Domain Name System.Of the many millions of the users of the Internet very few know of its origin, or why it appears in the now familiar format in which it appears. The following is a simple guide to the Domain system which explains how and why it works. The Editor The Domain Name System or DNS (Domain Name System) A system for converting host names and domain names into IP addresses on the Internet or on local networks that use the TCP/IP protocol. For example, when a Web site address is given to the DNS either by typing a URL in a browser or behind the is a system that stores information about host names and domain names in a kind of distributed database A database physically stored in two or more computer systems. Although geographically dispersed, a distributed database system manages and controls the entire database as a single collection of data. on networks, such as the Internet. Most importantly Adv. 1. most importantly - above and beyond all other consideration; "above all, you must be independent" above all, most especially , it provides an IP address for each host name, and lists the mail exchange servers accepting e-mail for each domain. The DNS provides a vital service on the Internet, because while computers and network hardware work with IP addresses to perform tasks such as addressing and routing, humans generally find it easier to work with host names and domain names, for example in URLs and e-mail addresses. The DNS therefore mediates between the needs and preferences of wetware A biological system. It typically refers to the human brain and nervous system. See liveware, grayware and wares. (jargon) wetware - /wet'weir/ (Probably from the novels of Rudy Rucker, or maybe Stanislav Lem) The human nervous system, as opposed to electronic computer and of software. A brief history of the DNS The practice of using a name as a more human-legible abstraction of a machine's address on the network predates even TCP/IP TCP/IP in full Transmission Control Protocol/Internet Protocol Standard Internet communications protocols that allow digital computers to communicate over long distances. , and goes back to the ARP Anet era. Originally, each computer on the network retrieved a file called HOSTS.TXT TXT Text TXT Text File (filename extension) TXT Textile TXT Teletext TXT Tecnologia per a Tothom TXT Textron Corporation (stock symbol) from SRI (now SRI International (company) SRI International - One of the world's largest contract research firms. Founded in 1946 in conjuction with Stanford University as the Stanford Research Institute, they later became fully independent and were incorporated as a non-profit organisation under U.S. ), which mapped an address to a name (technically, this file still exists - most modem operating systems Operating systems can be categorized by technology, ownership, licensing, working state, usage, and by many other characteristics. In practice, many of these groupings may overlap. either by default or through configuration, can check their Hosts file to match a URL URL in full Uniform Resource Locator Address of a resource on the Internet. The resource can be any type of file stored on a server, such as a Web page, a text file, a graphics file, or an application program. to an IP address before checking the DNS). However, such a system had inherent limitations, because of the obvious requirement that every time a given computer's address changed, every single system that wanted to communicate with that computer would need an update to its Hosts file. The growth of networking called for a more scalable system: one which recorded a change in a host's address only in one place, and in which other hosts would learn about the change dynamically. Enter the DNS. Paul Mockapetris Dr. Paul V. Mockapetris is the inventor of the Domain Name System. In 1983, he proposed a Domain Name System (DNS) architecture in RFCs 882 and 883 while at the Information Sciences Institute (ISI) of the University of Southern California. invented the DNS in 1983; the original specifications appear in RFC (Request For Comments) A document that describes the specifications for a recommended technology. Although the word "request" is in the title, if the specification is ratified, it becomes a standards document. 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 protocols. How the DNS works in theory Works in Theory is a radio program on SYN FM and the Community Radio Satellite in Australia. Overseas listeners can stream and send text messages live from SYN's website at www.syn.org. The practical operation of the DNS system consists of three players: * The DNS resolver, a DNS client program which runs on a user's computer, and which generates DNS requests on behalf of software programs; * The recursive See recursion. recursive - recursion DNS server A dedicated server or a service within a server that provides DNS name resolution in an IP network. It turns names for Web sites and network resources into numeric IP addresses. DNS servers are used in large companies, in all ISPs and within the DNS system in the Internet, a vital service , which searches through the DNS in response to queries from resolvers, and returns answers to those resolvers; and, * The authoritative DNS server which hands out answers to queries from recursors, either in the form of an answer, or in the form of a delegation (i.e. referral to another authoritative DNS server). Understanding the parts of a domain name A domain name usually consists of two or more parts (technically labels), separated by dots. * The rightinost label conveys the top-level domain (networking) top-level domain - The last and most significant component of an Internet fully qualified domain name, the part after the last ".". For example, host wombat.doc.ic.ac.uk is in top-level domain "uk" (for United Kingdom). (for example, the address www.wikipedia.org has the top-level domain org). * Each label to the left specifies a subdivision or subdomain of the domain above it. Note that "subdomain" expresses relative dependence, not absolute dependence: for example, wikipedia.org comprises a subdomain of the org domain, and en.wikipedia.org could form a subdomain of the domain wikipedia.org (in practice, however, en.wikipedia.org actually represents a hostname - see below). In theory, this subdivision can go down to 127 levels deep, and each label can contain up to 63 characters, as long as the whole domain name does not exceed a total length of 255 characters. But in practice some domain registries have shorter limits than that. * Finally, the leftmost left·most adj. Farthest to the left: in the leftmost lane of traffic. Adj. 1. leftmost - farthest to the left; "the leftmost non-zero digit" part of the domain name (usually) expresses the hostname. The rest of the domain name simply specifies a way of building a logical path to the information required; the hostname simply specifies a way of building a logical path to the information required; the host name is the actual target system for which an IP address is desired. For example, the domain name www.wikipedia.org has the hostname 'www' 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 In programming, the ability of a subroutine or program module to call itself. It is helpful for writing routines that solve problems by repeatedly processing the output of the same process. See recurse subdirectories. 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. * Before starting, the recursor has to know where to find the root servers; administrators of recursive DNS servers manually specify (and periodically update) a file called the root hints zone which specifies the IP addresses of these servers. * The process starts by the recursor asking one of these root servers - for example, the server with the IP address "198.41.0.4" - the question "what is the IP address for www.wikipedia.org?" * The root server replies with a delegation, meaning roughly: "I don't know Don't know (DK, DKed) "Don't know the trade." A Street expression used whenever one party lacks knowledge of a trade or receives conflicting instructions from the other party. the IP address of www.wikipedia.org, but I do know that the DNS server at 204.74.112.1 has information on the org domain." * The local DNS recursor then asks that DNS server (i.e. 204.74.112.1) the same question it had previously put to the root servers, i.e. "what is the IP address for www.wikipedia.org?". It gets a similar reply - essentially, "I don't know the address of www.wikipedia.org, but I do know that the DNS server at 207.142.131.234 has information on the wikipedia.org domain." * Finally the request goes to this third DNS server (207.142.131.234), which replies with the required IP address. This process utilises recursive searching. Understanding Domain registration and Glue records Reading the example above, you might reasonably wonder: "how does the DNS server 204.74.112.1 know what IP address to give out 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 only 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. Consequentially, 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. Usually, name servers appear listed by name, rather than by IP address. This generates another string of DNS requests to resolve the name of the name server, when an IP address of a name server has a registration at the parent zone, network programmers call this a glue record. DNS in practice When an application (such as a web browser The program that serves as your front end to the Web on the Internet. In order to view a site, you type its address (URL) into the browser's Location field; for example, www.computerlanguage.com, and the home page of that site is downloaded to you. ), wants 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 (1) (Time To Live) A parameter in a network packet that sets a time limit to its validity. In order to prevent an IP packet from propagating endlessly through the network, the value in the TTL field is reduced by each router. 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 until an administrator manually flushes the response from the resolver's memory) will the resolver contact the DNS server for the same information. Propagation time An important consequence of this distributed and caching architecture is that changes to the DNS are not necessarily immediately effective globally. This is best explained with an example: If an administrator has set a TTL of 6 hours for the host www.wikipedia.org, and then changes the IP address to which www.wikipedia.org resolves at 12:01pm, the administrator must consider that a person who cached a response with the old value at 12:00pm will not consult the DNS server again until 6:00pm. The period between 12:01pm and 6:00pm in this example is called propagation time, which is best defmed as a period of time that begins between whenever you make a change to a DNS record, and ends after the maximum amount of time specified by the TTL 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 (http://www.ietforg/rfc/rfc1537.txt) helps setting it. DNS in the real world In the real world, users do not interface directly with a DNS resolver--they interface with programs like web browsers The following is a list of web browsers. Historical Historically important browsers In order of release:
In North American Indian mythology, a powerful spirit in the form of a bird that watered the earth and made vegetation grow. Lightning was believed to flash from its eyes or beak, and the beating of its wings was thought to represent rolling thunder. etc.). When users make a request which requires a DNS lookup (in effect, virtually any request that uses the Internet), such programs send a request to the DNS resolver built into their operating system. The DNS resolver will almost invariably in·var·i·a·ble adj. Not changing or subject to change; constant. in·var  i·a·bil 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 Internet service provider Internet service provider (ISP)Company that provides Internet connections and services to individuals and organizations. For a monthly fee, ISPs provide computer users with a connection to their site (see data transmission), as well as a log-in name and password. to which the machine connects will usually supply this DNS server: such a user will either configure that server's address manually or allow DHCP (Dynamic Host Configuration Protocol) Software that automatically assigns temporary IP addresses to client stations logging into an IP network. It eliminates having to manually assign permanent "static" IP addresses. DHCP software runs in servers and routers. 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. 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 I minute. Other DNS applications The system outlined above provides a somewhat simplified scenario. The DNS includes several other functions: * Host names and IP addresses do not necessarily match on a one-to-one basis. Many host names may correspond to a single IP address: combined with virtual hosting, this allows a single machine to serve many web sites. Alternatively a single host name may correspond to many IP addresses: this can facilitate fault tolerance and load distribution, and also allows a site to move physical location seamlessly. * There are many uses of DNS besides translating names to IP addresses. For instance, Mail transfer agents use DNS to find out where to deliver E-mail for a particular address. The domain to mail exchanger mapping provided by MX records accommodates another layer of fault tolerance and load distribution on top of the name to IP address mapping. * Sender Policy Framework See SPF. controversially takes advantage of a DNS record type, the TXT record. * To provide resilience in the event of computer failure, multiple DNS servers provide coverage of each domain. In particular, thirteen root servers exist worldwide. DNS programs or operating systems have the IP addresses of these servers built in. The USA hosts, at least nominally, all but three of the root servers. However, because many root servers actually implement anycast, where many different computers can share the same IP address to deliver a single service over a large geographic region, most of the physical (rather than nominal) root servers now operate outside the USA. The DNS uses TCP (1) (Transmission Control Protocol) The reliable transport protocol within the TCP/IP protocol suite. TCP ensures that all data arrive accurately and 100% intact at the other end. and UDP UDP (uridine diphosphate): see uracil. (User Datagram Protocol) A protocol within the TCP/IP protocol suite that is used in place of TCP when a reliable delivery is not required. on port 53 to serve requests. Almost all DNS queries consist of a single LJDP 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. Types of DNS records Important categories of data stored in the DNS include the following: * An A record or address record maps a host name to its 32-bit IPv4 address. * An AAAA record or IPv6 address record maps a host name to its 128-bit IPv6 address. * A CNAME record or canonical name record makes one domain name an alias of another. The aliased domain gets all the subdomains and DNS records of the original. * An MX record or mail exchange record maps a domain name to a list of mail exchange servers for that domain. * A PTR record or pointer record maps a host name to the canonical name for that host. Setting up a PTR record for a host name in the in-addr.arpa domain that corresponds to an IP address implements reverse DNS lookup This article is about the Network process of Reverse DNS lookup. For the Java-like naming convention, see Reverse-DNS. Reverse DNS lookup (rDNS) is a process to determine the hostname or host associated with a given IP address or host address. for that address. For example (at the time of writing), www.icann.net has the IP address 192.0.34.164, but a PTR record maps 164.34.0.1 92. in-addr.arpa to its canonical name, referrals.icann.org. * An NS record or name server record maps a domain name to a list of DNS servers for that domain. Delegations depend on NS records. * An SOA (1) (Start Of Authority) The first record in a DNS zone file. See DNS records. (2) (Service Oriented Architecture) The modularization of business functions for greater flexibility and reusability. record or start ofauthority record specifies the DNS server providing authoritative information about an Internet domain. * A SRV record is a generalized service location record. * A TXT record allows an administrator to insert arbitrary text into a DNS record; this record is also used in the Sender Policy Framework specification. Other kinds of records simply provide information (for example, a LOC record gives the physical location of a host), or experimental data (for example, a WKS Lotus 1-2-3, Version 1A file extension. record gives a list of servers offering some well-known service such as HTTP or POP3 for a domain). Internationalised domain names Domain names must use only a subset of ASCII characters, preventing many languages from representing their names and words natively. ICANN has approved the Punycode-based IDNA IDNA Internationalized Domain Names in Applications (IETF) IDNA Integral Depletable Neutron Absorber IDNA Intelligent Dual Nano Architecture system, which maps Unicode strings into the valid DNS character set, as a workaround (jargon, programming) workaround - A temporary kluge used to bypass, mask or otherwise avoid a bug or misfeature in some system. Customers often find themselves living with workarounds for long periods of time rather than getting a bug fix. to this issue, and some registries have adopted IDNA. DNS software Various flavors of DNS software implement the DNS, including: * BIND (Berkeley Internet Name Domain (networking) Berkeley Internet Name Domain - (BIND) An implementation of a DNS server developed and distributed by the University of California at Berkeley. Many Internet hosts run BIND, and it is the ancestor of many commercial implementations. ) * djbdns (Daniel J.Bernstein's DNS) * MaraDNS * QIP QIP Quantum Information Processing QIP Quality Improvement Program QIP Quality Improvement Plan QIP Quality Imaging Products QIP Quality Insights of Pennsylvania QIP Quality Improvement Process QIP Quality Improvement Paradigm QIP Quantum Industrial Partners LDC (Lucent Technologies) * NSD NSD Nairobi sheep disease. (Name Server Daemon Pronounced "dee-mun" as in the word "demon," it is a Unix program that executes in the background ready to perform an operation when required. Functioning like an extension to the operating system, a daemon is usually an unattended process that is initiated at startup. ) * PowerDNS * Microsoft DNS (in the server editions of Windows 2000 and Windows 2003) 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 (1) (Network Interface Card) See network adapter. See also InterNIC. (2) (New Internet Computer) An earlier Linux-based computer from The New Internet Computer Company (NICC), Palo Alto, CA. ), or domain name registry A domain name registry, also called Network Information Centre (NIC), is part of the Domain Name System (DNS) of the Internet which converts domain names to IP addresses. . 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 domains (ccTLDs), the domain registries hold the authoritative WHOIS (Registrant, name servers, expiry dates etc). For instance, DENIC DENIC Deutsches Network Information Center , Germany NIC holds the authoritative WHOIS to a.DE domain name. However, some domain registries, such as VeriSign, use a registry-registrar model. 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 servers, expiry dates etc) at the registrars. Since about 2001, most GTLD (Generic Top-Level Domain) A top-level domain category on the Internet such as .com, .edu and .net. In 1997, the Internet Ad Hoc Committee (IAHC) suggested seven new gTLDs (.firm, .store, .web, .arts, .rec, .info & .nom) and recommended that .com, .net and . 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. Management functions delegated to the administrative contacts may include (for example): * the obligation to conform to the requirements of the domain registry in order to retain the right to use a domain name * authorisation to update the physical address, email address and telephone number etc in WHOIS Technical Contact A technical contact manages the name servers of a domain name. The many functions of a technical contact include: * making sure the configurations of the domain name conforms to the requirements of the domain registry * updating the domain zone * providing the 24X7 functionality of the name servers (that leads to the accessibility of the domain name) Billing Contact Self-explanatory, the party whom a NIC invoices Name Servers Namely the authoritative name servers 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 domains. The international body ICANN (the Internet Corporation for Assigned Names and Numbers See ICANN. (body, networking) Internet Corporation for Assigned Names and Numbers - (ICANN) The non-profit corporation that was formed to assume responsibility for IP address allocation, protocol parameter assignment, domain name system management, and root server system ) oversees the domain name industry. U.S. Truth in Domain Names Act The U.S. "Truth in Domain Names Act", in combination with the PROTECT Act, forbids knowingly using a misleading domain name with the intent of attracting people into viewing a visual depiction of sexually explicit conduct on the Internet. See also * cybersquatting Registering an Internet domain name for the purpose of reselling it for a profit. One of the more notable transactions was the domain name wallstreet.com, which was registered in 1994 for $70 and sold for one million in 1999. * dynamic DNS * DNSSEC (DNS SECurity) A set of extensions to the DNS system that are designed to prevent attacks agains the DNS system as well as DNS hijacking, which directs the user to an erroneous Web site. DNSSec uses a digital signature to ensure that the correct IP address is used. * ICANN * Root nameserver External links and documentation * DNS Setup & Configuration (http://www.adminschoice.com/docs/domain_name_service.htm) * DNS Resources Directory (http://www.dns.net/dnsrdl) * DNS & BIND Resources (htt://www.bind9.net/) * Understanding the Domain Name System (http://www.brainbliss.com/cat02/art05.html) * CircleID DNS Community (http://www.circlecom) * DNS Security Extensions (DNSSEC) (http://www.dnssec.net * root-servers.org (http://www.root-servers.org/) * RFC 1034 - domain names - concepts and facilities (http://ietf/org/rfc/IO34) * RFC 1035 - domain names - implementation and specification (http://ietf/org/rfc/rfc1035) * OpenNIC: A democratic and open alternative to the traditional DNS system (http:www.opennic.unrated.net) * DNS Poisoning, a practical example (http://keti.froyn.name/poison.html) * DNS Forgery (http://cryptoldjbdns/forgery.html) * DNS Forgery (http://wiki.hping.org/l42) * Sites supporting DNS LOC LOC - lines of code (http://www.ckdhr.com/dns-loc/sites.html * How to block ads using DNS and hosts file (http://www.mvps.org/winhelp2002/hosts.htm) * Signposts in Cyberspace: The Domain Name System and Internet Navigation (PDF (Portable Document Format) The de facto standard for document publishing from Adobe. On the Web, there are countless brochures, data sheets, white papers and technical manuals in the PDF format. format) (http://www.nap.edu/execsumm_pdf/11258.pdf) * Simple online lookup tools such as whois, reverse whois, and A and MX records, etc. (http://www.hashemian.com/toolsl) * DNS for Rocket Scientists by zytramcom (http://www.zytrax.coml/books/dns/) Retrieved from "http://en.wikipedia.org/wiki/Domain_Name_System" Categories: Internet standards | Internet protocols | Domain Name System Contact the editor at e-patterson@bt.connect.com |
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