How DHCP Interacts with Microsoft’s Dynamic Domain Name Service (DNS)

Posted by arlene

Microsoft’s version of DNS supports dynamic updates, as specified in RFC 2136, “Dynamic Updates in the Domain Name System (DNS UPDATE).” Windows 2000 clients can send dynamic updates after having received configuration information from a DHCP server. When a DHCP lease expires, the client will send an update to deregister the addressing information.

To register with DNS, the client first contacts a name server. If the name server is just a local serverand is not authoritative for the zone, it will return the address of the authoritative server to the client. The client then will contact the primary authoritative server to send it the updated addressing information. If it’s successful, a reply is sent back to the client.

The DHCP server also can be used to send dynamic updates to DNS. This is useful for pre-Windows 2000 clients that do not understand the dynamic update process. This also can be negotiated between the DHCP server and a Windows 2000 client during the initial DHCP process. This is done using a special FQDN (fully qualified domain name) DHCPREQUEST packet (using Option number 81). This packet has three possible flags that can be set:

• 0—This flag specifies that the client wants to be responsible for updating the A resource record
  on the DNS server, but would like for the DHCP server to update the PTR resource record.
• 1—This flag specifies that the client wants to perform both updates.
• 3—If this flag is set, the DHCP server will register both records, regardless of the client’s wishes. If the server sends a packet to the client with this flag set, the client does not attempt any updates.

These flags are not all that controls the process of which computer performs which updates. Instead, both the client and the server can be configured to perform (or not perform) this function. ‘Configuring Dynamic Updates on the DHCP Server

On the server side, you can specify in the properties page for the server how it will respond to dynamic update requests, and whether it will perform dynamic updates for clients that do not support this function (that is, pre-Windows 2000 clients). To configure the service for this functionality, follow these steps:

1. Click Start, Programs, Administrative Tools, DHCP (or Start, Administrative Tools, DHCP for Windows Server 2003).
2. In the left pane, expand the tree structure by clicking the plus sign next to the server’s name to expand the tree to show the scopes that belong to the server.
3. Right-click the scope you want to modify, and select Properties.
4. When the Properties page appears, click the DNS tab. You can see the properties found on this tab.
5. The first check box, Automatically Update DHCP Client Information in DNS, enables you to specify this function using two options. Click either the Update DNS Only If DHCP Client Requests radio button or the Always Update DNS radio button. If you select the second button, the server will always make updates for clients, ignoring their requests.
6. To enable the server to handle dynamic updates for clients that do not support dynamic DNS update, select the check box Enable Updates for DNS Clients That Do Not Support Dynamic Update.
7. Click the Apply button, and then click the OK button to dismiss the dialog box.

Configuring Dynamic Updates on the Client

You also can control how the client handles the dynamic DNS update function if you are using Windows 2000 clients. Remember from the preceding section that the server can override a client’s request if the appropriate selection is made on the scope’s DNS properties page.

Windows 2000 clients and servers, as well as Windows XP and Windows Server 2003, are already configured, by default, to send the FQDN packet with the Flags field set to zero. This means the client wants to update the A resource record and wants the server to update the PTR record. You can change this behavior by doing the following:

1. Right-click the My Network Places icon (or double-click the Network icon in the Control Panel). For Windows Server 2003, click on Start, Control Panel, Network Connections.
2. Right-click the icon for the network connection you want to configure. From the menu that pops up, select Properties.
3. Highlight Internet Protocol (TCP/IP) and click the Properties button.
4. Click the Advanced button, and then select the DNS tab. At the bottom of the properties page, you’ll notice that Register This Connection’s Addresses in DNS is selected, and Use This Connection’s DNS Suffix in DNS Registration is not. The second option is the default.
5. To disable the client from performing DNS updates, deselect the check box labeled Register This Connection’s Addresses in DNS.
6. When finished, dismiss the properties page by clicking OK in each dialog box.

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