Guide to TCP/IP, Third Edition

Chapter 8: The Dynamic Host Configuration

Protocol

IP Addressing and Related Topics 2

Objectives

• Understand the basic services DHCP offers to its clients and explain its background

• Understand the specifics of IP address management using DHCP

• Explain the DHCP Discovery, renewal, and release processes

• Understand the basic DHCP packet structure and types of DHCP messages in use

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Objective (continued)

• Describe broadcast and unicast addressing• Describe relay agent communications• Discuss Microsoft DHCP scopes and classes• Use DHCP troubleshooting utilities

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Introducing DHCP

• DHCP – Service that provides a way for client computer to

request IP address assignment – Delivers necessary configuration information to

clients – Traces its origins back to an earlier protocol named

BOOTP– Servers can manage one or more ranges of IP

addresses

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How DHCP Works

• When TCP/IP is configured on client computer – The Obtain an IP address automatically option is the

only necessary set-up element• Next time workstation attempts to access network

– It broadcasts a DHCP address request to the network because it has no IP address

• All DHCP servers present on same broadcast domain– Receive request and send back message that

indicates a willingness to grant an address lease

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How DHCP Works (continued)

• Client – Accepts address lease offer and sends packet to

server that extended offer• In reply

– Server proffers an IP address for a specific period of time that the client uses thereafter

• When half the lease period expires– Client attempts to renew the lease

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The Role of Leases

• Leases – “Loans” of an address for a specific amount of time

• Leases that range from one to three weeks – Typical on networks in which machines seldom move

and the workforce is stable• Lease periods

– Average between one and three days for networks on which roving workers come and go regularly

• Lease periods of four to eight hours – Common on ISP networks

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DHCP Software Elements

• DHCP client– Enabled at client machine when you select the

Obtain an IP address automatically option • DHCP server

– Manages address pools and configuration data• DHCP relay agent

– Intercepts address requests on local cable segment – Repackages requests as a unicast to one or more

DHCP servers

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DHCP Lease Types

• Manual address lease– Administrator assigns an IP address manually

• DHCP server – Assigns addresses for specific periods of time

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More About DHCP Leases

• How DHCP integrates with DNS– Server addresses are advertised using DNS– All address updates must be entered manually– Client addresses usually come into play only when

• e-mail addresses of the form user@domain.name must be resolved

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Understanding IP Address Management with DHCP

• DHCP Discovery– Occurs when DHCP client broadcasts a request for

an IP address• DHCP Discovery

– Relies on an initial DHCP broadcast• DHCP servers

– PING an address before offering it to the client• Windows clustering

– Allows two or more servers to be managed as a single system

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DHCP Address Discovery

• Uses four packets– DHCP Discover packet– DHCP Offer packet– DHCP Request packet– DHCP Acknowledgment packet

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The Discover Packet

• During DHCP Discovery process– Client broadcasts Discover packet that identifies

client’s hardware address– Header contains

• Source IP address 0.0.0.0

• Preferred address– Typically the last address the client used

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The Offer Packet

• Send by DHCP server to offer IP address to DHCP client

• Offer packet – Includes IP address that is offered to the client and, – Sometimes, answers to the requested options in the

DHCP Discover packet

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The Request Packet

• Once Offer packet is received– Client can either accept offer by issuing a DHCP

Request packet, or – Reject offer by sending a DHCP Decline packet

• Typically– Client sends a Decline only if it receives more than

one Offer

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The Acknowledgment Packet

• Sent from server to client to indicate the completion of the four-packet DHCP Discovery process

• Response – Contains answers to any configuration options

requested by client in the previous Request packet

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The Address Renewal Process

• When DHCP client receives an address from a DHCP server– Client also receives a lease time and notes the time

that the address was received• Lease time

– Defines how long the client can keep the address• In middle of the lease period

– Client starts a renewal process

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The Renewal Time (T1)

• T1– Defined as the time that the client tries to renew its

network address • The Renewal packet

– Is unicast directly to the DHCP server• Default value for T1

– 0.5 * duration_of_lease (i.e., lease time)

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The Rebinding Time (T2)

• The time that client begins to – Broadcast a renewal request for an extended lease

time from another DHCP server• DHCP specification, RFC 2131, defines default

value for T2 as– 0.875 * duration_of_lease

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The DHCP Address Release Process

• Client should release its address by – Sending a DHCP Release packet to the server

• DHCP Release packet– Sent over UDP– DHCP server does not send any acknowledgment

• If client does not send the DHCP Release packet– DHCP server automatically releases the address at

the lease expiration time

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DHCP Header Fields

Opcode

• This 1-byte field indicates whether this packet is a DHCP request (0x01) or DHCP reply (0x02)

Hardware Type

• This 1-byte field identifies the hardware address type, and matches the values assigned for ARP hardware type definitions

DHCP Header Fields

Hardware Address Length• This 1-byte field indicates the length of the hardware

address

Hop Count• This field is set to zero by the client, and may be

used by relay agents as they assist a client in obtaining an IP address and/or configuration information

DHCP Header Fields

Transaction ID• This 4-byte field contains a random number

selected by the client, and is used to match requests and responses between the client and server

Number of Seconds• This 2-byte field indicates the number of

seconds that elapsed since the client began requesting a new address, or renewal of an existing address

DHCP Header Fields

Flags• The first bits of the 2-byte Flags field can be

toggled to one to indicate that the DHCP client cannot accept unicast MAC layer datagrams before the IP software is completely configured

Client IP Address• The DHCP client fills in this 4-byte field with its

IP address after it is assigned and bound to the IP stack

DHCP Header Fields

Your IP Address• This 4-byte field contains the address being offered

by the DHCP server

Server IP Address• This 4-byte field contains the IP address of the

DHCP server to use in the boot process

DHCP Header Fields

Gateway IP Address

• This 4-byte field contains the address of the DHCP relay agent, if one is used

Client Hardware Address

• This 16-byte field contains the hardware address of the client

DHCP Header Fields

Server Host Name• This 64-byte field can contain the server host

name, but such information is optional• This field can contain a null-terminated string (all

zeroes)

Boot Filename• This field contains an optional boot filename or null-

terminated string

DHCP Data Field• DHCP Data Field is often refered to as the Options

Field – This field is used to expand the data that is included in

the DHCP packet.– This field always starts with the “Magic Cookie” number

(0x63.82.53.63) and ends with the “End” identifier (0xFF)

• DHCP Option 53: Message Type - This required option indicates the general purpose of any DHCP message

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DHCP Option 53: Message Type

• Only DHCP option required in all DHCP packets• Indicates general purpose of any DHCP message• DHCP boot sequence uses these message types:

– DHCP MessageType 1: Discover (client to server)– DHCP MessageType 2: Offer (server to client)– DHCP MessageType 3: Request (client to server)– DHCP MessageType 5: ACK (server to client)

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Broadcast and UNICAST in DHCP

• DHCP clients – Must broadcast service requests until they obtain IP

addresses • DHCP clients

– Use unicast addressing after they obtain an address for a local DHCP server or relay agent

• DHCP servers – Examine DHCP packets coming from clients

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Communications with a DHCP Relay Agent

• DHCP boot up process r– Relies heavily on broadcasts, but most routers do

not forward broadcasts• Relay agent function

– Typically loaded on a router connected to the segment containing DHCP clients

• Relay agent device – Configured with the address of the DHCP server– Can communicate using unicast packets directly with

that server

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Microsoft DHCP Scopes and Classes

• Address scopes– Define a set of addresses that a DHCP server can

assign to clients• Superscopes

– A collection of scopes that contain sets of non-consecutive IP addresses

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Setting up a Simple DHCP Server

• Field values in the Dynamic Pool Settings– IP Assigned From– The Range of Pool (1-255):– SubMask– Gateway– Wins

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Setting up a Simple DHCP Server (continued)

• Field values in the Dynamic Pool Settings– DNS– Domain Name– Lease Time (60 – 31536000 sec)– Status

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Troubleshooting DHCP

• One good way to troubleshoot DHCP– Use a protocol analyzer, such as Ethereal

• Analyzer – Can display the sequence of messages that occurs

on a network• Commands to release and/or renew an IP address

– ipconfig /release– ipconfig /renew

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Summary

• DHCP – Provides a way for computers to obtain usable,

unique IP addresses and necessary TCP/IP configurations

• From the administrative side– DHCP makes it easy to define and manage pools of

IP addresses• DHCP’s origins

– Lie in an earlier TCP/IP Application layer protocol, called BOOTP

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Summary (continued)

• DHCP – Supports manual or static address allocation

• When a DHCP client starts– It begins the process of DHCP Discovery, during

which the client receives an IP address and lease• DHCP

– Supports a variety of message types and options– Only MessageType 53 (DHCP Message) is

mandatory for any given DHCP message

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Summary (continued)

• Because DHCP can ferry a surprisingly large range of configuration information– The protocol makes use of several message options

• A protocol analyzer is especially effective when diagnosing DHCP difficulties