21-05-2014, 04:00 PM
DOMAIN NAME SYSTEM
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The DNS translates Internet domain and host names to IP addresses. DNS automatically converts the names we type in our Web browser address bar to the IP addresses of Web servers hosting those sites.
DNS implements a distributed database to store this name and address information for all public hosts on the Internet. DNS assumes IP addresses do not change (are statically assigned rather than dynamically assigned).
The DNS database resides on a hierarchy of special database servers. When clients like Web browsers issue requests involving Internet host names, a piece of software called the DNS resolver (usually built into the network operating system) first contacts a DNS server to determine the server's IP address. If the DNS server does not contain the needed mapping, it will in turn forward the request to a different DNS server at the next higher level in the hierarchy. After potentially several forwarding and delegation messages are sent within the DNS hierarchy, the IP address for the given host eventually arrives at the resolver, that in turn completes the request over Internet Protocol.
DNS additionally includes support for caching requests and for redundancy. Most network operating systems support configuration of primary, secondary, and tertiary DNS servers, each of which can service initial requests from clients. Internet Service Providers (ISPs) maintain their own DNS servers and use DHCP to automatically configure clients, relieving most home users of the burden of DNS configuration.
The Internet, and many larger private Internet Protocol (IP) networks, rely on the Domain Name System (DNS) to help direct traffic. The DNS maintains a distributed database of network names and addresses
and it provides methods for computers to remotely query the database. Some people call DNS the "phone book of the Internet."
DNS and the World Wide Web
All public Web sites run on servers connected to the Internet with public IP addresses. The Web servers at About.com, for example, have addresses like 207.241.148.80. Although people can type address information like http://207.241.148.80/ into their Web browser to visit sites, being able to use proper names like http://www.about is much more practical.
The Internet utilizes DNS as a worldwide name resolution service for public Web sites. When someone types a site's name into their browser, DNS looks up the corresponding IP address for that site, the data required to make the desired network connections between Web browsers and Web servers.
DNS Servers and Name Hierarchy
DNS uses a client/server network architecture. DNS servers are the computers designated to store DNS database records (names and addresses), while clients of the DNS include PCs, phones and other devices of end users. DNS servers also interface with each other, acting as clients to each other when needed.
The DNS organizes its servers into a hierarchy. For the Internet, so-called root name servers reside at the top of the DNS hierarchy. The Internet root name servers manage DNS server information for the Web's top-level domains (TLD) (like ".com" and ".uk"), specifically the names and IP addresses of the original (called authoritative) DNS servers responsible for answering queries about each TLD individually. Servers at the next lower level of the DNS hierarchy track second-level domain names and addresses (like "about.com") , and additional levels manage Web domains (like "compnetworking.about.com").
DNS servers are installed and maintained by private businesses and Internet governing bodies around the world. For the Internet, 13 root name servers (actually redundant pools of machines around the world) support the hundreds of Internet top-level domains, while About.com provides authoritative DNS server information for the sites within its network. Organizations can similarly deploy DNS on their private networks separately, on the smaller scale.
Domain name space
The domain name space consists of a tree of domain names. Each node or leaf in the tree has zero or more resource records, which hold information associated with the domain name. The tree sub-divides into zones beginning at the root zone. A DNS zone may consist of only one domain, or may consist of many domains and sub-domains, depending on the administrative authority delegated to the manager.
Domain name syntax
The definitive descriptions of the rules for forming domain names appear in RFC 1035, RFC 1123, and RFC 2181. A domain name consists of one or more parts, technically called labels, that are conventionally concatenated, and delimited by dots, such as example.com.
• The right-most label conveys the top-level domain; for example, the domain name www.example.com belongs to the top-level domain com.
• The hierarchy of domains descends from right to left; each label to the left specifies a subdivision, or subdomain of the domain to the right. For example: the label example specifies a subdomain of the com domain, and www is a sub domain of example.com. This tree of subdivisions may have up to 127 levels.
• Each label may contain up to 63 characters. The full domain name may not exceed the length of 253 characters in its textual representation.[1] In the internal binary representation of the DNS the maximum length requires 255 octets of storage, since it also stores the length of the name.[3] In practice, some domain registries may have shorter limits.[citation needed]
• DNS names may technically consist of any character representable in an octet. However, the allowed formulation of domain names in the DNS root zone, and most other sub domains, uses a preferred format and character set. The characters allowed in a label are a subset of the ASCII character set, and includes the characters a through z, A through Z, digits 0 through 9, and the hyphen. This rule is known as the LDH rule (letters, digits, hyphen). Domain names are interpreted in case-independent manner.[8] Labels may not start or end with a hyphen.[9] There is an additional rule that essentially requires that top-level domain names not be all-numeric.[10]
• A hostname is a domain name that has at least one IP address associated. For example, the domain names www.example.com and example.com are also hostnames, whereas the com domain is not.
Internationalized domain names
The permitted character set of the DNS prevented the representation of names and words of many languages in their native alphabets or scripts. ICANN has approved the Internationalizing Domain Names in Applications (IDNA) system, which maps Unicode strings into the valid DNS character set using Punycode. In 2009 ICANN approved the installation of IDN country code top-level domains. In addition, many registries of the existing top level domain names (TLD)s have adopted IDNA.
Name servers
The Domain Name System is maintained by a distributed database system, which uses the client-server model. The nodes of this database are the name servers. Each domain has at least one authoritative DNS server that publishes information about that domain and the name servers of any domains subordinate to it. The top of the hierarchy is served by the root nameservers, the servers to query when looking up (resolving) a TLD.
Authoritative name server
An authoritative name server is a name server that gives answers that have been configured by an original source, for example, the domain administrator or by dynamic DNS methods, in contrast to answers that were obtained via a regular DNS query to another name server. An authoritative-only name server only returns answers to queries about domain names that have been specifically configured by the administrator.
An authoritative name server can either be a master server or a slave server. A master server is a server that stores the original (master) copies of all zone records. A slave server uses an automatic updating mechanism of the DNS protocol in communication with its master to maintain an identical copy of the master records.
A set of authoritative name servers has to be assigned for every DNS zone. An NS record about addresses of that set must be stored in the parent zone and servers themselves (as self-reference).
When domain names are registered with a domain name registrar, their installation at the domain registry of a top level domain requires the assignment of a primary name server and at least one secondary name server. The requirement of multiple name servers aims to make the domain still functional even if one name server becomes inaccessible or inoperable.[11] The designation of a primary name server is solely determined by the priority given to the domain name registrar. For this purpose, generally only the fully qualified domain name of the name server is required, unless the servers are contained in the registered domain, in which case the corresponding IP address is needed as well.
Primary name servers are often master name servers, while secondary name servers may be implemented as slave servers.
An authoritative server indicates its status of supplying definitive answers, deemed authoritative, by setting a software flag (a protocol structure bit), called the Authoritative Answer (AA) bit in its responses.] This flag is usually reproduced prominently in the output of DNS administration query tools (such as dig) to indicate that the responding name server is an authority for the domain name in question.
DNS Caches
To better process high volumes of requests, the DNS utilizes caching. DNS caches store local copies of recently-accessed DNS records while the originals continue to be maintained on their designated servers. Having local copies of DNS records avoids having to generate network traffic up and through the DNS server hierarchy. However, if a DNS cache becomes outdated, network connectivity issues can result. DNS caches have also been prone to attack by network hackers. Network administrators can flush a DNS cache if needed using ipconfig and similar utilities.
Dynamic DNS
Standard DNS requires all IP address information stored in the database to be fixed. This works fine for supporting typical Web sites but not for devices using dynamic IP addresses such as Internet Web cams or home Web servers. Dynamic DNS (DDNS) adds network protocol extensions to DNS to enable name resolution service for dynamic clients.
Various third-party providers offer dynamic DNS packages designed for those wanting to remotely access their home network via the Internet. Setting up an Internet DDNS environment requires signing up with the chosen provider and installing additional software on the local network. The DDNS provider remotely monitors subscribed devices and makes the required DNS name server updates.
Alternatives to DNS
The Microsoft Windows Internet Naming Service (WINS) supports name resolution similar to DNS but works only on Windows computers and using a different name space. WINS is used on some private networks of Windows PCs.
Dot-BIT is an open source project based based on BitCoin technology that is working to add support for a ".bit" top-level domain to the Internet DNS.