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Chapter Overview
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Understanding DHCP
Configuring a DHCP Server
Troubleshooting DHCP
1
Understanding DHCP
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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
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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.)
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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
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Configuring TCP/IP manually on client
computers has many limitations.
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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
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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.
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This method is necessary for some computers that
require permanently assigned IP addresses.
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DHCP Address Allocation Types
(Cont.)
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Automatic allocation: the DHCP server
supplies clients with IP addresses taken from
a common pool, and the clients permanently
retain the assigned addresses.
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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.
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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
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A typical transaction between a DHCP client
and server has four phases:
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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
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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.)
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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
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The address of the server that offered the address
The offered IP address
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DHCP Communications (Cont.)
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When the server receives the DHCPREQUEST, it
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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
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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.)
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If the DHCP server is available, it responds with one
of the following messages:
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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
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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
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A DHCP client can terminate an IP address
lease at any time by transmitting a
DHCPRELEASE message to the DHCP server.
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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
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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.
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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.
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If it contacts a DHCP server, the client abandons its
autoconfigured IP address.
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Installing a DHCP Server
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Before installing DHCP on a server, consider these
questions:
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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.
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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
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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
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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
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As an administrator, you need to know
how to
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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
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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.
2.
3.
4.
Install the Microsoft DHCP Server service.
Authorize the DHCP server.
Create an IP address scope and activate it.
Configure DHCP options.
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Authorizing a DHCP Server
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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
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When creating a DHCP scope, consider the
following:
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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.)
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Depending on the starting and ending IP
addresses for your scope, the DHCP console
suggests a default subnet mask.
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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
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A new scope in the DHCP console has four
subheadings:
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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
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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
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003
006
015
044
046
Router
DNS Servers
DNS Domain Name
WINS/NBNS Servers
WINS/NBT Node Type
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Configuring Additional Options
(Cont.)
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Options can be configured at the server or
scope level.
Use the DHCP console to configure additional
DHCP options.
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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
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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
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If your internetwork requires multiple DHCP
servers
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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
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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
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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
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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
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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
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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
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Many DHCP problems involve incorrect or
missing configuration details.
To prevent the most common DHCP problems
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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.)
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To prevent the most common DHCP problems
(Cont.)
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Create reservations on all DHCP servers that can
potentially service the reserved client
For server performance, remember that DHCP is diskintensive 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
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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
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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
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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
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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
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If clients can access the network but cannot
obtain IP addresses from a DHCP server,
there are several possible causes:
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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
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When a DHCP server fails, clients experience
TCP/IP problems or errors.
First, ensure that the DHCP Server service is
running on the server.
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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
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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
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When the DHCP console displays the lease
expiration time for reserved clients for a scope, it
indicates one of the following:
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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.
47
DHCP Server Uses Broadcasts to
Respond to All Client Messages
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By default, a Windows 2000 DHCP server
attempts to transmit all DHCP responses to
the limited broadcast address
(255.255.255.255).
You can
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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
48
DHCP Server Fails to Issue Address
Leases for a New Scope
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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:
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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.
49
Monitoring Server Performance
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Monitoring the performance of a Windows
2000 DHCP server can help in troubleshooting
performance problems.
The Windows 2000 DHCP server performance
counters can monitor
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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
50
Lesson Summary
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Most DHCP problems present themselves as a failure
of clients to obtain IP addressing information from a
DHCP server.
When troubleshooting clients, ensure that
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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.
51