Dynamic Host Configuration Protocol: Frequently Asked Questions

DHCP is an Internet Engineering Task Force (IETF) standard specified in Request for Comments (RFC) 2131 for simplifying management of host Internet Protocol (IP) configuration. The DHCP standard defines the use of DHCP servers as a way to manage dynamic allocation of IP addresses and other related configuration details for DHCP-enabled clients on your network.

DHCP provides the following benefits for administering your TCP/IP-based network:

• Reliable configuration DHCP avoids configuration errors caused by the need to manually type in values at each computer. In addition, DHCP helps prevent address conflicts caused by a previously assigned IP address being reused to configure a new computer on the network.
• Reduces configuration management Using DHCP servers can greatly decrease time spent configuring and reconfiguring computers on your network. Servers can be configured to supply a full range of additional configuration values when assigning address leases. These values are assigned using DHCP options.

The DHCP lease renewal process helps assure that where client configurations need to be updated often (such as users with mobile or portable computers who change locations frequently), these changes can be made efficiently and automatically by clients communicating directly with DHCP servers.

DHCP is defined in the following IETF RFCs:

• RFC 2131: Dynamic Host Configuration Protocol
• RFC 2132: DHCP Options and BOOTP Vendor Extensions

For additional standards related to DHCP, see the Dynamic Host Configuration IETF working group.

DHCP in Windows Vista, Windows XP, Windows Server 2008, and Windows Server 2003 is IETF standards compliant.

DHCP documentation is included with Windows Server 2003 and Windows Server 2008 (click Start, then click Help and Support). There are also DHCP sections of the Windows Server 2003 Deployment Guide and Windows Server 2003 Technical Reference.

For the product documentation resources available for DHCP in Windows Server 2008, see the Windows Server 2008 DHCP TechCenter.

For the product documentation resources available for DHCP in Windows Server 2003, see the Windows Server Dynamic Host Configuration Protocol (DHCP) TechCenter.

For a list of all the resources for DHCP in Windows Server 2008, Windows Server 2003, and Windows 2000 Server, see the Dynamic Host Configuration Protocol Web site.

For private networks, DHCP is used to provide centralized IP address configuration information to DHCP client computers across small, medium, or enterprise networks.

For public networks, DHCP is used to provide centralized IP address configuration to DHCP client computers connecting to the Internet, such as through broadband Internet connections or through a wireless hotspot.

Current versions of Windows support the three elements of a DHCP infrastructure:

• Computers running Windows XP, Windows Server 2008, or Windows Server 2003 can use the DHCP Client service to act as a DHCP client and automatically obtain an IP address configuration from a DHCP server.
• Computers running Windows Server 2008 or Windows Server 2003 can use the DHCP Server service to act as a DHCP server and automatically assign IP address configurations to DHCP client computers.
• Computers running Windows Server 2008 or Windows Server 2003 can use the Routing and Remote Access service to act as a DHCP relay agent (also known as a Bootstrap protocol [BOOTP] relay agent) and listen for broadcast and unicast DHCP messages and relay them between DHCP servers and clients.

A DHCP server running Windows Server 2008 or Windows Server 2003:

• Is integrated with the Active Directory directory service to prevent unauthorized or rogue DHCP servers from starting on your network.
• Is integrated with the Domain Name System (DNS) and performs DNS dynamic update for DHCP clients on their behalf.
• Can take advantage of Windows clustering for high availability.

The DHCP Server and DHCP Client services include the following enhancements:

• Dynamic Host Configuration Protocol for IPv6 (DHCPv6) support



DHCPv6, defined in RFC 3315, provides stateful address configuration for IPv6 hosts on a native IPv6 network. The DHCP Client service in Windows Vista and Windows Server 2008 supports DHCPv6. A computer running Windows Vista or Windows Server 2008 can perform both DHCPv6 stateful and stateless configuration on a native IPv6 network. The DHCP Server service in Windows Server 2008 supports DHCPv6 stateful (both addresses and configuration settings) and stateless (configuration settings only) operation.

For more information about DHCPv6, see The DHCPv6 Protocol.

• Network Access Protection enforcement support



DHCP enforcement in the Network Access Protection (NAP) platform requires a DHCP client to prove its system health before receiving an address configuration for unlimited access. For more information, see the Network Access Protection Web page.

For more information, see the Microsoft Windows DHCP Team Blog.

DHCPv6 is the IPv6 counterpart to DHCP for IPv6 networks. DHCPv6 can be used to assign stateful IPv6 addresses or stateless configuration settings to IPv6 hosts. DHCPv6 is defined in RFC 3315.

The DHCP Client service in Windows Vista and Windows Server 2008 supports DHCPv6. A computer running Windows Vista or Windows Server 2008 can perform both DHCPv6 stateful and stateless configuration on a native IPv6 network. The DHCP Server service supports both DHCPv6 stateless and stateful operation.

For more information, see The DHCPv6 Protocol.

DHCP is a client-server protocol in which a DHCP client requests an IP address configuration from a DHCP server on the network. DHCP servers store the set of available IP addresses and related configuration information in a database. When DHCP clients start up on a network, they must discover the DHCP servers on the network and request IP configuration information. The DHCP servers respond with an offered IP address configuration. The DHCP client then requests the use of a specific IP address configuration from among those offered, which is acknowledged by the DHCP server that offered it.

Because the DHCP server typically only allocates an IP address configuration for a specific amount of time known as the lease time, the DHCP client must renew the lease time of its IP address configuration on an ongoing basis. If the DHCP client does not renew the lease time before it expires or changes the subnet to which it is connected, it must obtain a new IP address configuration.

For a detailed technical explanation of how DHCP works, see Chapter 6 - Dynamic Host Configuration Protocol of the TCP/IP Fundamentals for Microsoft Windows online book.

DHCP clients and servers use the following messages to communicate during the DHCP configuration process:

• DHCPDiscover Sent from client to server to initially discover the presence of DHCP servers on the network.
• DHCPOffer Sent from server to client to respond to the DHCPDiscover message. The DHCPOffer message contains an IP address configuration offered to the requesting DHCP client.
• DHCPRequest Sent from client to server to request a specific IP address configuration from a specific DHCP server.
• DHCPAck Sent from server to client to acknowledge that the client has been allocated a specific IP address configuration.
• DHCPNak Sent from server to client to indicate that the client cannot use a specific IP address configuration. For example, DHCP servers send DHCPNak messages when a wireless client has moved to a different subnet and attempts to renew the lease on its previous address.
• DHCPDecline Sent from client to server to indicate that the offered IP address configuration is invalid. For example, DHCP clients send DHCPDecline messages when they discover that the offered IP address is a duplicate.
• DHCPRelease Sent from client to server to indicate that the DHCP client is no longer using the IP address configuration.
• DHCPInform Sent from client to server to request additional configuration settings.

In Windows Server 2008 and Windows Server 2003, the DHCP Server service is integrated with Active Directory to provide authorization for DHCP servers. An unauthorized DHCP server on a network can disrupt network operations by allocating incorrect addresses or configuration options. A DHCP server that is a domain controller or a member of an Active Directory domain queries Active Directory for the list of authorized servers (identified by IP address). If its own IP address is not in the list of authorized DHCP servers, the DHCP Server service does not complete its startup sequence and automatically shuts down.

This is a common issue for network administrators who attempt to install and configure a DHCP server in an Active Directory environment without first authorizing the server.

For a DHCP server that is not a member of the Active Directory domain, the DHCP Server service sends a broadcast DHCPInform message to request information about the root Active Directory domain in which other DHCP servers are installed and configured. Other DHCP servers on the network respond with a DHCPAck message, which contains information that the querying DHCP server uses to locate the Active Directory root domain. The starting DHCP server then queries Active Directory for a list of authorized DHCP servers and starts the DHCP Server service only if its own address is in the list.

The authorization process for DHCP server computers depends on the installed role of the server on your network. There are three roles or server types for which each server computer can be installed:

• Domain controller The computer keeps and maintains a copy of the Active Directory database and provides secure account management for domain member users and computers.
• Member server The computer is not operating as a domain controller but has joined a domain in which it has a membership account in the Active Directory database.
• Stand-alone server The computer is not operating as a domain controller or a member server in a domain. Instead, the server computer is made known to the network through a specified workgroup name, which can be shared by other computers, but is used only for computer browsing purposes and not to provide secured logon access to shared domain resources.

If you deploy Active Directory, all computers operating as DHCP servers must be either domain controllers or domain member servers before they can be authorized and provide DHCP service to clients.

Although it is not recommended, you can use a stand-alone server as a DHCP server as long as it is not on a subnet with any authorized DHCP servers. When a stand-alone DHCP server detects an authorized server on the same subnet, it automatically stops leasing IP addresses to DHCP clients.

To authorize a DHCP server, do the following:

1. Log on as a member of the Enterprise Admins group for the enterprise where the server is being added.
2. Click Start, click Settings, click Control Panel, double-click Administrative Tools, and then double-click DHCP.
3. In the console tree, click DHCP.
4. On the Action menu, click Manage authorized servers. The Manage Authorized Servers dialog box appears.
5. Click Authorize.
6. When prompted, type the name or IP address of the DHCP server to be authorized, and then click OK.

No. Remote access clients use the Internet Protocol Control Protocol (IPCP), defined in RFC 1332, to obtain an IP address from their remote access server. However, remote access clients running Windows Vista, Windows XP, Windows Server 2008, or Windows Server 2003 send a DHCPInform message after they have been configured with IPCP to obtain additional configuration parameters that are not included with IPCP. The DHCP Relay Agent component of Routing and Remote Access on the remote access server forwards the DHCPInform message to a DHCP server and forwards the response back to the remote access client.

The remote access server, such as a computer running either Windows Server 2008 or Windows Server 2003 and Routing and Remote Access, can use DHCP to obtain IP addresses to assign to remote access clients with IPCP.

For more information, see Using Routing and Remote Access servers with DHCP.

To configure Routing and Remote Access to use DHCP to obtain IP addresses for remote access clients, do the following:

1. Click Start, point to Programs, point to Administrative Tools, and then click Routing and Remote Access.
2. Right-click the server name, and then click Properties.
3. On the IP tab, click Dynamic Host Configuration Protocol (DHCP)
4. In Use the following adapter to obtain DHCP, DNS, and WINS address for dial-up clients, click the correct adapter.
5. Click OK.

You must configure the DHCP Relay Agent component of Routing and Remote Access with the address of at least one DHCP server. Do the following:

1. Click Start, point to Programs, point to Administrative Tools, and then click Routing and Remote Access.
2. In the console tree, right-click DHCP Relay Agent, and then click Properties.
3. On the General tab, in Server address, type the IP address of your DHCP server, and then click Add.
4. Repeat step 3 for each DHCP server you need to add, and then click OK.

The DHCP Server service in Windows Server 2008 and Windows Server 2003 can perform updates on behalf of its DHCP clients to any DNS servers that support dynamic updates. This behavior can perform DNS dynamic updates for DHCP clients that do not support DNS dynamic updates. To configure DNS dynamic update behavior for a DHCP server, do the following:

1. Click Start, click Settings, click Control Panel, double-click Administrative Tools, and then double-click DHCP.
2. In the console tree, right-click the DHCP server name, and then click Properties.
3. From the properties dialog box for the DHCP server, click the DNS tab.
4. On the DNS tab, configure the appropriate settings.

Windows Server 2008 and Windows Server 2003 provides a set of DHCP server performance counters that can be used to measure and monitor various aspects of server activity, such as the following:

• All types of DHCP messages sent and received by the DHCP service.
• The average amount of processing time spent by the DHCP server per message sent and received.
• The number of messages dropped because of internal delays at a DHCP server computer.

Use the System Monitor snap-in. For more information, see DHCP performance monitoring reference.

See the Microsoft Knowledgebase article How to move a DHCP database from a computer that is running Windows NT Server 4.0, Windows 2000, or Windows Server 2003 to a computer that is running Windows Server 2003.

See the following topics:

• Troubleshooting DHCP clients
• Troubleshooting DHCP servers
• Troubleshooting the DHCP Relay Agent

The tools you use for troubleshooting DHCP servers are the following:

• DHCP console
• Event Viewer
• Network Monitor
• System Monitor counters