Dynamic Host Configuration Protocol

DHCP and DNS

Domain Name System (DNS) servers provide name resolution for network clients. DNS maintains (among other things) information that links a computer's fully qualified domain name (FQDN) to its assigned IP address(es).

While DHCP provides a powerful mechanism for automatically configuring client IP addresses, until recently DHCP did not notify the DNS service to update the DNS records on the client; specifically, updating the client name to an IP address, and IP address to name mappings maintained by a DNS server.

Without a way for DHCP to interact with DNS, the information maintained by DNS for a DHCP client may be incorrect. For example, a client may acquire its IP address from a DHCP server, but the DNS records would not reflect the IP address acquired nor provide a mapping from the new IP address to the computer name (FQDN).

In Windows 2000, DHCP servers and clients can register with DNS to provide this update service if the DNS server supports DNS with dynamic updates. The Windows 2000 DNS service supports dynamic updates. For more information, see the chapter "Windows 2000 DNS" in this book

A Windows 2000 DHCP server can register with a DNS server and update pointer (PTR) and address (A) resource records on behalf of its DHCP-enabled clients using the DNS dynamic update protocol.

The ability to register both A and PTR type records lets a DHCP server act as a proxy for clients using Microsoft Windows 95 and Windows NT 4.0 for the purpose of DNS registration. DHCP servers can differentiate between Windows 2000 and other clients. An additional DHCP option code (option code 81) enables the return of a client's FQDN to the DHCP server. If implemented, the DHCP server can dynamically update DNS to modify an individual computer's resource records with a DNS server using the dynamic update protocol. This DHCP option permits the DHCP server the following possible interactions for processing DNS information on behalf of DHCP clients that include Option Code 81 in the DHCPRequest message they send to the server:

The DHCP server always registers the DHCP client for both the forward (A-type records) and reverse lookups (PTR-type records) with DNS.
The DHCP server never registers the name-to-address (A-type records) mapping information for DHCP clients.
The DHCP server registers the DHCP client for both forward (A-type records) and reverse lookups (PTR-type records) only when requested to by the client

DHCP and static DNS service are not compatible for keeping name-to-address mapping information synchronized. This might cause problems with using DHCP and DNS together on a network if you are using older, static DNS servers, which are incapable of interacting dynamically when DHCP client configurations change.

To avoid failed DNS lookups for DHCP-registered clients when static DNS service is in effect, do the following steps:

If WINS servers are used on the network, enable WINS lookup for DHCP clients that use NetBIOS.
Assign IP address reservations with an infinite lease duration for DHCP clients that use DNS only and do not support NetBIOS.

Wherever possible, upgrade or replace older, static-based DNS servers with DNS servers supporting updates. Dynamic updates are supported by the Microsoft DNS service, included in Windows 2000.

Additional Recommendations

When using DNS and WINS together, consider the following options for interoperation:

If a large percentage of clients use NetBIOS and you are using DNS, consider using WINS lookup on your DNS servers. If WINS lookup is enabled on the Microsoft DNS service, WINS is used for final resolution of any names that are not found using DNS resolution. The WINS forward lookup and WINS-R reverse lookup records are supported only by DNS. If you use servers on your network that do not support DNS, use DNS Manager to ensure that these WINS records are not propagated to DNS servers that do not support WINS lookup.
If you have a large percentage of computers running Windows 2000 on your network, consider creating a pure DNS environment. This involves developing a migration plan to upgrade older WINS clients to Windows 2000. Support issues involving network name service are simplified by using a single naming and resource locator service (such as WINS and DNS) on your network. For more information, see "Windows Internet Name Service" and "Windows 2000 DNS" in this book.