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TCP/IP Stack Configuration Parameters ..........................................136
IP Layer Parameters for the Client
.........................................136
Options Defining IP Layer Parameters for a Specific Interface
....138
Link Layer Parameters Options
..............................................141
TCP Parameters Options
......................................................141
Service Parameter Options
...........................................................143
time server ......................................................................143
name server ......................................................................143
domain server ...................................................................143
log server........................................................................143
quotes server ...................................................................144
LPR server........................................................................144
Impress server..................................................................144
RLP server........................................................................144
SMTP server ......................................................................145
POP server........................................................................145
NTP server........................................................................145
finger server ...................................................................145
WWW server........................................................................146
NNTP server ......................................................................146
IRC server........................................................................146
X Window System Options
..................................................147
mobile IP home agent
.........................................................147
NIS and NIS+ Options
.........................................................147
NetBIOS over TCP/IP Options
...............................................148
StreetTalk Options
..............................................................150
NDS Options
......................................................................150
NetWare/IP Options
............................................................152
SLP Options
.......................................................................155
subnet selection
.................................................................156
authentication
...................................................................157
relay agent information
.......................................................157
Summary
..................................................................................158
The DHCP Handbookx
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10 Failover Protocol Operation 159
Failover Protocol Overview
...........................................................160
Database Synchronization
....................................................160
Address Allocation Constraints
..............................................161
Communication Between Failover Peers
.................................162
Lease Handling with Failover
........................................................163
IP Address Binding States
.....................................................163
Assigning Lease Durations with Failover
.................................165
Failover Operational States
...........................................................166
Normal Operation, Primary/Backup Configuration,
and Load Balancing
...........................................................167
Operation in the
COMMUNICATIONS-INTERRUPTED State................168
Operation in the
PARTNER-DOWN State......................................168
Operation in the
STARTUP State .............................................170
Operation in the
RECOVER State .............................................170
Operation in the
POTENTIAL-CONFLICT State ............................171
Operation in the
CONFLICT-DONE State ....................................172
Operation in the
RESOLUTION-INTERRUPTED State ......................172
Binding Update Conflicts
.............................................................173
Pool Rebalancing
........................................................................173
Complex Failover Configurations
..................................................174
Summary
..................................................................................175
11 DHCP–DNS Interaction 177
The Domain Name System
...........................................................177
DHCP and DNS
..........................................................................178
Dynamic Updates to the DNS Database
..........................................179
Dynamic Updates and DHCP
........................................................180
DHCP Client DNS Name Selection
.........................................180
Responsibility for Performing DNS Updates
.............................182
DHCP Client Name Collision
................................................183
Lease Expiration
.................................................................184
Client Relocation
................................................................185
Client Name Change
...........................................................186
DNS Dynamic Update Security Issues
.....................................187
How the DHCP Server Updates the DNS
.........................................188
Summary
..................................................................................189
Contents xi
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Part III DHCP Servers and Clients 191
12 Theory of the Operation of a DHCP Server 193
Address Allocation Strategy
..........................................................193
Allocation in Response to
DHCPDISCOVER or
BOOTREQUEST Messages .......................................................194
Address Assignment
............................................................195
Allocation and Renewal in Response to a DHCPREQUEST Message
.....196
Lease Extensions
.................................................................197
Address Use Denied
.............................................................197
DNS Updates
......................................................................198
DHCP Message Handling
.............................................................199
DHCPDECLINE Message Handling.............................................199
DHCPRELEASE Message Handling.............................................199
DHCPINFORM Message Handling ..............................................199
DHCPLEASEQUERY Message Handling ........................................200
Abandoned Lease Address Reclamation
..........................................200
Summary
..................................................................................201
13 The Microsoft DHCP Server 203
Installing the Microsoft DHCP Service
............................................203
Managing DHCP Servers
..............................................................205
Configuring DHCP Servers
...........................................................205
Configuring Scopes
.............................................................207
Configuring Superscopes
......................................................212
Adding a Scope to an Existing Superscope
...............................214
Configuring Reservations
.....................................................214
Configuring Options
...........................................................215
Client Options
...................................................................217
Vendor-Specific Options
......................................................217
User Class Options
..............................................................218
Defining New Options
.........................................................219
Controlling the Windows DHCP Server
..........................................221
Activating Scope
.................................................................221
Starting the DHCP Server Automatically and Manually
.............221
Uninstalling the DHCP Service
..............................................221
Summary
..................................................................................221
The DHCP Handbookxii
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14 The ISC DHCP Server 223
Obtaining the ISC DHCP Server
....................................................223
Support for the ISC DHCP Server
...........................................224
Installing the ISC DHCP Distribution
.....................................226
Configuring System Logging for the ISC DHCP Distribution
......231
Prerequisites to Operation of the ISC DHCP Server
...........................231
The Lease Database
.............................................................231
The Configuration File
.........................................................233
Configuring the ISC DHCP Server
.................................................234
Server Control Parameters
....................................................235
Client Options and Parameters
..............................................235
Configuring Connections to Other Services
.............................238
Network Configuration Information
......................................240
Scopes
..............................................................................240
IP Address Assignments
.......................................................244
Invoking the ISC DHCP Server
......................................................244
Command-Line Arguments
..................................................244
Specifying Interfaces
............................................................247
Server Operation
........................................................................247
Starting the Server Automatically
..........................................247
Updating the Server Configuration
........................................249
Modifying the Lease Database
...............................................249
RPM Packages for DHCP
......................................................249
Summary
..................................................................................252
15 Configuring a DHCP Server 253
Configuring a DHCP Server to Be Authoritative
...............................253
Configuring an Individual Subnet
.................................................254
Address Allocation
..............................................................255
Client Configuration Information
.........................................256
A Complete Subnet Configuration
.........................................257
Supporting Multiple Network Segments
.........................................258
Multiple Network Interfaces
.................................................258
Using DHCP Relay Agents
....................................................259
Configuring Multiple IP Subnets on Each Network Segment
..............266
Address Allocation on Shared Networks
..................................266
Option Scoping with Shared Networks
...................................267
Avoiding Routing on a Shared-Network Segment
.....................268
Pitfalls of Shared-Network Configurations
...............................270
Summary
..................................................................................273
Contents xiii
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16 Client Identification and Fixed-Address Allocation 275
Identifying Clients
......................................................................275
Using the
dhcp-client-identifier Option.............................276
Using the Link-Layer Address as an Identifier
..........................277
How the DHCP Server Identifies Clients
.................................278
Specifying Client Identification in a DHCP Server
....................278
Client Identification Name Collisions
....................................280
Static Allocation
.........................................................................280
Mixing Static and Dynamic Allocation
...........................................281
Moving a Client from Dynamic to Static Address Allocation
......282
Converting a DHCP Server from Static to Dynamic Allocation
...284
Automatic Allocation
..................................................................286
Access Control
...........................................................................286
Summary
..................................................................................288
17 Setting Up a Reliable DHCP Service 289
Determining Your Level of DHCP Service Reliability
.........................289
The Effects of Loss of Service
................................................289
Specific Failures in DHCP Service
..................................................291
Server Failures
....................................................................291
Planned Outages
.................................................................293
Resource Starvation
.............................................................293
Network Infrastructure Failures
.............................................294
Improving Reliability by Using Long Leases
....................................295
Setting Up a Secondary DHCP Server
.............................................295
Dynamic Address Allocation
.................................................296
Static Address Allocation
......................................................296
Hybrid Allocation Models
....................................................297
Problems with Setting Up Redundant Servers
..................................298
Address Consistency Rule Violations
......................................298
Loss of Address While in Use
................................................298
Dynamic Allocation Pool Starvation
......................................298
Duplicate Responses from Redundant Servers
..........................298
Summary
..................................................................................299
18 Configuring a Failover Server 301
Types of Failover Relationships
.....................................................301
The Cooperating Partners Relationship
...................................302
The Failover Relationship
.....................................................302
The Backing Store Relationship
.............................................303
The DHCP Handbookxiv
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Setting Up Failover Service for the First Time ..................................304
Configuring the ISC DHCP Server to Do Failover
.............................305
Merging Configuration Files
.................................................305
Configuring the Cooperating Partners Relationship
..................309
Operating a Failover Pair
..............................................................314
Starting the Servers for the First Time
.....................................314
Normal Operations
.............................................................315
Operational Problems
..........................................................315
Issues Specific to the ISC DHCP Failover Implementation
..................318
The Version of ISC Software to Use
........................................319
Ad hoc DNS Updates
...........................................................319
Known Problems with the ISC DHCP Server and Failover
..........319
Summary
..................................................................................320
19 Tuning a DHCP Service 321
Network Device Configuration and Address Assignment Strategies
......321
Manual Configuration
.........................................................321
Strategies for Supporting BOOTP Devices
................................322
Configuring Lease Lengths
...........................................................325
Examples of Long and Short Lease Times
................................325
One Lease per Client
...........................................................328
Tradeoffs Between Number of Clients and Lease Length
............329
The Effect of Lease Length on DHCP Server Load
.....................329
The Effect of Lease Length on Reliability
................................329
DHCP Leases with DDNS Updates
.........................................330
DHCP Renew Time
.............................................................330
Customizing Lease Durations
................................................331
Configuring the Lease Length on the DHCP Server
...................331
Monitoring the Server
.................................................................332
Gathering and Using Traffic Statistics
.....................................332
Verifying Server Configuration
..............................................333
DHCP Address Pool Depletion
..............................................334
Summary
..................................................................................334
20 Programmable DHCP Server Customization 337
Differentiating Between Clients
....................................................338
Conditional Statements
.......................................................338
Client Classing
...................................................................340
Contents xv
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Controlling Address Allocation .....................................................342
Pool-Based Address Allocation
...............................................343
Class-Based Address Allocation
..............................................344
Automatic Generation of Subclasses
.......................................346
Differentiation Between Similar Sets of Subclasses
....................346
Client Class Options
...................................................................347
The
user class Option .......................................................347
The
vendor class identifier Option...................................348
The
vendor-specific information Option.............................350
Lease Events
..............................................................................352
Lease Variables
...........................................................................352
Summary
..................................................................................353
21 DHCP Clients 355
The Theory of DHCP Client Operation
...........................................355
Getting an IP Address
..........................................................355
When the Client Fails to Get an Address
.................................356
Using an IP Address After It Is Acquired
..................................357
Maintaining a Lease on an IP Address
....................................357
When the Lease Expires
.......................................................358
Multiple Network Interfaces
.................................................358
More Than One IP Address Per Interface
.................................358
The Microsoft DHCP Client
..........................................................359
Installing and Enabling the Microsoft DHCP Client
..................359
The Windows DHCP Client User Interface
..............................362
Behavior Specific to the Microsoft DHCP Client
.......................363
The dhcpcd DHCP Client
.............................................................365
The pump DHCP Client
..............................................................366
The ISC DHCP Client
..................................................................367
ISC DHCP Client Installation
................................................367
ISC DHCP Client Operation
..................................................367
ISC DHCP Client Configuration
............................................368
Microsoft DHCP Client Emulation
.........................................369
ISC DHCP Client Network Setup Script Customization
..............370
ISC DHCP Client Debugging
.................................................370
Controlling the ISC DHCP Client
..........................................372
The Apple MacOS X DHCP Client
.................................................374
Controlling the MacOS X DHCP Client from the Command
Line
................................................................................376
Summary
..................................................................................378
The DHCP Handbookxvi
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22 Setting Up DHCP in a Small Office 381
Small Office Network Architectures
...............................................382
IP Address Translation
.................................................................383
Running a DHCP Server and Client on the Same Computer
...............384
Running the DHCP Server and Client on Different Interfaces
.....384
Running the DHCP Client and Server on the Same Interface
......386
Running the DHCP Server on Your Firewall
....................................388
Filtering Rules
....................................................................388
Server Identifiers
.................................................................389
Problems with DSL Routers
..........................................................389
Configuring an Integrated Router/Server
........................................389
Configuring a WAN Port
..............................................................390
Configuring a LAN Port and Small Network Services
........................390
Summary
..................................................................................392
23 Updating DNS with DHCP 393
Overview of Updating DNS with DHCP
..........................................393
The Motivation for Doing DNS Updates from DHCP
........................394
The Domain Name Update Policy
.................................................395
The Threat Model for PTR Record Updates
..............................395
Client-Supplied Hostnames
..................................................396
No Supplied Hostname
........................................................397
Name Clashes
............................................................................397
Dual Booting
.....................................................................397
DNS Update Security
...................................................................397
Generating a TSIG Key
.........................................................399
Configuring the Servers
...............................................................400
Configuring the DHCP Server to Do Updates
...........................400
Configuring the DNS Server to Allow Updates
.........................402
Completing the DHCP Server Configuration
...........................402
Configuring the DHCP Client to Do Updates
..........................404
Configuring the DNS Server for Client Updates
........................405
Updating the Client’s
A Record from a Script ...........................406
DNS Record Removal
..................................................................407
The Threat Model for Dangling
A Records ...............................408
Time to Live on Client
A Records...........................................408
Debugging Problems with DNS Updates
.........................................410
Summary
..................................................................................411
Contents xvii
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The DHCP Handbookxviii
24 Debugging Problems with DHCP 413
The Debugging Process
................................................................413
Discovering That You Have a Problem
....................................413
Determining What the Problem Is
.........................................415
Solving the Problem
............................................................416
Connectivity Problems
................................................................417
Local Connectivity
..............................................................417
Relay Agent Connectivity
.....................................................419
Server Connectivity
.............................................................419
When the Server Does Not Respond
..............................................422
No Available IP Addresses
.....................................................423
Server Not Configured for Client’s Network Segment
................423
BOOTP Clients and DHCP Servers
.........................................423
Server
DHCPNAK Message Behavior ..................................................423
The Server Sends
DHCPNAK Message When Inappropriate ............424
Rogue DHCP Servers
...........................................................424
Configuration Drift Between Cooperating DHCP Servers
...........424
Server Fails to Send
DHCPNAK Messages When Appropriate ..........425
Incorrect Option Values
...............................................................426
The Uniqueness of Client Identifiers
..............................................426
Dual-Boot Client Systems
.............................................................427
Duplicate IP Addresses
.................................................................427
When a Client Fails to Get a Reserved IP Address
.............................428
Summary
..................................................................................429
25 DHCP for IPv6 431
An Introduction to IPv6
..............................................................432
IPv6 Addressing
..................................................................432
IPv6 Auto-Configuration
......................................................434
Fragmentation and Path MTU Discovery
................................436
The Motivations for DHCPv6
.......................................................436
The Design of DHCPv6
................................................................436
Differences Between DHCPv6 and DHCPv4
.............................436
Client/Server Transactions in DHCPv6
....................................438
Interaction with IPv6 Auto-Configuration
...............................440
Summary
..................................................................................440
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Contents xix
Part IV Appendixes 441
AMicrosoft DHCP Server Examples 443
Examples 3.1, 3.2, 3.3, and 3.4 ................................................................443
Examples 3.5 and 3.6 ...............................................................................445
Example 3.7 ..............................................................................................446
Examples 3.8 and 3.9 ...............................................................................446
Example 3.10 ............................................................................................447
Example 3.11 ............................................................................................447
Examples 12.1 and 12.2 ...........................................................................448
Example 13.1 ............................................................................................448
Examples 14.1 through 14.6 ....................................................................448
Example 14.7 ............................................................................................448
Example 14.8 ............................................................................................448
Example 14.9 ............................................................................................448
Examples 14.10 and 14.11 .......................................................................449
Example 14.12 ..........................................................................................449
Example 14.13 ..........................................................................................449
Example 14.14 ..........................................................................................449
Examples 14.15 through 14.17 ................................................................449
Examples 15.1 and 15.2 ...........................................................................449
Examples 15.3 through 15.7 ....................................................................450
Example 15.8 ............................................................................................450
Example 15.9 ............................................................................................452
Example 15.10 ..........................................................................................452
Examples 15.11 and 15.12 .......................................................................452
Example 15.13 ..........................................................................................452
Example 15.14 ..........................................................................................452
Example 15.15 ..........................................................................................452
Example 15.16 ..........................................................................................452
Example 15.17 ..........................................................................................452
Example 15.18 ..........................................................................................452
Example 15.19 ..........................................................................................453
Example 16.1 and 16.2.............................................................................453
Example 16.3 ............................................................................................453
Example 16.4 ............................................................................................453
Example 16.5 ............................................................................................454
Example 16.6 ............................................................................................454
Examples 16.7 and 16.8 ...........................................................................455
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