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DNS on Windows 2000, Second Edition by Cricket Liu, Matt Larson

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Aging and Scavenging

Zones with dynamic update enabled are prone to stale records; that is, A or PTR records that are dynamically added but 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 2000 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 host sends a dynamic update message for its A record (and PTR record, if configured with a static address) every 24 hours by default. Windows 2000 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 section. The DNS server counts such a message as a refresh, too.) A refresh is the signal to the server that a particular client is still alive and using its records.

The idea behind aging and scavenging is to remove records that haven’t been ...

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