DNS Name Server Caching, Negative Caching and Data Persistence
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Most of the grunt work done by name servers is responding to name resolution requests. Busy servers, like the root name servers, the ones that carry zone information for the top-level domains, and ones that serve very busy zones, have to handle hundreds or even thousands of name resolution requests each second. Each of these requests takes time and resources to resolve; it also takes internetwork bandwidth away from the business of transferring data. It is essential, therefore, that DNS server implementations employ mechanisms to improve their efficiency and cut down on unnecessary name resolution requests. One of the most important of these is caching.
The word cache refers to a store, or a place where something is kept. In the computer world, the term usually refers to an area of memory set aside for storing information that has been recently obtained so it can be used again. In the case of DNS, caching is used by DNS name servers to store the results of recent name resolution and other requests, so that if the request occurs again it can be satisfied from the cache without requiring another complete run of the name resolution process. Due to how most people use computers, a particular request is often followed by another request for the same name, so caching can significantly reduce the number of requests that result in complete name resolution procedures.
An example is the best way to illustrate this. Suppose you are using a host on your company's local network. This host is probably configured to use your company's DNS name server to handle resolution requests. You type www.xyzindustries.com into your Web browser, which causes a resolution attempt to be made for that address. Most likely, your local DNS server doesn't know that name, so it will follow the complete name resolution process to get its address. After doing this, your local DNS server will cache the name www.xyzindustries.com and the address associated with it.
If you click on a link for a page at that Web site, that new page will also probably be somewhere at the www.xyzindustries.com site. This will result in another DNS resolution request being sent off to your local DNS server again. However, this time, the local server will not have to perform a resolution; it remembers that this name is in its cache and returns the saved address for the name immediately. Voila! You get your answer faster, and unnecessary Internet traffic is avoided.
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