Droms R. The DHCP handbook

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The DHCP Handbookxx

Examples 16.9, 16.10, and 16.11 .............................................................455

Example 17.1 ...........................................................................................456

Example 18.1 through 18.9......................................................................456

Example 19.1 ............................................................................................456

Examples 20.1 through 20.14 ..................................................................456

Example 22.1 ............................................................................................456

Example 22.2 ............................................................................................457

Example 22.3 ............................................................................................457

Example 22.4 ............................................................................................457

Examples 22.5 through 22.7 ....................................................................457

Examples in Appendixes ..........................................................................457

B ISC DHCP Server Configuration File Reference 459

How to Use This Appendix

...........................................................459

File Organization

........................................................................460

The

shared-network Declaration...................................................463

The

subnet Declaration ...............................................................463

The

range Declaration.................................................................464

The

host Declaration ..................................................................464

The

hardware Declaration ....................................................465

The

dhcp-client-identifier option Statement......................465

The

fixed-address Declaration.............................................466

The

pool Declaration ..................................................................466

The

known clients Permit ...................................................467

The

unknown clients Permit ................................................467

The

members of Permit........................................................467

The

dynamic bootp clients Permit.......................................467

The

all clients Permit ......................................................467

The

class Declaration.................................................................468

The

match if Statement ......................................................468

The

match Statement ...........................................................468

The

spawn with Statement ...................................................468

The

lease limit Statement..................................................469

The

subclass Declaration ............................................................469

The

group Declaration.................................................................470

The

option space Declaration......................................................470

The

include Directive .................................................................471

The

key Declaration ....................................................................471

The

zone Declaration ..................................................................472

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Contents xxi

The failover peer Declaration ....................................................472

The

primary Statement ........................................................473

The

secondary Statement.....................................................473

The

address Statement .......................................................473

The

peer address Statement ................................................473

The

port Statement.............................................................473

The

peer port Statement.....................................................474

The

max-response-delay Statement.......................................474

The

max-unacked-updates Statement .....................................474

The

mclt Statement.............................................................474

The

hba and split Statements ..............................................474

The

load balance max secs Statement....................................475

Programming Statements

.............................................................475

The

if Statement................................................................475

The

else Clause .................................................................475

The

elsif Clause................................................................476

The

switch Statement .........................................................476

The

on Statement................................................................477

The

log Statement ..............................................................477

The

set Statement ..............................................................477

The

unset Statement ...........................................................478

Expressions

...............................................................................478

Indeterminate Operators

......................................................479

Boolean Operators

..............................................................479

Data Operators

...................................................................480

Numeric Operators

..............................................................484

DNS Operators

...................................................................485

Parameter Statements

..................................................................486

The

allow, deny, and ignore Statements.................................486

The

default-lease-time Statement.......................................487

The

max-lease-time Statement .............................................487

The

min-lease-time Statement .............................................487

The

min-secs Statement......................................................488

The

dynamic-bootp-lease-cutoff Statement..........................488

The

dynamic-bootp-lease-length Statement ..........................489

The

get-lease-hostnames Statement .....................................489

The

authoritative Statement...............................................489

The

always-reply-rfc1048 Statement....................................490

The

use-lease-addr-for-default-route Statement .................490

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The DHCP Handbookxxii

The server-identifier Statement ........................................490

The

vendor-option-space Statement .....................................491

The

site-option-space Statement ........................................491

The

always-broadcast Statement ..........................................491

The

ddns-domainname Statement ...........................................492

The

ddns-hostname Statement...............................................492

The

ddns-rev-domainname Statement .....................................492

The

lease-file-name Statement ...........................................492

The

pid-file-name Statement...............................................492

The

ddns-updates Statement ................................................492

The

omapi-port Statement ...................................................493

The

omapi-key Statement.....................................................493

The

stash-agent-options Statement .....................................493

The

ddns-ttl Statement ......................................................493

The

update-optimization Statement .....................................493

The

ping-check Statement ...................................................494

The

update-static-leases Statement....................................494

The

log-facility Statement ................................................494

Statements That Define Values to Send to Clients

............................494

The

filename Statement ......................................................495

The

server-name Statement..................................................495

The

next-server Statement..................................................495

The

option Statement .........................................................495

The option Definition

.................................................................496

The

boolean Type ...............................................................496

The

integer Type ...............................................................497

The

ip-address Type ..........................................................497

The

text Type....................................................................497

The

string Type.................................................................497

Arrays

...............................................................................498

Records

.............................................................................498

The Standard DHCP Options

........................................................498

C The DHCP Message Format 503

The

fixed-format Section............................................................503

The

htype Field ..................................................................505

The

variable-format Section .......................................................506

D DHCP Options Summary 507

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Contents xxiii

EBibliography and Other Resources 519

Web Resources for DHCP

.............................................................519

RFCs Related to DHCP

.................................................................520

Other RFCs of Interest

.................................................................521

Additional Reading

.....................................................................523

F DHCP Server and Operating System Versions 525

Choosing a DHCP Server

.............................................................525

Operating System Platforms

..................................................525

User Interface

.....................................................................526

Database Formats

................................................................527

Support for BOOTP Clients

...................................................527

ISC DHCP Server Operating System Dependencies

...........................528

Problems with the 255.255.255.255 Broadcast Address

..............528

Linux Difficulties

................................................................528

HP-UX Difficulties

..............................................................530

Solaris Difficulties

...............................................................530

GGlossary 531

Index 545

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Preface to the Second Edition

Since the publication of the first edition of this book, DHCP has continued to grow

in importance as a network management tool. Service providers have expanded the

use of DHCP as a part of automated subscriber management systems that reduce

their cost of operation. Inexpensive routers used for home and small office Internet

connections now often include DHCP services that provide plug-and-play operation

of computers that use private IP addressing.

In this second edition of The DHCP Handbook, the authors have updated their

description of the details of DHCP to include new DHCP options and messages.

Several new DHCP options have been defined since the publication of the first

edition, including a mechanism for the authentication of DHCP messages, the Relay

Agent Information option, through which relay agents can provide additional infor-

mation to a DHCP server and the User Class option, through which a host gives

more information about itself to the DHCP server. There is also a new message,

DHCPFORCERENEW, through which a DHCP server can cause a DHCP client to

contact the server for a new IP address and other configuration information.

Redundant operation of DHCP servers can now be implemented through the DHCP

failover protocol.

Another change in this edition is the use of the Microsoft Windows 2000 DHCP

server in several examples. The Windows 2000 server has some new features, which

are explained in detail. There is also a new user interface, which is illustrated in the

screen shots of the operation of the Windows 2000 server throughout the book.

Although the fundamental definition of DHCP, as specified in RFC 2131 and RFC

2132 has not changed, there have been many developments in the function and use

of DHCP since the first edition. The authors have updated many chapters and added

several new chapters in this second edition. Chapters 10 and 18 describe the DHCP

failover protocol and how to use it. Chapters 11 and 23 describe the details and

configuration of DHCP–Dynamic DNS (DDNS) interaction. Our intention is to

provide the most current and useful guide to the operation and use of DHCP in this

second edition of The DHCP Handbook.

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An Introduction to DHCP

Early in the development of the TCP/IP protocols, little motivation existed for

automating the configuration of devices that use TCP/IP. Few computers used TCP/IP,

and the computers that were networked weren’t very portable. Perhaps most signifi-

cantly, the majority of computers were shared among many users and had desig-

nated administrators who managed many operational details, including TCP/IP

configuration.

Today, everything is different. An organization may have thousands—in some cases

tens of thousands—of computers on its internal network. Devices ranging from

mainframes to desktop computers to personal digital assistants (PDAs) to embedded

processors are networked. Computers are highly mobile so that laptops, PDAs, and

similar devices can move between network segments many times during the course

of a day. And today’s computers are not typically managed by trained system admin-

istrators. Most computers are set up and installed by users who aren’t familiar with

(and who probably don’t want to know about) the arcane details of the TCP/IP

protocol suite.

To meet the demands of plug-and-play operation through automating the configura-

tion of networked computers, the IETF developed DHCP. DHCP provides automated,

managed configuration of computers and other devices that use TCP/IP. Through

DHCP, a network administrator can assign a network address and supply a subnet

mask and a default router. DHCP is built around a client/server model in which

networked computers (the clients) contact a centralized configuration server for

configuration parameters. The administrator supplies the server with a description of

the network infrastructure, along with rules about how to assign addresses and other

configuration parameters. The server interacts directly with clients, according to the

rules the administrator provides. Thus, the DHCP server acts as the network adminis-

trator’s agent for managing the configurations of DHCP clients.

Through the DHCP server, you can control the assignment of addresses and the

configuration of other TCP/IP protocol parameters in whatever way is appropriate for

the network and the organization. You can use fully dynamic address assignment,

preassign a specific address to each computer, or use a mixed strategy in which your

server computers are assigned fixed addresses and other computers are assigned

addresses on demand.

The bottom line is that DHCP allows you to build a networking system that enables

your users to freely add new computers, replace existing computers, and move

computers between networked locations, all without explicit intervention on the

part of the users or of a network administrator. In fact, this introduction was written

on a laptop that was connected to a campus network from Ralph Droms’ home

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through an ADSL link, a campus office, and two campus classrooms (while he gave

two final exams). Although each of these locations is serviced by a different part of

the campus network, Ralph was able to simply turn on his laptop in each location

and use the network immediately.

DHCP is currently a Draft Standard of the IETF. It is an open, vendor-independent

standard. The specifications for DHCP are written in RFC 2131 and RFC 2132, which

are available from

www.rfc-editor.org. DHCP clients and servers are widely avail-

able from major software vendors, as well as from the ISC.

DHCP is a product of the Dynamic Host Configuration Working Group (DHCWG) of

the IETF. The DHCWG first met at the IETF meeting in Cocoa Beach, Florida, in April

1989. At that meeting, the DHCWG defined the problem that it would address to be

the automated configuration of TCP/IP hosts, including allocation of a network

address and transmission of other parameters, such as the subnet mask and a default

router.

DHCP is loosely based on BOOTP (RFC 951). DHCP retains the basic message format

of BOOTP and the operation of BOOTP relay agents, and it shares the UDP ports

initially assigned to BOOTP (67 and 68). This backward compatibility with BOOTP

allows DHCP to use the installed base of BOOTP relay agents and avoid the require-

ment of a DHCP server on every network segment.

DHCP is still a work in progress. The DHCWG has several additional functions under

development for DHCP, which are described in the last few chapters of this book. For

current information on the status of DHCP and the activities of the DHCWG, visit

www.dhcp.org or www.ietf.org/html.charters/dhc-charter.html.

Objectives of This Book

As we wrote this book, we set as our goal the development of a complete resource for

understanding DHCP, designing DHCP services, and debugging problems with DHCP

clients and servers. We start with the background and theory of DHCP, including

message exchanges between clients and server, message formats, and an introduction

to the ISC DHCP server. Next, we describe the implementation and operation of

DHCP servers and clients. We spend more time describing the DHCP server than the

client; the DHCP server is more interesting because it is the component that the

network administrator usually interacts with, whereas the DHCP client simply runs

automatically and in the background. We also discuss practical aspects of DHCP—

why you should use it, when to use it, and how to design and run an efficient DHCP

service.

We include examples and case studies of DHCP in operation throughout the book.

We drew the case studies from our experience with DHCP in real IP networks,

and we constructed the examples to illustrate specific concepts and ideas. Along with

The DHCP Handbookxxvi

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An Introduction to DHCP xxvii

these examples and case studies, we included notes, tips, and warnings based on

experience with the design of DHCP, the implementation of DHCP clients and

servers, and the application of DHCP to production networks. We included this

material to flesh out the framework of the theory and principles of DHCP with as

much information about DHCP in practice as we could.

This Book’s Audience

This book is intended for network planners, implementers, and administrators; in

short, it is for anyone who must design, implement, manage, or debug a network

that uses DHCP. Planners considering the use of DHCP or designing a DHCP service

will find the protocol description and design guidelines of particular value. If you are

not already familiar with the use and architecture of DHCP, be sure to read the first

three chapters, which introduce DHCP through an example and explain some of the

details of DHCP.

The discussion of DHCP and its applications assumes some familiarity with the

details of the TCP/IP protocols. In particular, we assume that you understand hard-

ware and IP addressing, subnetting, routing, and some of the application-layer serv-

ices, such as DNS. We review some aspects of TCP/IP that are specific to

understanding and using DHCP in Chapter 4, “Configuring TCP/IP Stacks.” For a

more comprehensive introduction to TCP/IP, we recommend either Internetworking

with TCP/IP, by Doug Comer, or TCP/IP Illustrated, by W. Richard Stevens.

Readers who are already using DHCP will find the material in this book on configur-

ing and tuning a DHCP server of particular interest. Anyone running a large installa-

tion will want to read about reliable DHCP service.

The later chapters are intended for anyone who is tracking and planning for future

developments in DHCP. We, the authors, are both participants in the IETF working

group that is responsible for DHCP, and we have included material on current work

within the IETF in areas such as authentication, interserver communication,

DHCP/LDAP integration, and DHCP for IPv6.

Organization of This Book

The book is written in three main parts. Part I, “Introduction to DHCP,” introduces

DHCP through examples and provides some background on configuring TCP/IP

protocol stacks.

Part II, “DHCP Theory of Operation,” focuses on the specification and operation of

DHCP. This section of the book begins with an explanation of the objectives of

DHCP and the motivation behind the design decisions in DHCP. Part II also includes

detailed descriptions of the DHCP message formats and message exchanges between

clients and servers, as well as the role of relay agents in those message exchanges.

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Part III, “DHCP Servers and Clients,” begins with a description of the operation of

the ISC and Microsoft DHCP servers and clients. Next, we explain how to configure

the ISC server, with several specific examples. In the following chapters, we discuss

more advanced topics in DHCP service design, such as customized client configura-

tions, reliable DHCP service, tuning a DHCP service, and setting up DHCP in a small

office. Part III concludes with material on current work in DHCP, including authenti-

cation, interaction between DHCP and DNS, communication between DHCP servers,

and the development of DHCP for IPv6.

Throughout the book, we give examples that use the ISC DHCP server. This server is

freely available and therefore accessible even to readers whose employers may

already have purchased a commercial DHCP server and, thus, are not in a position to

purchase whatever commercial DHCP server we might have used in our examples.

Appendix A, “Microsoft DHCP Server Examples,” includes a list of examples and

expository text for the Microsoft DHCP server, keyed to the examples throughout

the book that use the ISC DHCP server. This server is the commercial server that a

reader is most likely to have ready access to.

We would have liked to provide examples for a wider variety of DHCP servers, but

unfortunately, every DHCP server has a different configuration syntax, and we

simply couldn’t provide examples for all of them. We believe that by showing exam-

ples presented for two DHCP servers with very different configuration mechanisms,

a reader using a third DHCP server will most likely understand each example as it

relates to whatever configuration mechanism that server uses.

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About the Authors

The authors of this text, Ralph Droms and Ted Lemon, bring extensive expertise and

experience with DHCP and IP networking to this book. In this text, the authors

combine their insights to create a unique perspective on the theory and design of

the DHCP specification, as well as the practical aspects of implementing a DHCP

server and running a DHCP service.

Ralph Droms, Ph.D., organized the DHCWG with Phil Gross in 1989. He has

chaired the working group since its inception and is a key contributor to the design

and development of DHCP. Ralph is also editor of the DHCP RFCs and continues to

participate in the evolution of DHCP.

Since joining Cisco in 2000, Ralph has continued his work on DHCP and network

management. Previously, he was a member of the Computer Science Department

faculty at Bucknell University, where he guided students through the study of TCP/IP

internetworking, operating systems, and computer architecture. Ralph has also been

a member of the computer science faculty at Pennsylvania State University, and he

was on the research staff at IBM and Burroughs (now Unisys).

As a consultant in network architecture and infrastructure design, Ralph has worked

with large and small companies on a variety of TCP/IP issues, including network

architecture, server strategies and configurations, and the use of DHCP, DNS, and

other technologies in network management. Ralph served as co-director of the

computer center at Bucknell, where he supervised the design and implementation of

the campuswide multiprotocol network.

Ralph lives with his wife and two daughters in Westford, Massachusetts. You can

reach him at

rdroms@cisco.com.

Ted Lemon first encountered DHCP while working as a network administrator at

Digital Equipment Corporation in the early 1990s. In 1996 Paul Vixie of the Internet

Software Consortium became concerned that there was no high-quality open-source

implementation of DHCP, and he asked Ted if he would be willing to produce one.

The ISC DHCP distribution was the result.

As part of the work of producing the ISC DHCP distribution, Ted has been active in

the IETF DHCWG since 1996. Along with Ralph, Bernie Volz, and Jim Bound, Ted is

working on a new version of DHCP for IPv6, as well as extensions to the DHCP

protocol for IPv4.

One of the important ways that open-source projects are improved is through exami-

nation of user feedback for ways to do things better and for common problems that

users have. Ted has had a great deal of experience helping people with common

problems with the various aspects of DHCP. His motivation in working on this book

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