DNS resolvers See also: resolv.conf The client-side of the DNS is called a DNS resolver. It is responsible for initiating and sequencing the queries that ultimately lead to a full resolution (translation) of the resource sought, e.g., translation of a domain name into an IP address. A DNS query may be either a non-recursive query or a recursive query: * A non-recursive query is one in which the DNS server provides a record for a domain for which it is authoritative itself, or it provides a partial result without querying other servers. * A recursive query is one for which the DNS server will fully answer the query (or give an error) by querying other name servers as needed. DNS servers are not required to support recursive queries. The resolver, or another DNS server acting recursively on behalf of the resolver, negotiates use of recursive service using bits in the query headers. Resolving usually entails iterating through several name servers to find the needed information. However, some resolvers function more simply by communicating only with a single name server. These simple resolvers (called "stub resolvers") rely on a recursive name server to perform the work of finding information for them. [edit] Operation [edit] Address resolution mechanism Domain name resolvers determine the appropriate domain name servers responsible for the domain name in question by a sequence of queries starting with the right-most (top-level) domain label. A DNS recursor consults three nameservers to resolve the address www.wikipedia.org. The process entails: 1. A network host is configured with an initial cache (so called hints) of the known addresses of the root nameservers. Such a hint file is updated periodically by an administrator from a reliable source. 2. A query to one of the root servers to find the server authoritative for the top-level domain. 3. A query to the obtained TLD server for the address of a DNS server authoritative for the second-level domain. 4. Repetition of the previous step to process each domain name label in sequence, until the final step which returns the IP address of the host sought. The diagram illustrates this process for the host www.wikipedia.org. The mechanism in this simple form would place a large operating burden on the root servers, with every search for an address starting by querying one of them. Being as critical as they are to the overall function of the system, such heavy use would create an insurmountable bottleneck for trillions of queries placed every day. In practice caching is used in DNS servers to overcome this problem, and as a result, root nameservers actually are involved with very little of the total traffic. [edit] Circular dependencies and glue records Name servers in delegations are identified 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. If the name given in the delegation is a subdomain of the domain for which the delegation is being provided, there is a circular dependency. In this case the nameserver providing the delegation must also provide one or more IP addresses for the authoritative nameserver mentioned in the delegation. This information is called glue. The delegating name server provides this glue in the form of records in the additional section of the DNS response, and provides the delegation in the answer section of the response. For example, if the authoritative name server for example.org is ns1.example.org, a computer trying to resolve www.example.org first resolves ns1.example.org. Since ns1 is contained in example.org, this requires resolving example.org first, which presents a circular dependency. To break the dependency, the nameserver for the org top level domain includes glue along with the delegation for example.org. The glue records are address records that provide IP addresses for ns1.example.org. The resolver uses one or more of these IP addresses to query one of domain's authoritative servers, which allows it to complete the DNS query. [edit] Record caching Because of the large volume of DNS requests generated for the public Internet, the designers wished to provide a mechanism to reduce the load on individual DNS servers. To this end, the DNS resolution process allows for caching of records for a period of time after an answer. This entails the local recording and subsequent consultation of the copy instead of initiating a new request upstream. The time for which a resolver caches a DNS response is determined by a value called the time to live (TTL) associated with every record. The TTL is set by the administrator of the DNS server handing out the authoritative response. The period of validity may vary from just seconds to days or even weeks. As a noteworthy consequence of this distributed and caching architecture, changes to DNS records do not propagate throughout the network immediately, but require all caches to expire and refresh after the TTL. RFC 1912 conveys basic rules for determining appropriate TTL values. Some resolvers may override TTL values, as the protocol supports caching for up to 68 years or no caching at all. Negative caching, i.e. the caching of the fact of non-existence of a record, is determined by name servers authoritative for a zone which must include the Start of Authority (SOA) record when reporting no data of the requested type exists. The value of the MINIMUM field of the SOA record and the TTL of the SOA itself is used to establish the TTL for the negative answer. [edit] Reverse lookup A reverse lookup is a query of the DNS for domain names when the IP address is known. Multiple domain names may be associated with an IP address. The DNS stores IP addresses in the form of domain names as specially formatted names in pointer (PTR) records within the infrastructure top-level domain arpa. For IPv4, the domain is in-addr.arpa. For IPv6, the reverse lookup domain is ip6.arpa. The IP address is represented as a name in reverse-ordered octet representation for IPv4, and reverse-ordered nibble representation for IPv6. When performing a reverse lookup, the DNS client converts the address into these formats, and then queries the name for a PTR record following the delegation chain as for any DNS query. For example, the IPv4 address 208.80.152.2 is represented as a DNS name as 2.152.80.208.in-addr.arpa. The DNS resolver begins by querying the root servers, which point to ARIN's servers for the 208.in-addr.arpa zone. From there the Wikimedia servers are assigned for 152.80.208.in-addr.arpa, and the PTR lookup completes by querying the wikimedia nameserver for 2.152.80.208.in-addr.arpa, which results in an authoritative response. [edit] Client lookup DNS resolution sequence Users generally do not communicate directly with a DNS resolver. Instead DNS resolution takes place transparently in applications programs such as web browsers, e-mail clients, and other Internet applications. When an application makes a request that requires a domain name lookup, such programs send a resolution request to the DNS resolver in the local 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 one or more designated DNS servers. In the case of most home users, the Internet service provider to which the machine connects will usually supply this DNS server: such a user will either have configured that server's address manually or allowed DHCP to set it; however, where systems administrators have configured systems to use their own DNS servers, their DNS resolvers point to separately maintained nameservers of the organization. In any event, the name server thus queried will follow the process outlined above, 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. [edit] Broken resolvers An additional level of complexity emerges 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 by disobeying TTLs, or by indicating that a domain name does not exist just because one of its name servers does not respond.[13] As a final level of complexity, some applications (such as web-browsers) also have their own DNS cache, in order to reduce the use of the DNS resolver library itself. This practice can add extra difficulty when debugging DNS issues, as it obscures the freshness of data, and/or what data comes from which cache. These caches typically use very short caching times—on the order of one minute.[citation needed] Internet Explorer represents a notable exception: versions up to IE 3.x cache DNS records for 24 hours by default. Internet Explorer 4.x and later versions (up to IE 8) decrease the default time out value to half an hour, which may be changed in corresponding registry keys.[14] [edit] Other applications

DNS resolvers

See also: resolv.conf

The client-side of the DNS is called a DNS resolver. It is responsible for initiating and sequencing the queries that ultimately lead to a full resolution (translation) of the resource sought, e.g., translation of a domain name into an IP address.
A DNS query may be either a non-recursive query or a recursive query:

• A non-recursive query is one in which the DNS server provides a record for a domain for which it is authoritative itself, or it provides a partial result without querying other servers.
• A recursive query is one for which the DNS server will fully answer the query (or give an error) by querying other name servers as needed. DNS servers are not required to support recursive queries.

The resolver, or another DNS server acting recursively on behalf of the resolver, negotiates use of recursive service using bits in the query headers.
Resolving usually entails iterating through several name servers to find the needed information. However, some resolvers function more simply by communicating only with a single name server. These simple resolvers (called "stub resolvers") rely on a recursive name server to perform the work of finding information for them.

[edit] Operation

[edit] Address resolution mechanism

Domain name resolvers determine the appropriate domain name servers responsible for the domain name in question by a sequence of queries starting with the right-most (top-level) domain label.

A DNS recursor consults three nameservers to resolve the address www.wikipedia.org.

The process entails:

1. A network host is configured with an initial cache (so called hints) of the known addresses of the root nameservers. Such a hint file is updated periodically by an administrator from a reliable source.
2. A query to one of the root servers to find the server authoritative for the top-level domain.
3. A query to the obtained TLD server for the address of a DNS server authoritative for the second-level domain.
4. Repetition of the previous step to process each domain name label in sequence, until the final step which returns the IP address of the host sought.

The diagram illustrates this process for the host www.wikipedia.org.
The mechanism in this simple form would place a large operating burden on the root servers, with every search for an address starting by querying one of them. Being as critical as they are to the overall function of the system, such heavy use would create an insurmountable bottleneck for trillions of queries placed every day. In practice caching is used in DNS servers to overcome this problem, and as a result, root nameservers actually are involved with very little of the total traffic.

[edit] Circular dependencies and glue records

Name servers in delegations are identified 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. If the name given in the delegation is a subdomain of the domain for which the delegation is being provided, there is a circular dependency. In this case the nameserver providing the delegation must also provide one or more IP addresses for the authoritative nameserver mentioned in the delegation. This information is called glue. The delegating name server provides this glue in the form of records in the additional section of the DNS response, and provides the delegation in the answer section of the response.
For example, if the authoritative name server for example.org is ns1.example.org, a computer trying to resolve www.example.org first resolves ns1.example.org. Since ns1 is contained in example.org, this requires resolving example.org first, which presents a circular dependency. To break the dependency, the nameserver for the org top level domain includes glue along with the delegation for example.org. The glue records are address records that provide IP addresses for ns1.example.org. The resolver uses one or more of these IP addresses to query one of domain's authoritative servers, which allows it to complete the DNS query.

[edit] Record caching

Because of the large volume of DNS requests generated for the public Internet, the designers wished to provide a mechanism to reduce the load on individual DNS servers. To this end, the DNS resolution process allows for caching of records for a period of time after an answer. This entails the local recording and subsequent consultation of the copy instead of initiating a new request upstream. The time for which a resolver caches a DNS response is determined by a value called the time to live (TTL) associated with every record. The TTL is set by the administrator of the DNS server handing out the authoritative response. The period of validity may vary from just seconds to days or even weeks.
As a noteworthy consequence of this distributed and caching architecture, changes to DNS records do not propagate throughout the network immediately, but require all caches to expire and refresh after the TTL. RFC 1912 conveys basic rules for determining appropriate TTL values.
Some resolvers may override TTL values, as the protocol supports caching for up to 68 years or no caching at all. Negative caching, i.e. the caching of the fact of non-existence of a record, is determined by name servers authoritative for a zone which must include the Start of Authority (SOA) record when reporting no data of the requested type exists. The value of the MINIMUM field of the SOA record and the TTL of the SOA itself is used to establish the TTL for the negative answer.

[edit] Reverse lookup

A reverse lookup is a query of the DNS for domain names when the IP address is known. Multiple domain names may be associated with an IP address. The DNS stores IP addresses in the form of domain names as specially formatted names in pointer (PTR) records within the infrastructure top-level domain arpa. For IPv4, the domain is in-addr.arpa. For IPv6, the reverse lookup domain is ip6.arpa. The IP address is represented as a name in reverse-ordered octet representation for IPv4, and reverse-ordered nibble representation for IPv6.
When performing a reverse lookup, the DNS client converts the address into these formats, and then queries the name for a PTR record following the delegation chain as for any DNS query. For example, the IPv4 address 208.80.152.2 is represented as a DNS name as 2.152.80.208.in-addr.arpa. The DNS resolver begins by querying the root servers, which point to ARIN's servers for the 208.in-addr.arpa zone. From there the Wikimedia servers are assigned for 152.80.208.in-addr.arpa, and the PTR lookup completes by querying the wikimedia nameserver for 2.152.80.208.in-addr.arpa, which results in an authoritative response.

[edit] Client lookup

DNS resolution sequence

Users generally do not communicate directly with a DNS resolver. Instead DNS resolution takes place transparently in applications programs such as web browsers, e-mail clients, and other Internet applications. When an application makes a request that requires a domain name lookup, such programs send a resolution request to the DNS resolver in the local 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 one or more designated DNS servers. In the case of most home users, the Internet service provider to which the machine connects will usually supply this DNS server: such a user will either have configured that server's address manually or allowed DHCP to set it; however, where systems administrators have configured systems to use their own DNS servers, their DNS resolvers point to separately maintained nameservers of the organization. In any event, the name server thus queried will follow the process outlined above, 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.

[edit] Broken resolvers

An additional level of complexity emerges 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 by disobeying TTLs, or by indicating that a domain name does not exist just because one of its name servers does not respond.^[13]
As a final level of complexity, some applications (such as web-browsers) also have their own DNS cache, in order to reduce the use of the DNS resolver library itself. This practice can add extra difficulty when debugging DNS issues, as it obscures the freshness of data, and/or what data comes from which cache. These caches typically use very short caching times—on the order of one minute.^{[citation needed]}
Internet Explorer represents a notable exception: versions up to IE 3.x cache DNS records for 24 hours by default. Internet Explorer 4.x and later versions (up to IE 8) decrease the default time out value to half an hour, which may be changed in corresponding registry keys.^[14]

[edit] Other applications

Apa kepanjangan dari DNS ....
a. Domain Name System
b. Domain Name Service
c. Domain Network System
d. Domain Network Service
e. Domain Net System

Siapa yang menemukan DNS ....
a. David Mockapetris
b. Mchael Mockapetris
c. James Mockapetris
d. Paul Mockapetris
e. Daniel Mockapetris

Pada tahun berapa Paul Mockapetris menemukan DNS ....
a.1981
b.1982
c.1983
d.1984
e.1985

DNS adalah sebuah sistem yang menyimpan informasi tentang ....
a. nama file
b. nama jaringan
c. nama host
d. nama house
e. nama judul

Apa yang disebut sebagai server nama otoritatif yang mengasuh zona nama domain dari sebuah nama domain ....
a. Name Servers
b. Name System
c. Name Service
d. Name Susun
e. Semua benar