The Domain Name System (DNS) is one of the most important services in the Internet. Major task is the dissolution of names to answer i.e. on name inquiries with the associated IP address.

The DNS is a hierachische data base distributed world-wide on thousands of servers, which administers the name area of the Internets. This name area is divided into so-called zones, for which independent in each case administrators are responsible. For local requirements - approximately within a Firmenetzes - it is to be operated also possible, a DNS independent of the Internet.

Mainly the DNS is used for the conversion by Domainnamen in IP addresses (forward lookup). This is comparable with a directory, which dissolves the names of the participants into their telephone number. The DNS offers thus a simplification, because humans can note names by far better than number columns. So one can note the Domainnamen www.wikipedia.org more easily, than the appropriate IP address 145.97.39.155.

A further advantage is that IP addresses - for instance of Web servers - can be changed relatively without risk. There Internet participants only (unchanged) the DNS names respond, remain for them changes of the subordinated IP level as far as possible hidden. Since also several IP addresses can be assigned to a name, even a rudimentary distribution of load (load Balancing) can be realized.

With the DNS also a reverse dissolution of IP addresses is in name (reverse lookup) possible. In analogy to the directory this corresponds to a search for the name of a participant to a well-known call number, what admits inverse search within the telecommunications industry under the name is.

The DNS was sketched to 1983 by Paul Mockapetris and described in the RFC 882. The RFC 882 in the meantime from RFC 1034 and RFC 1035 one replaced and one supplemented by numerous further standards. Original task was it to replace the local host files which were responsible for the dissolution of name up to then and which had not any longer been up to to the exponentially increasing number of new entries. Due to the prove-measured high reliability and flexibility, gradually further volume of data was integrated into the DNS and put so to the Internet users at the disposal (see below: Extension of the DNS).

DNS draws out through:

• decentralized administration
• hierarchical structuring of the name area in tree form
• Clarity of the names
• Expandability

Components of the DNS

The DNS consists of three main components:

• Domain Namensraum
• Name servers
• Resolver

Domain Namensraum

The Domain Namensraum has a structure. The sheets and knot of the tree are called labels. A complete Domainname of an object consists of the concatenation all label label is character strings (alphanumeric, when only special character is "‚- `permits), which are long at least one indication and maximally 63 indications. The individual label by points are separated. A Domain Name is locked with one point (the rear point normally omitted, belonged purely formally however to a complete Domain name to it). A correct, complete Domain Name (also Fully Qualified Domain Name (FQDN) mentioned) read for instance www.wikipedia.de. (the last point belongs to the Domain Name).

A Domain Name may be including all points maximally 255 indications long.

A Domain Name is always from right to left delegated and dissolved, i.e. the further on the right of a label stands, the more highly it stands in the Type the point at the right end of a Domainnamens separates the label for the first hierarchy level from the root. This first level is called also Top level Domain (TLD).

The DNS objects of a domain (for example the computer names) are usually held as sentence of resource record in a zone file, which is present on one or more autoritativen name servers. In place of zone file the somewhat more general expression zone is usually used.

Name server

Name servers are programs, which answer inquiries to the Domain Namensraum. One differentiates between autoritativen and not autoritativen name servers.

A autoritativer name server is responsible for a zone. Its information about this zone is therefore regarded as secured. For each zone at least a autoritativer server, the Primary name server exists. This is specified in the SOA resource record of a zone file. For redundancy and distribution of load reasons autoritative name servers are nearly always realized as server cluster, whereby the zone data are appropriate for one or more Secondary identically on name servers. The synchronisation between Primary and Secondary name servers takes place by zone transfer.

A not autoritativer name server refers its information about a zone of other name servers as it were out second or third hand. Its information becomes as not secured outstandingly. Since DNS data change normally only very rarely, not autoritative name servers store the information in the local RAM, once inquired by a resolver, so that this with a renewed inquiry faster are present. This technology is called Caching. Each of these entries possesses its own expiration date (TTL Time to Live), at whose expiration the entry from the Cache is deleted. The TTL is specified thereby by a autoritativen name server for this entry and determined according to the probability of change of the entry (frequently changing DNS data keep a low TTL). That can mean perhaps in addition, that the name server can supply wrong information in this time, if the data changed in the meantime.

A special case is the Caching Only name server. In this case the name server is responsible for no zone and must dissolve all arriving inquiries over further name servers. But different strategies are available:

Strategies

So that a not autoritativer name server information about other parts of the name area can find, it avails itself of the following strategies.

Parts of the name area of a Domain are often paged out at Subdomains with then particularly responsible name servers. A name server of a domain knows the responsible name servers for this Subdomains from its zone file and delegates inquiries to this subordinated name area at one this name server.

If the inquired name area lies outside of the own domain, the inquiry is passed on to a name server configured firmly.

If no forwarding server one configured or to these does not answer, the root servers are asked. In addition in form of a static file the names and IP addresses of the root servers are deposited. There are 13 root servers (server A to M). The root servers answer excluding iterative inquiries. They would be otherwise overloaded with the number of inquiries simply.

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