Editing Domain Name System (section)

===Client lookup===
[[File:DNS Architecture.svg|right|thumb|400px|DNS resolution sequence]]
Users generally do not communicate directly with a DNS resolver. Instead DNS resolution takes place transparently in applications such as [[web browser]]s, [[e-mail client]]s, 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 [[Dynamic Host Configuration Protocol|DHCP]] to set it; however, where systems administrators have configured systems to use their own DNS servers, their DNS resolvers point to separately maintained name servers of the organization. In any event, the name server thus queried will follow the process outlined [[#Address resolution mechanism|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.

====Broken resolvers====
Some large ISPs have configured their DNS servers to violate rules, 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.<ref>{{cite web
|url = http://ask.slashdot.org/story/05/04/18/198259/providers-ignoring-dns-ttl
|title = Providers ignoring DNS TTL?
|publisher = [[Slashdot]]
|year = 2005
|access-date = 2012-04-07
}}</ref>

Some applications such as web browsers maintain an internal DNS cache to avoid repeated lookups via the network. This practice can add extra difficulty when debugging DNS issues as it obscures the history of such data. These caches typically use very short caching times on the order of one minute.<ref>{{cite web|url=http://dyn.com/web-browser-dns-caching-bad-thing/|title=Ben Anderson: Why Web Browser DNS Caching Can Be A Bad Thing|author=Ben Anderson|date=7 September 2011|access-date=20 October 2014}}</ref>

[[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 timeout value to half an hour, which may be changed by modifying the default configuration.<ref>{{cite web
|url = http://support.microsoft.com/default.aspx?scid=KB;en-us;263558
|title = How Internet Explorer uses the cache for DNS host entries
|publisher = [[Microsoft Corporation]]
|year = 2004
|access-date = 2010-07-25
}}</ref>

When [[Google Chrome]] detects issues with the DNS server it displays a specific error message.