The Microsoft Windows Server™ 2003 network operating system includes an enhanced implementation of the Dynamic Host Configuration Protocol (DHCP). This paper outlines the features of DHCP, the Windows Server 2003 implementation of the technology, basic deployment information, and best practices for use. Based on industry standards, DHCP for Windows Server 2003 supports RFCs 2131 and 2132.
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The Microsoft® Windows Server™ 2003 network operating system builds on the longstanding Microsoft support for the Dynamic Host Configuration Protocol (DHCP)—an open, industry standard that reduces the complexity of administering TCP/IP-based networks. Each host computer connected to a TCP/IP network must have a unique IP address. DHCP frees network administrators from manually configuring IP addresses for individual desktop computers.
Internet Engineering Task Force (IETF) Requests for Comments (RFCs) 2131 and 2132 define DHCP as open and standards-based. A DHCP server stores all available IP addresses in a central database along with associated configuration information, including the subnet mask, gateways, and the addresses of DNS servers. This database enables automatic IP address configuration for hosts as they start up.
DHCP saves network administrative time—and the larger the network, the greater the savings. Without dynamic address assignment, network administrators must manage IP addresses to avoid duplicate use and apply configuration changes to workstations manually. The resulting lack of centralized configuration information makes it difficult for the administrator to ensure consistent client configurations.
By contrast, DHCP in Windows Server 2003 provides benefits and features that make it a compelling solution to the networking needs of organizations today.
DHCP Features in Windows Server 2003
DHCP in Windows Server 2003 includes enhancements to RFCs 2131 and 2132, which make DHCP easier to deploy and manage than other implementations. These enhancements include:
Integration of DHCP with Domain Name System (DNS).
DNS servers provide domain name resolution for network resources. They associate the TCP/IP address assigned by DHCP to a client with its fully qualified domain name (FQDN). This association, or mapping, of an IP address to a domain name requires that a change in either the address or the name necessitates an update of the information in DNS. The DHCP protocol does not automatically update DNS in the event that the DHCP server changes the IP address of a client. To facilitate this interaction, servers running Windows Server 2003 and DHCP and clients running DHCP can register with DNS, allowing cooperation between the two. When DHCP changes IP address information, corresponding DNS updates synchronize name-to-address associations for the computer.
When a DHCP server registers and updates DNS pointer (PTR) and address (A) resource records on behalf of its DHCP-enabled clients, it uses the information contained within an additional DHCP option: the Client FQDN option (option 81), which permits a client to provide its FQDN and any instructions to the DHCP server that is used to process DNS dynamic updates on its behalf.
The following reasons or events can trigger a dynamic update:
Added, removed, or modified IP addresses in the TCP/IP properties configuration for any of the installed network connections.
An IP address lease changes or renews any of the installed network connections with the DHCP server. For example, when a computer starts or after use of the ipconfig /renew command.
Upon use of the ipconfig /registerdns command, which manually forces a refresh of the client name registration in DNS.
When one of these events triggers a dynamic update, the DHCP Client service (not the DNS Client service) sends updates. The DHCP Client service performs this function for all network connections on the client, including any that are not configured to use DHCP.
When a qualified DHCP client issues an update, such as a DHCP-enabled computer running Microsoft Windows® 2000 or Windows XP Professional, servers running Windows Server 2003 and DHCP process the update to determine in which of three ways the server will initiate updates on behalf of the client:
The DHCP server always registers the DHCP client for both the forward (A resource records) and reverse lookup or pointer (PTR resource records) with DNS.
The DHCP server never registers the name-to-address (A resource records) for DHCP clients.
The DHCP server registers the DHCP client for both forward (A resource records) and reverse lookup or pointer (PTR resource records) when requested to do so by the client.
The ability to register both A and PTR resource records enables a DHCP server to act as a proxy for clients running other operating systems, such as Microsoft Windows Millennium Edition, Windows 98, and Windows NT® Workstation 4.0, for the purpose of DNS dynamic update registration. The DHCP server can automatically differentiate between Windows 2000 and Windows XP Professional, and other clients.
DHCP requires the use of DNS dynamic update to keep name-to-address mapping information synchronized. Using DHCP and DNS together on a network might cause problems when using older, static DNS servers, which cannot interact dynamically when DHCP client configurations change. You can avoid failed DNS lookups for DHCP-registered clients when using static DNS service by doing the following:
If you are using Windows Internet Name Service (WINS) servers on a 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 DNS servers with DNS servers that support DNS dynamic updates, Windows 2000 and Windows Server 2003 DNS dynamic updates.