InformationWeek
Defining The Business Value Of Technology
March 13, 2000
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But times have changed, and use of the Dynamic Host Configuration Protocol, which doles out IP addresses to clients, demands a technique for automatically adding and deleting records to DNS servers each time an IP is assigned or recovered. Of course, frequent changes to DNS zone files in turn require a more efficient mechanism for replicating changes to secondary DNS servers.
The Windows 2000 DNS server supports two relatively new DNS protocols that address these needs. The older of the protocols, incremental zone transfers, lets DNS servers exchange the most recent changes to a zone file rather than the entire database. The DNS server that's responsible for the zone maintains serial numbers for each change to the database and includes this serial number when updating secondary servers.
When replicating changes, DNS servers compare serial numbers and the primary server will send all the incremental transfers made since the secondary server last updated itself. In some situations, the primary server may no longer have all of the incremental changes still in cache, in which case the primary server sends the entire zone file.
Dynamic DNS lets DHCP servers or clients add and delete records to a DNS database (using the new Update message). Dynamic DNS also supports the notion of assertions, which let clients spell out the prerequisites for an Update message, such as ensuring that the DNS server doesn't already have an assignment with the new IP address or host name. The particulars of who updates which records on which servers, however, varies depending on the type of IP assignment (a static vs. dynamically assigned address, for example) and the type of client (Windows 2000 vs. Windows 95 or NT).
Normally, registering the name of a system entails two records: an A resource record maps a name (such as www.informationweek.com) to an IP address, and a corresponding PTR record, which maps an IP address to a host name for "reverse" lookups. In a pure Windows 2000 environment, the DHCP client normally initiates the creation and deletion of "A" records and the DHCP server takes care of the "PTR" records. Clients running most other operating systems don't automatically initiate Dynamic DNS updates, so you may need to configure your Windows 2000 DHCP service to manage the entire process for other operating systems.
When integrating non-Windows 2000 DNS servers in a Windows 2000 network, you'll need to consider how support for these two protocols will affect your zone management and replication efficiency. For more information, consult the Internet Engineering Task Forces request for proposals 1995 (incremental transfers) and 2136 (Dynamic DNS) at www.ietf.org.
he original domain name system design clearly envisioned a relatively static naming system. Updates typically involved an administrator opening a "zone" file (a database containing all the DNS records for which the DNS server has responsibility), and manually editing it. Administrators made these changes on the primary DNS server and that server then transferred the entire zone file to secondary servers using the AXFER command.
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