DNS on Windows Server 2003, 3rd Edition

Aging and Scavenging

Now let’s shift gears a bit and discuss one other aspect of maintenance that may be applicable to some of your servers. If you have any zones with dynamic update enabled, they are prone to stale records. (See Chapter 11 for a description of dynamic update.) Stale records are A or PTR records that were dynamically added but were not properly removed when no longer necessary. Most DHCP clients—including Windows clients—don’t release their addresses on shutdown, which means they don’t send the corresponding dynamic update message to remove their A records (nor does the DHCP server send a dynamic update message to remove the PTR record). Imagine a transient host, such as a laptop, that receives but never releases an address, leaving A and PTR records in DNS. Microsoft refers to these records as stale, and the DNS server in Windows Server 2003 can track their age and remove, or scavenge, them when they are no longer necessary.

The DNS server knows a record is not stale when it receives a dynamic update request for it. A Windows 2000 or Windows XP host sends a dynamic update message for its A record (and PTR record, if configured with a static address) every 24 hours by default. These same Windows hosts also send dynamic updates on lease renewal. An update of an existing record is called a refresh. (Before sending the update to make any changes, clients actually probe for a record’s existence by sending a dynamic update message with only a prerequisite ...