to serve requests. Almost all DNS queries consist of a single UDP request from the client followed by a single UDP reply from the server. TCP typically comes into play only when the response data size exceeds 512 bytes, or for such tasks as Zone Transfer . Some operating systems such as HP-UX are known to have resolver implementations that use TCP for all queries, even when UDP would suffice. STANDARDS
TYPES OF DNS RECORDS Important categories of data stored in the DNS include the following:
Other types of records simply provide information (for example, a LOC Record gives the physical ''location'' of a host), or experimental data (for example, a WKS record gives a list of servers offering some ''well known service'' such as HTTP or POP3 for a domain). INTERNATIONALIZED DOMAIN NAMES See Also: Internationalized domain name Domain names must use only a subset of ASCII characters—the Roman Alphabet in upper and lower case, the digits 0 through 9, and the Hyphen . This prevented the representation of names and words of many languages natively. ICANN has approved the Punycode -based IDNA system, which maps Unicode strings into the valid DNS character set, as a workaround to this issue. Some Registries have adopted IDNA. DNS SOFTWARE DNS-oriented Utilities include:
LEGAL USERS OF DOMAINS Registrant No one in the world really "owns" a domain name except the Network Information Centre (NIC), or Domain Name Registry . Most of the NICs in the world receive an annual fee from a legal user in order for the legal user to utilise the domain name (i.e. a sort of a leasing agreement exists, subject to the registry's terms and conditions). Depending on the various naming convention of the registries, legal users become commonly known as "registrants" or as "domain holders". ICANN holds a complete list of domain registries in the world. One can find the legal user of a domain name by looking in the WHOIS database held by most domain registries. For most of the more than 240 Country Code Top-level Domain s (ccTLDs), the domain registries hold the authoritative WHOIS (Registrant, name servers, expiry dates etc). For instance, DENIC , Germany NIC holds the authoritative WHOIS to a .DE domain name. However, some domain registries, such as VeriSign , use a registry-registrar model. There are hundreds of Domain Name Registrars that actually perform the domain name registration with the end-user, such as ENom . By using this method of distribution, the registry only has to manage the relationship with the registrar, and the registrar maintains the relationship with the end-users, or 'registrants'. For .COM, .NET domain names, the domain registries, VeriSign holds a basic WHOIS (registrar and name servers etc). One can find the detailed WHOIS (Registrant, Name Server s, expiry dates etc) at the registrars. Since about 2001, most GTLD registries (.ORG, .BIZ, .INFO) have adopted a so-called "thick" registry approach, i.e. keeping the authoritative WHOIS with the various registries instead of the registrars. Administrative contact A registrant usually designates an administrative contact to manage the domain name. In practice, the administrative contact usually has the most immediate power over a domain. Management functions delegated to the administrative contacts may include (for example):
Technical contact A technical contact manages the name servers of a domain name. The many functions of a technical contact include:
Billing contact Self-explanatory, the party whom a NIC invoices. Name servers Namely the authoritative Name Server s that host the domain name zone of a domain name. POLITICS Many investigators have voiced criticism of the methods used currently to control ownership of domains. Most commonly, critics claim abuse by monopolies or near-monopolies, such as VeriSign , Inc., and problems with assignment of Top-level Domain s. The international body ICANN (the Internet Corporation for Assigned Names and Numbers) oversees the domain name Industry . Truth in Domain Names Act In the United States , the "Truth in Domain Names Act", in combination with the PROTECT Act , forbids the use of a misleading domain name with the intention of attracting people into viewing a Visual Depiction Of Sexually Explicit Conduct on the Internet SEE ALSO
REFERENCES EXTERNAL LINKS AND DOCUMENTATION
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