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Example: How Dynamic Update Works

For Windows 2000, dynamic updates are typically requested when either a DNS name or IP address changes on the computer. For example, suppose that a client named "oldhost" is first configured in System properties with the following names:

Computer name

oldhost

DNS domain name of computer

example.microsoft.com

Full computer name

oldhost.example.microsoft.com

In this example, no connection-specific DNS domain names are configured for the computer. Later, the computer is renamed from "oldhost" to "newhost", resulting in the following name changes on the system:

Computer name

newhost

DNS domain name of computer

example.microsoft.com

Full computer name

newhost.example.microsoft.com

Once the name change is applied in System properties, Windows 2000 prompts you to restart the computer. When the computer restarts Windows, the DHCP Client service performs the following sequence to update DNS:

  1. The DHCP Client service sends a start of authority (SOA) type query using the DNS domain name of the computer.

  2. The client computer uses the currently configured FQDN of the computer (such as "newhost.example.microsoft.com") as the name specified in this query.

  3. The authoritative DNS server for the zone containing the client FQDN responds to the SOA-type query.

  4. For standard primary zones, the primary server (owner) returned in the SOA query response is fixed and static. It always matches the exact DNS name as it appears in the SOA RR stored with the zone. If, however, the zone being updated is directory-integrated, any DNS server loading the zone can respond and dynamically insert its own name as the primary server (owner) of the zone in the SOA query response.

  5. The DHCP Client service then attempts to contact the primary DNS server.

  6. The client processes the SOA query response for its name to determine the IP address of the DNS server authorized as the primary server for accepting its name. It then proceeds to perform the following sequence of steps, as needed to contact and dynamically update its primary server:

    a. It sends a dynamic update request to the primary server determined in the SOA query response.

    If the update succeeds, no further action is taken.

    b. If this update fails, the client next sends an NS-type query for the zone name specified in the SOA record.

    c. When it receives a response to this query, it sends an SOA query to the first DNS server listed in the response.

    d. After the SOA query is resolved, the client sends a dynamic update to the server specified in the returned SOA record.

    If the update succeeds, no further action is taken.

    e. If this update fails, the client repeats the SOA query process by sending to the next DNS server listed in the response.

  7. Once the primary server is contacted that can perform the update, the client sends the update request, and the server processes it.

  8. The contents of the update request include instructions to add A (and possibly PTR) RRs for "newhost.example.microsoft.com" and remove these same record types for "oldhost.example.microsoft.com", the name that was previously registered.

    The server also checks to ensure that updates are permitted for the client request. For standard primary zones, dynamic updates are not secured, so any client attempt to update succeeds. For Active Directory-integrated zones, updates are secured and performed using directory-based security settings.

Dynamic updates are sent or refreshed periodically. By default, Windows 2000 sends a refresh once every 24 hours. If the update results in no changes to zone data, the zone remains at its current version, and no changes are written. Updates result in actual zone changes or increased zone transfer only if names or addresses actually change.

Note that names are not removed from DNS zones if they become inactive or are not updated within the refresh interval (24 hours). DNS does not use a mechanism to release or tombstone names, although DNS clients do attempt to delete or update old name records when a new name or address change is applied.

When the DHCP Client service registers A and PTR resource records for a Windows 2000 computer, it uses a default caching Time-To-Live (TTL) of 15 minutes for host records. This determines how long other DNS servers and clients cache a computer's records when they are included in a query response.

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