Download ch08

Guide to TCP/IP, Third
Edition
Chapter 8:
The Dynamic Host Configuration
Protocol
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)
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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 [email protected]
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 Packet Structures
• Fields
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–
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–
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Operation Code (OPCODE) or Message Type Field
Hardware Type Field
Hardware Length Field
Hops Field
Transaction ID Number Field
Seconds Since Boot or Seconds Elapsed Field
Flags Field
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DHCP Packet Structures (continued)
• Fields
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Client IP Address Field
Your IP Address Field
Server IP Address Field
Gateway or Relay Agent IP Address Field
Client Hardware Address Field
Server Host Name Field
Boot File Field
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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:
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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 nonconsecutive IP addresses
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Setting up a Simple DHCP Server
• Field values in the Dynamic Pool Settings
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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
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–
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
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