DNS Name Resolution Efficiency Improvements: Caching and Local Resolution
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One other area where resolution efficiency can be improved is the special case where we are trying to resolve the names of computers in our own organizations. Suppose that I, an employee at XYZ Industries, want to get some sales information using FTP from sales.xyzindustries.com. My FTP client will invoke my local resolver to resolve that name, by sending it to our local DNS server. Now, would it be smart for that server, which is here inside the company, to start the resolution process up at the root name server? Not really.
The local DNS server that accepts local resolution requests from resolvers on the network may in fact be the authoritative name server for sales.xyzindustries.com. In other cases, it may know how to answer certain resolution requests directly. Obviously, it makes sense for the server to check to see if it can answer a resolver's query before heading up to the root server, since this provides a faster answer to the client and saves internetwork traffic. This is called local resolution.
Most DNS servers will perform this check to see if they have the information needed for a request before commencing the formal top-down resolution process. The exception is DNS servers that do not maintain information about any zones: caching-only servers. In some cases, DNS resolvers on client machines may also have access to certain local zone information, in which case they can use it instead of sending a resolution query at all.
There's one more thing I also want to point out: most operating systems also support the use of the old host table mechanism, which can be useful for local machines on a network. If a host has a host table, the resolver will check the host table to see if it can find a mapping for a name before it will bother with the more time-consuming DNS resolution process. This is not technically part of DNS, but is often used in conjunction with it.
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