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Chapter Overview

Understanding DHCP Configuring a DHCP Server Troubleshooting DHCP

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Understanding DHCP Dynamic Host Configuration Protocol

(DHCP) is a service and a protocol that work together to automatically assign Internet Protocol (IP) addresses and other Transmission Control Protocol/Internet Protocol (TCP/IP) configuration settings to computers on a network.

DHCP overcomes the limitations of manually configuring TCP/IP clients and tracking IP addresses.

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DHCP Overview DHCP dynamically assigns IP addresses to

clients from a pool. Each time a DHCP client starts, it requests IP

addressing information from a DHCP server. When a DHCP server receives the request, it

selects IP addressing information from a pool of addresses (called a scope) and offers it to the DHCP client.

If the client accepts the offer, the IP addressing information is leased to the client for a specified time.

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DHCP Overview (Cont.)

As the lease interval progresses, the client computer renews the address assignment each time the computer is started.

If the lease expires without a renewal, the IP address is returned to the pool for reassignment.

DHCP is based on open standards published by the Internet Engineering Task Force (IETF).

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DHCP Client/Server Interaction

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Manual vs. Automatic Configuration

Configuring TCP/IP manually on client computers has many limitations. Errors in assigning or typing IP addresses can

lead to difficult-to-trace network problems. Manual configuration requires a large amount

of administrative maintenance. Using DHCP to automatically configure IP

addressing information overcomes all of these limitations.

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DHCP Address Allocation Types

The core function of DHCP is to assign IP addresses.

There are three types of IP address allocation: manual, automatic, and dynamic.

Manual allocation: an administrator assigns a specific IP address for the DHCP server to provide to a specific DHCP client. This method is necessary for some computers

that require permanently assigned IP addresses.

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DHCP Address Allocation Types (Cont.) Automatic allocation: the DHCP server

supplies clients with IP addresses taken from a common pool, and the clients permanently retain the assigned addresses. This method works well when computers are rarely

moved. Dynamic allocation: the DHCP server supplies

IP addresses to clients from a pool on a leased basis. The client must periodically renew the lease or the

address expires and is returned to the pool. Most DHCP installations use dynamic allocation.

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How DHCP Works A typical transaction between a DHCP

client and server has four phases: DHCPDISCOVER: the DHCP client requests

configuration parameters from a DHCP server. DHCPOFFER: the DHCP server offers IP

addressing information to the requesting client. DHCPREQUEST: the client accepts or renews an

IP address assignment. DHCPACK: the DHCP server acknowledges the

client’s acceptance of an offered IP address.

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DHCP Client/Server Communications

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DHCP Communications When a DHCP client computer is booted,

it initiates communication with a DHCP server by generating a series of DHCPDISCOVER broadcast messages.

At this point, the client has no IP address and is said to be in the init state.

The client’s broadcasts are limited to the local subnet unless a DHCP relay agent is used, in which case the broadcasts are relayed to DHCP servers on other subnets.

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DHCP Communications (Cont.) When a DHCP server receives a

DHCPDISCOVER message from a client, it generates a DHCPOFFER message containing IP addressing information and other optional TCP/IP parameters.

The client might receive DHCPOFFER messages from multiple DHCP servers.

The client accepts one of the offered IP addresses by generating a DHCPREQUEST broadcast message that contains The address of the server that offered the address The offered IP address

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DHCP Communications (Cont.) When the server receives the DHCPREQUEST, it

Commits the offered IP addressing information to its database

Sends a DHCPACK message to the client, acknowledging the completion of the process

If the server cannot complete the address assignment, it transmits a DHCPNAK message to the client and the process begins again.

Finally, the client performs an Address Resolution Protocol (ARP) test to ensure that no other system on the network is using the assigned IP address.

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DHCP Leasing When a DHCP server dynamically allocates

settings, the client leases its IP address for a specified time and must renew the lease to continue using it.

An IP address lease is typically measured in days.

The lease renewal process begins when a bound client reaches renewal time value (or T1 value) of its lease and begins generating DHCPREQUEST unicast messages to the server that holds the lease.

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DHCP Leasing (Cont.) If the DHCP server is available, it responds with one

of the following messages: A DHCPACK message, which renews the lease and restarts

the lease timer A DHCPNAK message, which terminates the lease and

forces the client to begin again If the server does not respond, the client begins

transmitting DHCPREQUEST broadcast messages, soliciting an address assignment from any DHCP server on the network.

If the lease expires The client’s IP address is released All of the client’s TCP/IP communication ceases, except for

DHCPDISCOVER broadcast transmissions

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The DHCP Lease Renewal Process

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Releasing an IP Address

A DHCP client can terminate an IP address lease at any time by transmitting a DHCPRELEASE message to the DHCP server. On a computer running Microsoft Windows

2000, use the Ipconfig.exe utility to manually release the client computer’s IP address.

On a computer running Microsoft Windows Me, Microsoft Windows 98, or Microsoft Windows 95, use the Winipcfg.exe utility.

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When No DHCP Servers Are Online If there is no response from a DHCP server, a

client computer running Windows 2000 uses Automatic Private IP Addressing to automatically assign itself an IP address and subnet mask.

The IP address is from the range 169.254.0.1 through 169.254.255.254.

The subnet mask is 255.255.0.0. The client continues to check for a DHCP

server every five minutes. If it contacts a DHCP server, the client abandons

its autoconfigured IP address.

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Installing a DHCP Server Before installing DHCP on a server, consider these

questions: Will all of the computers on the network be DHCP clients? Will a DHCP server supply IP addresses to multiple local

area networks (LANs)? How many DHCP servers are required? What IP addressing options will clients obtain from a

DHCP server? Use Add/Remove Programs to install a DHCP

server. The computer running the DHCP Server service should

not be a DHCP client. You should manually configure a DHCP server.

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DHCP Relay Agent When a router running a DHCP relay agent

receives broadcasts from DHCP clients, it relays them to DHCP servers on other subnets.

A DHCP relay agent enables a single DHCP server to support clients on multiple subnets.

You should not configure a server running the DHCP Server service to function as a DHCP relay agent as well.

To add the DHCP Relay Agent to a computer running Windows 2000 Server that acts as a router, use Routing And Remote Access.

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Lesson Summary DHCP is a service and a protocol that work

together to automatically assign IP addresses and other TCP/IP configuration settings to computers on a network.

DHCP can allocate IP addresses in three ways: manual allocation, automatic allocation, and dynamic allocation.

A typical transaction between a DHCP client and server has four phases: DHCPDISCOVER, DHCPOFFER, DHCPREQUEST, and DHCPACK.

A DHCP relay agent relays DHCP and Bootstrap Protocol (BOOTP) messages between clients and servers located on different subnets.

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Configuring a DHCP Server

As an administrator, you need to know how to Authorize a DHCP server Create and manage DHCP scopes Configure additional DHCP options Activate a scope Implement multiple DHCP servers

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Installing and Configuring a DHCP Server

After you install the DHCP Server service on a computer running Windows 2000 Server, you must configure the server before it can assign TCP/IP settings to clients.

To implement DHCP: 1. Install the Microsoft DHCP Server service. 2. Authorize the DHCP server. 3. Create an IP address scope and activate it. 4. Configure DHCP options.

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Authorizing a DHCP Server When a misconfigured or unauthorized DHCP

server (called a rogue server) is introduced on a network, it can cause problems.

To avoid the problem of rogue DHCP servers, DHCP servers on a Windows 2000 network must be authorized in the Active Directory service before they can service clients.

All Windows 2000 DHCP servers must be either domain controllers or member servers to be authorized in Active Directory and provide DHCP service to clients.

You can authorize a DHCP server by using the DHCP console.

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The Manage Authorized Servers Dialog Box

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Creating a DHCP Scope When creating a DHCP scope, consider

the following: You must create a least one scope for every

DHCP server. You must exclude any static IP addresses you

manually assign from the scope. You must create a separate scope for each

subnet. You can create only one scope for each

subnet on a DHCP server. DHCP servers do not share scope information.

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Creating a DHCP Scope (Cont.)

Depending on the starting and ending IP addresses for your scope, the DHCP console suggests a default subnet mask. If your network requires a different subnet

mask, you can modify this value. Use the DHCP console to create a DHCP

scope.

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The IP Address Range Page of the New Scope Wizard

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The Lease Duration Page of the New Scope Wizard

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Managing DHCP Scopes A new scope in the DHCP console has four

subheadings: Address Pool: lists the IP addresses in the scope Address Leases: lists the IP addresses that are

currently leased by clients on the network Reservations: contains IP address assignments

for specific computers on the network Scope Options: contains DHCP options that are

assigned to clients obtaining IP addresses from the scope

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Configuring Additional Options You can configure DHCP options to

supply client computers with TCP/IP configuration parameters in addition to IP addresses and subnet masks.

Commonly used DHCP options are 003 Router 006 DNS Servers 015 DNS Domain Name 044 WINS/NBNS Servers 046 WINS/NBT Node Type

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Configuring Additional Options (Cont.) Options can be configured at the server

or scope level. Use the DHCP console to configure

additional DHCP options. To configure options for a server, right-click

the Server Options heading, and then click Configure Options.

To configure options for a scope, right-click the Scope Options heading, and then click Configure Options.

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The DHCP Scope Options Dialog Box

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Configuring the Router in the Scope Options Dialog Box

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Activating a Scope

You must activate the scope before it can begin servicing clients.

If you have not already done so with the New Scope Wizard, you can use the DHCP console to activate a scope.

In the DHCP console, right-click the scope you want to activate, and then click Activate.

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Implementing Multiple DHCP Servers If your internetwork requires multiple DHCP

servers Create a unique scope for each subnet on each

server Use a router that functions as a DHCP relay

agent to connect the networks Each DHCP server should have

A scope containing about 75 percent of the available IP addresses for the local subnet

A scope for each remote subnet containing about 25 percent of the available IP addresses for a subnet

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Lesson Summary On a Windows 2000 network, a DHCP

server must be authorized in Active Directory before it can service clients.

DHCP must be installed on a Windows 2000 domain controller or member server in order to be authorized.

Use the DHCP console to Create and activate a scope Configure DHCP options, such as routers and

Domain Name System (DNS) servers, for a scope or for a server

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Troubleshooting DHCP

The most common DHCP client problem is a failure to obtain an IP address from a DHCP server.

The most common DHCP server problems are The inability to start the DHCP Server

service on the network The failure of clients to obtain TCP/IP

configuration settings from a working server

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Preventing DHCP Problems Many DHCP problems involve incorrect or

missing configuration details. To prevent the most common DHCP

problems Use the 75/25 design rule for balancing scope

distribution of addresses when multiple DHCP servers are used to service the same scope

Use superscopes for multiple DHCP servers on each subnet in a LAN environment

Use server-side conflict detection on DHCP servers only when it is needed

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Preventing DHCP Problems (Cont.) To prevent the most common DHCP problems

(Cont.) Create reservations on all DHCP servers that can

potentially service the reserved client For server performance, remember that DHCP is

disk-intensive and purchase appropriate hardware Keep audit logging enabled for troubleshooting use Integrate DHCP with other services, such as

Windows Internet Name Service (WINS) and DNS Use the appropriate number of DHCP servers for the

number of DHCP-enabled clients on the network

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Troubleshooting DHCP Clients

Most DHCP-related problems present themselves as a failed IP configuration attempt by a client, so you should start DHCP troubleshooting at the client.

If a DHCP-related problem does not originate at the client, check the system event log and DHCP server audit logs for clues.

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Invalid IP Address Configuration If a DHCP client does not have an IP address

configured, or has an IP address of 169.254.x.x, the client cannot contact a DHCP server to obtain an IP address.

Ensure that The client has a functioning network connection:

check cables, network interface adapters, etc. Networking software components are installed The Obtain An IP Address Automatically option is

selected in the Internet Protocol (TCP/IP) Properties dialog box

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Missing Configuration Settings

If a DHCP client is missing configuration settings, the DHCP server might not be configured to supply those options, or the client might not support the options distributed by the server.

Verify that the options have been configured at either the server or scope level.

Ensure that the DHCP option selections are appropriate for your clients.

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DHCP Servers Do Not Provide IP Addresses If clients can access the network but

cannot obtain IP addresses from a DHCP server, there are several possible causes: The IP address of the DHCP server might

have changed. If DHCP clients must go through a router to

use a DHCP server, a DHCP relay agent might not be available.

If multiple DHCP servers are used, they might have overlapping scopes.

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Troubleshooting DHCP Servers When a DHCP server fails, clients experience

TCP/IP problems or errors. First, ensure that the DHCP Server service is

running on the server. If the service is not started, try to start it

manually. Notice the service’s startup type (Manual or

Automatic). Check the logs in Event Viewer to determine

if the service failed to start or stopped because of a problem elsewhere in the computer.

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DHCP Relay Agent Service Is Installed But Not Working

Because both the DHCP Relay Agent and the DHCP Server service listen for and respond to BOOTP and DHCP messages sent using User Datagram Protocol (UDP) ports 67 and 68, neither service works reliably if both are installed on the same computer.

Install the DHCP Relay Agent and the DHCP Server service on separate computers running Windows 2000 Server.

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DHCP Console Incorrectly Reports Lease Expirations

When the DHCP console displays the lease expiration time for reserved clients for a scope, it indicates one of the following:

If the scope lease time is set to an infinite lease time, the reserved client’s lease is also shown as infinite.

If the scope lease time is set to a finite length of time, the reserved client’s lease uses this same lease time.

To create reserved clients with unlimited lease durations, create a scope with an unlimited lease duration and add reservations to that scope.

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DHCP Server Uses Broadcasts to Respond to All Client Messages By default, a Windows 2000 DHCP server

attempts to transmit all DHCP responses to the limited broadcast address (255.255.255.255).

You can Configure the server’s registry so that it will

recognize the broadcast flag of a DHCP client Adjust its transmission behavior to send a

unicast or broadcast response to the client, depending on the flag setting in the client request

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DHCP Server Fails to Issue Address Leases for a New Scope When superscopes are not used, only a

single DHCP scope can be active on the network at one time, even if more than one scope is defined and activated.

If you want clients to receive IP addresses from a new scope, do one of the following: Configure the DHCP server to use a superscope

that includes the old scope and the new scope. Change the primary IP address for the DHCP

server so that it has the same network identifier as the new scope.

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Monitoring Server Performance Monitoring the performance of a Windows

2000 DHCP server can help in troubleshooting performance problems.

The Windows 2000 DHCP server performance counters can monitor All types of DHCP messages sent and received

by the DHCP Server service The average amount of processing time spent by

the DHCP server per message packet sent and received

The number of message packets dropped because of internal delays on the DHCP server

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Lesson Summary Most DHCP problems present themselves as a

failure of clients to obtain IP addressing information from a DHCP server.

When troubleshooting clients, ensure that The network connection is functioning The client is configured to obtain its IP address by using a

DHCP server When troubleshooting a DHCP server, make sure

that the DHCP Server service is running. To ensure that both services operate properly,

install the DHCP Relay Agent service and the DHCP Server service on separate computers running Windows 2000 Server.