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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
IP Addressing and Related Topics 3
Objective (continued)
• Describe broadcast and unicast addressing• Describe relay agent communications• Discuss Microsoft DHCP scopes and classes• Use DHCP troubleshooting utilities
IP Addressing and Related Topics 4
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
IP Addressing and Related Topics 5
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
IP Addressing and Related Topics 6
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
IP Addressing and Related Topics 7
IP Addressing and Related Topics 8
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
IP Addressing and Related Topics 9
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
IP Addressing and Related Topics 10
DHCP Lease Types
• Manual address lease– Administrator assigns an IP address manually
• DHCP server – Assigns addresses for specific periods of time
IP Addressing and Related Topics 11
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
IP Addressing and Related Topics 12
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
IP Addressing and Related Topics 13
DHCP Address Discovery
• Uses four packets– DHCP Discover packet– DHCP Offer packet– DHCP Request packet– DHCP Acknowledgment packet
IP Addressing and Related Topics 14
IP Addressing and Related Topics 15
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
IP Addressing and Related Topics 16
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
IP Addressing and Related Topics 17
IP Addressing and Related Topics 18
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
IP Addressing and Related Topics 19
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
IP Addressing and Related Topics 20
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
IP Addressing and Related Topics 21
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)
IP Addressing and Related Topics 22
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
IP Addressing and Related Topics 23
IP Addressing and Related Topics 24
IP Addressing and Related Topics 25
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
IP Addressing and Related Topics 26
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
IP Addressing and Related Topics 35
IP Addressing and Related Topics 36
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)
IP Addressing and Related Topics 37
IP Addressing and Related Topics 38
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
IP Addressing and Related Topics 39
IP Addressing and Related Topics 40
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
IP Addressing and Related Topics 41
IP Addressing and Related Topics 42
IP Addressing and Related Topics 43
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
IP Addressing and Related Topics 44
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
IP Addressing and Related Topics 45
Setting up a Simple DHCP Server (continued)
• Field values in the Dynamic Pool Settings– DNS– Domain Name– Lease Time (60 – 31536000 sec)– Status
IP Addressing and Related Topics 46
IP Addressing and Related Topics 47
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
IP Addressing and Related Topics 48
IP Addressing and Related Topics 49
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
IP Addressing and Related Topics 50
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
IP Addressing and Related Topics 51
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