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The TCP/IP Guide  9  TCP/IP Application Layer Protocols, Services and Applications (OSI Layers 5, 6 and 7)       9  Name Systems and TCP/IP Name Registration and Name Resolution            9  TCP/IP Name Systems: Host Tables and Domain Name System (DNS)                 9  TCP/IP Domain Name System (DNS)                      9  DNS Name Servers and Name Resolution                           9  DNS Name Server Concepts and Operation

DNS Name Server Load Balancing 1 2 3 4 DNS Resolution Concepts and Resolver Operations

Automating Zone Transfers: DNS Notify

The first problem that many DNS administrators wanted to tackle was the reliance on polling for updating slave name servers. Imagine that you placed an order for a new music CD at your favorite online music store, but it was out of stock—backordered. Which makes more sense: having you call them every 6 hours to ask if it has arrived yet until it gets there, or having the store simply call you when it shows up?

The answer is so obvious that the question seems ridiculous. Yet DNS uses the first model: slave name servers must constantly “call up” their zone masters and ask them “has anything changed yet?” This both generates unnecessary traffic, and results in the slave name server being out of date from the time the master does change until the next poll is performed. Tweaking the Refresh time for the zone only allows one to choose between more polls or more staleness when changes happen; neither is really “good”.

To improve upon this situation, a new technique was developed; it was formalized in RFC 1996, published in 1996 (weird coincidence!) This standard, A Mechanism for Prompt Notification of Zone Changes (DNS NOTIFY), defines a new DNS message type called Notify, and describes a protocol for its use. The Notify message is a variation on the standard DNS message type, with some of the fields redefined to support this new feature.

When both the master and slave name servers support this feature, when a modification is made to a resource record, the master server will automatically send a Notify message to its slave server(s), saying “your CD has arrived!” er… “the database has changed”. J The slave then acts as if its Refresh timer has just expired. Enabling this feature allows the Refresh interval to be dramatically increased, since slave servers don't need to constantly poll the master for changes.

DNS Name Server Load Balancing 1 2 3 4 DNS Resolution Concepts and Resolver Operations