DNS Resolver Functions and General Operation
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Resolution Functions Performed By Name Resolvers
To accomplish its resolution duties, name resolvers perform a number of related functions:
This seems fairly simple, and it is in some ways, but implementation can become rather complicated. Bear in mind that the resolver may need to juggle several outstanding name resolutions simultaneously. It has to keep track of the different requests, queries and responses and make sure everything is kept straight.
The user interface is a very important part of a name resolvers job. We want users to be able to just use a name and have their software automatically treat it like an address. For this reason, normal name resolution usually doesn't involve explicitly running a piece of resolver software. Consider again your Web browser. You don't have to say please find the IP address for www.xyzindustries.com and then say please connect to this IP address for XYZ Industries. You just type in www.xyzindustries.com and the name resolution happens magically.
There is no magic, of course. The resolver is just called implicitly instead of explicitly. The Web browser recognizes that a name has been entered instead of an IP address and feeds it to the resolver, saying I need you to resolve this name, please. (Hey, it never hurts to be polite.) The resolver then takes care of resolution and provides the IP address back to the Web browser, which connects to the site. Thus, the resolver is the interface between the user (both the human user and the software user, the browser) and the DNS system.
Name resolvers don't have to perform nearly as many administrative jobs as name servers do; clients are usually simpler than servers in this regard. One important support function that many name resolvers do perform, however, is caching. Like name servers, name resolvers can cache the results of the name resolutions they perform to save time if the same resolution is required again (not all resolvers perform caching, however.)
I should point out that even though resolvers are the DNS components that are most associated with name resolution, name servers can also act as clients in certain types of name resolution. In fact, it is possible to set up a network so that the resolvers on each of the client machines do nothing more than hand resolution requests to a local DNS server and let the server take care of it. In this case, the client resolver becomes little more than a shell, sometimes called a stub resolver. This has the advantage of centralizing name resolution for the network, but a potential disadvantage of performance reduction.
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