DNS on Windows Server 2003, 3rd Edition

Changing TTLs

An experienced zone administrator needs to know how to set the time to live on his zone’s data to his best advantage. The TTL on a resource record, remember, is the time for which any server can cache that record. So if the TTL for a particular resource record is 3,600 seconds and a server outside your network caches that record, it will have to remove the entry from its cache after an hour. If it needs the same data after the hour is up, it’ll have to query your name servers again.

When we introduced TTLs, we emphasized that your choice of a TTL would dictate how current you would keep copies of your data, at the cost of increased load on your name servers. A low TTL would mean that name servers outside your network would have to get data from your name servers often and that the data would therefore be kept current. On the other hand, your name servers would be peppered by the name servers’ queries.

You don’t have to choose a TTL once and for all, though. You can—and experienced administrators do—change TTLs periodically to suit your needs.

Suppose we know that one of our hosts is about to be moved to another network. This host houses the movie.edu film library, a large collection of files our site makes available to hosts on the Internet. During normal operation, outside name servers cache the address of our host according to the minimum (default) TTL in the SOA record. (We set the movie.edu TTL to be one day in our sample files.) A name server caching the old ...