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Contents 3
Introduction 1
DHCP Features in Windows Server 2003 2
DHCP Overview 10
DHCP Deployment 14
Best Practices 18
Summary 24
Related Links 25
Appendix A: Predefined Options for DHCP Clients 26
Introduction
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).
Enhanced monitoring and statistical reporting.
DHCP vendor-specific and class ID option support.
Multicast address allocation.
Unauthorized DHCP server detection.
Windows clustering for high availability.
Automatic and alternate client configuration.
Command-line management
Integration of DHCP with 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.
Enhanced Monitoring and Statistical Reporting
Enhanced monitoring and statistical reporting provides notification when the number of available IP addresses falls below a 90 percent threshold. To alert network managers, icon color, which is associated with the remaining addresses falling below the defined level, changes to yellow. The icon color changes to red whenever the addresses become completely depleted.
The DHCP snap-in, which supports Simple Network Management Protocol (SNMP) and Management Information Bases (MIBs) for DHCP statistics, provides the graphical display of statistical data. This interface helps administrators monitor system status, such as the number of available addresses versus depleted addresses, or the number of leases processed per second. Additional statistical information includes the number of messages and offers processed, as well as the number of requests, acknowledgements (DHCPACKS), declines, negative status acknowledgment messages (DHCPNACKS), and releases received.
The DHCP snap-in can display the total number of scopes and addresses on a server, the number used, and the number available for a particular scope.
DHCP Vendor-Specific and Class ID Option Support
In Windows Server 2003, DHCP supports vendor-specific defining of options. When an option is identified as vendor-specific, DHCP looks up the configuration as specified for the specific vendor. This feature enables the quick introduction of compelling custom applications for enterprise networks. Equipment from multiple vendors on a network can also use different option numbers for different functions. RFC 2132 describes the vendor class and vendor options.
Most implementations of DHCP treat clients equally, rendering the server unaware of the specific types of clients with which it interacts. This behavior necessitates a common configuration issued by the server to all DHCP clients. It also restricts the server to assigning only an address from a scope, along with the options available within that scope.
In Windows Server 2003, the implementation of user classes allows DHCP clients to distinguish themselves by specifying a client type, such as a desktop or laptop. An administrator can then configure a DHCP server to assign different options, depending on the type of client receiving them. For example, the server could assign short leases to laptop clients, while desktop clients on the same network might require other settings. User class support gives administrators greater flexibility in configuring clients.
Vendor and user classes are made available for use by DHCP clients and servers in a variety of ways. The following table compares and contrasts the features of these classes.
Table 1. Vendor and User Classes Available for Use by DHCP Clients and Servers
User Classes
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Vendor Classes
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Part of proposed DHCP standard drafts. For updated information, obtain the latest draft from the Request for Comments Web site at http://ietf.org/.
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Part of accepted DHCP standard drafts (RFC 2131, 2132).
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Clients identify user class membership with user class information.
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Clients identify vendor class membership by using the vendor class identifier option.
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User class IDs can be set and viewed at DHCP clients running Windows 2000, Windows XP, or Windows Server 2003 operating systems by using the ipconfig command.
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Vendor class IDs are configured internally by DHCP client system vendors, such as Microsoft and other vendors.
When adding a vendor class at the server, you need to specify class data at the server that exactly matches the class ID used by same-vendor clients. For more information, contact the applicable vendor.
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With user classes, you can take advantage of predefined classes that can be used to support specialized groups of clients, such as Bootstrap Protocol (BOOTP) or Routing and Remote Access clients.
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Vendor classes provide classification of DHCP clients by Windows 98, Windows 2000, or Microsoft DHCP clients, which share the use of the Microsoft-reserved vendor prefix (MSFT) in any class-related data.
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User classes are typically created for administrative reasons, such as to identify all DHCP clients in a specific site or location. For example, all computers and printers located on the same floor in an office building might be assigned or configured as part of the same user class.
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Vendor classes are typically created for the enhancement of vendor-specific DHCP functionality. For example, Microsoft provides several additional vendor-specific options, such as the ability to disable NetBIOS over TCP/IP for DHCP clients running Windows 2000 or Windows XP.
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User classes are most helpful to users for managing DHCP option assignments based on their needs. If your DHCP network is large enough to benefit from user classes, you can freely customize your DHCP environments with them. In most cases, the user class provides a way to assign and override standard DHCP option values set at either the server, scope, or reserved client level.
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Vendor classes are most helpful to vendors for managing DHCP option assignments based on vendor-specific needs without disturbing other non-vendor DHCP clients. With a vendor class, options are passed to clients using the vendor specific information option, which encapsulates the options it sends. This behavior helps non-vendor clients to recognize the information as vendor-specific so they can ignore and discard it as needed.
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