Droms R. The DHCP handbook
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201 West 103rd Street, Indianapolis, Indiana 46290 USA
The DHCP
Handbook
Second Edition
Ralph Droms and Ted Lemon
000 3273 FM 10/3/02 5:02 PM Page i
The DHCP Handbook, Second Edition
Copyright © 2003 by Sams Publishing
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ity for errors or omissions. Nor is any liability assumed for damages
resulting from the use of the information contained herein.
International Standard Book Number: 0-672-32327-3
Library of Congress Catalog Card Number: 2001095796
Printed in the United States of America
First Printing: October 2002
05 04 03 02 4321
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Page Layout
Ayanna Lacey
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Contents at a Glance
Preface to the Second Edtion .......................................................xxiv
Part I Introduction to DHCP
1 An Introduction to DHCP
...............................................................3
2 An Example of DHCP in Operation
.................................................19
3 Configuring the DHCP Server
.........................................................29
4 Configuring TCP/IP Stacks
.............................................................43
II DHCP Theory of Operation
5 The DHCP Client/Server Model
......................................................59
6 The Format of DHCP Messages
.......................................................69
7 Transmitting DHCP Messages
.........................................................83
8 DHCP Message Exchanges
............................................................103
9 DHCP Options
...........................................................................123
10 Failover Protocol Operation
..........................................................159
11 DHCP–DNS Interaction
...............................................................177
III DHCP Servers and Clients
12 Theory of the Operation of a DHCP Server
.....................................193
13 The Microsoft DHCP Server
..........................................................203
14 The ISC DHCP Server
..................................................................223
15 Configuring a DHCP Server
..........................................................253
16 Client Identification and Fixed-Address Allocation
...........................275
17 Setting Up a Reliable DHCP Service
...............................................289
18 Configuring a Failover Server
........................................................301
19 Tuning a DHCP Service
...............................................................321
20 Conditional Behavior
..................................................................337
21 DHCP Clients
............................................................................355
22 Setting Up DHCP in a Small Office
................................................381
23 Updating DNS with DHCP
...........................................................393
24 Debugging Problems with DHCP
...................................................413
25 DHCP for IPv6
...........................................................................431
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IV Appendixes
A Microsoft DHCP Server Examples
..................................................441
B ISC DHCP Server Configuration File Reference
................................459
C The DHCP Message Format
..........................................................503
D DHCP Options Summary
.............................................................507
E Bibliography and Other DHCP Resources
........................................519
F DHCP Server and Operating System Versions
..................................525
G Glossary
....................................................................................531
Index
.......................................................................................545
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Table of Contents
Part I Introduction to DHCP 1
1 An Introduction to DHCP 3
Configuring Devices on a Network
...................................................3
IP Address Allocation
..............................................................4
Configuration Information
......................................................5
Configuring Network Devices
...................................................5
Moving Devices to Different Network Segments
..........................5
Moving or Adding Network Services
..........................................6
Renumbering the Network
.......................................................6
Reclaiming Disused IP Addresses
...............................................6
A First Attempt at Automating Device Configuration
...........................7
The Benefits of DHCP
.....................................................................7
Availability of DHCP Clients
....................................................8
DHCP on Large Networks
........................................................8
Mobility
...............................................................................8
DHCP on Small Networks
........................................................9
Assigning IP Addresses Using DHCP
..................................................9
DHCP Server as Agent
.............................................................9
Address Leasing
....................................................................10
Address Reclamation with DHCP
............................................10
Renumbering with DHCP
......................................................10
Describing Network Services with DHCP
..................................10
Moving or Adding Network Services with DHCP
........................11
Perceived Problems of DHCP
..........................................................11
Excess Broadcast Traffic
.........................................................11
Server Load
.........................................................................12
DHCP Reliability
..................................................................13
When Not to Use DHCP
........................................................14
Address Allocation Policies
............................................................14
Static Allocation
...................................................................15
Dynamic Allocation
..............................................................15
Automatic Allocation
............................................................15
Hybrid Allocation Policies
......................................................15
Summary
....................................................................................16
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2 An Example of DHCP in Operation 19
Setting Up the GSI Network
...........................................................19
Using DHCP to Configure Computers
..............................................21
Using the DHCP Server to Obtain a New IP Address
...................21
Restarting
desktop1 ..............................................................22
Moving
desktop1 to a New Network Segment ...........................23
Retiring
desktop1 from Service ...............................................25
Leases on IP Addresses in DHCP
.....................................................25
Two Alternative Mechanisms to Leasing
...................................26
Benefits of DHCP Leases
........................................................27
Summary
....................................................................................28
3 Configuring the DHCP Server 29
Specifying the Basic Network Architecture
........................................29
Subnet Declarations
..............................................................30
Subnet Address Allocation
......................................................31
Required Configuration Parameters
.................................................32
Configuration Options
..........................................................33
Specifying Leases
..........................................................................34
Lease Durations
....................................................................35
GSI Subnet Leases
.................................................................36
Other DHCP Options
....................................................................37
Subnet Options
....................................................................38
Global Values for Options
......................................................39
Extending a Lease and Moving Between Subnets
...............................40
Extending a GSI Lease
...........................................................40
Moving Between GSI Subnets
.................................................41
Other Configuration Information
...................................................41
Summary
....................................................................................41
4 Configuring TCP/IP Stacks 43
The TCP/IP Protocol Suite
..............................................................43
The Physical Layer
........................................................................44
The Data Link Layer
.....................................................................44
Address Translation
...............................................................45
The Internet Layer
........................................................................46
Network (IP) Addresses
..........................................................46
Subnetting
..........................................................................48
Datagram Delivery
................................................................49
The DHCP Handbookvi
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Multiple IP Networks on a Network Segment ............................50
Multicast
.............................................................................50
Other Internet-Layer Parameters
.............................................51
Summary of IP Software Parameters
.........................................51
The Transport Layer
......................................................................53
UDP
...................................................................................53
TCP
....................................................................................53
The Application Layer
...................................................................54
The Client/Server Model
........................................................54
Summary
....................................................................................55
Part II DHCP Theory of Operation 57
5 DHCP Client/Server Model 59
DHCP Goals and Design Decisions
..................................................59
Administrative Control, Correctness, and Reliability
...................59
Avoiding Manual Configuration and Reducing Changes
to Configuration
................................................................61
Identifying Clients
................................................................62
New Functions in DHCP
........................................................62
Functions Not Included in DHCP
............................................63
Related TCP/IP Protocols
...............................................................64
The DHCP Client/Server Architecture
..............................................66
Summary
....................................................................................67
6The Format of DHCP Messages 69
DHCP Message Format Overview
....................................................69
The Fixed-Format Section
..............................................................70
The
options Section .....................................................................71
The
DHCP message type Option..............................................72
The
subnet mask Option .......................................................73
The
router Option ...............................................................74
The
DNS server Option.........................................................74
The
requested IP address Option .........................................75
The
end Option....................................................................75
Examples of Message Formats
.........................................................76
The
DHCPREQUEST Message Format ...........................................76
The
DHCPACK Message Format..................................................77
Design Constraints
.......................................................................80
Summary
....................................................................................81
Contents vii
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7Transmitting DHCP Messages 83
Using UDP for DHCP
....................................................................83
Broadcast Messages
...............................................................84
Unicast Messages
..................................................................85
Server Response Messages
......................................................85
Using Broadcast for Delivery to Clients
....................................86
Using the Broadcast Flag
........................................................86
Relay Agents
................................................................................86
Relay Agent Options
.............................................................87
Forwarding Destinations
........................................................88
Response Delivery
................................................................88
Multiple Relay Agents
...........................................................89
Relay Agent Implementation
..................................................90
Reliable Delivery of DHCP Messages
................................................90
Avoiding Message Collisions
..................................................91
Transaction IDs
....................................................................91
Other Transmission Methods
.........................................................92
DHCPREQUEST Messages...........................................................92
DHCPINFORM Messages ............................................................93
DHCPRELEASE Messages...........................................................94
DHCPFORCERENEW Messages ......................................................94
DHCPLEASEQUERY Messages ......................................................94
Authenticated DHCP Messages
.......................................................94
Protocol Design
....................................................................95
The Authentication Token Protocol
.........................................96
The Delayed Authentication Protocol
......................................97
Summary
..................................................................................101
8 DHCP Message Exchanges 103
Client States
..............................................................................104
Obtaining an Initial Configuration
........................................105
Confirming an IP Address When Restarting
.............................109
Extending a Lease
...............................................................112
Extending a Lease from a Different Server
...............................114
When a Lease Expires
..........................................................115
Moving to a New Network
...................................................115
Working with Multiple Servers
......................................................116
Obtaining an Initial Address
.................................................117
Restarting
..........................................................................118
Broadcasting to Extend a Lease
.............................................118
The DHCP Handbookviii
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Other Message Exchanges ............................................................119
Obtaining Configuration Information with an IP Address
Not Obtained Through DHCP
.............................................119
Terminating a Lease
............................................................120
Updating a Client’s Configuration
.........................................120
Summary
..................................................................................120
9 DHCP Options 123
DHCP-Specific Options
................................................................124
DHCP message type.............................................................124
client identifier .............................................................125
server identifier .............................................................126
requested address .............................................................126
lease time........................................................................127
lease renewal time (T1) ...................................................127
lease renewal time (T2) ...................................................127
vendor class identifier ...................................................128
vendor-specific information .............................................128
parameter request list .....................................................129
message.............................................................................129
maximum DHCP message size ................................................129
option overload ................................................................130
TFTP server name ..............................................................130
bootfile name ...................................................................130
pad...................................................................................131
end...................................................................................131
Host Configuration Parameters Options
.........................................131
host name .........................................................................131
domain name ......................................................................132
client FQDN ......................................................................132
time offset ......................................................................134
bootfile size ...................................................................134
root path .........................................................................134
swap server ......................................................................134
extensions path ................................................................135
merit dump file................................................................135
user class........................................................................135
Contents ix
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