2.7. Caching

The whole resolution process may seem awfully convoluted and cumbersome to someone accustomed to simple searches through the host table. Actually, though, it's usually quite fast. One of the features that speeds it up considerably is caching.

A name server processing a recursive query may have to send out quite a few queries to find an answer. However, it discovers a lot of information about the domain name space as it does so. Each time it's referred to another list of name servers, it learns that those name servers are authoritative for some zone, and it learns the addresses of those servers. And at the end of the resolution process, when it finally finds the data the original querier sought, it can store that data for future reference. With Version 4.9 and all Version 8 and 9 BINDs, name servers even implement negative caching: if an authoritative name server responds to a query with an answer that says the domain name or datatype in the query doesn't exist, the local name server will temporarily cache that information, too. Name servers cache all this data to help speed up successive queries. The next time a resolver queries the name server for data about a domain name the name server knows something about, the process is shortened quite a bit. If the name server has cached the answer, positive or negative, it simply returns the answer to the resolver. Even if it doesn't have the answer cached, it may have learned the identities of the name servers that are authoritative for the zone the domain name is in and be able to query them directly.

For example, say our name server has already looked up the address eecs.berkeley.edu. In the process, it cached the names and addresses of the eecs.berkeley.edu and berkeley.edu name servers (plus eecs.berkeley.edu's IP address). Now if a resolver were to query our name server for the address of baobab.cs.berkeley.edu, our name server could skip querying the root name servers. Recognizing that berkeley.edu is the closest ancestor of baobab.cs.berkeley.edu that it knows about, our name server would start by querying a berkeley.edu name server, as shown in Figure 2-16. On the other hand, if our name server had discovered that there was no address for eecs.berkeley.edu, the next time it receives a query for the address, it could simply respond appropriately from its cache.

Figure 2-16. Resolving baobab.cs.berkeley.edu

In addition to speeding up resolution, caching obviates a name server's need to query the root name servers to answer queries it can't answer locally. This means that it's not as dependent on the roots, and the roots won't suffer as much from all its queries.

---