Network Administration: DNS Caching
DNS servers don’t really like doing all that work to resolve DNS names, but they’re not stupid. They know that if a user visits www.wiley.com today, he’ll probably do it again tomorrow. As a result, name servers keep a cache of query results. The next time the user visits www.wiley.com, the name server is able to resolve this name without having to query all those other name servers.
The Internet is constantly changing, however, so cached data can quickly become obsolete. For example, suppose that Wiley Publishing, Inc., switches its website to a different server? It can update its name servers to reflect the new IP address, but any name servers that have a cached copy of the query will be out of date.
To prevent this from being a major problem, DNS data is given a relatively short expiration time. The expiration value for DNS data is called the TTL (Time to Live). TTL is specified in seconds. Thus, a TTL of 60 means the data is kept for one minute.