1. The Dynamic Host Configuration Protocol (DHCP)
A Guide to TCP/IP
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2. Objectives
After reading this chapter and completing the exercises you will be able to:
Understand the basic services DHCP offers to its clients
Explain DHCP’s background, history, and origins
Describe the basic software components that permit DHCP to function
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3. Objectives
After reading this chapter and completing the exercises you will be able to:
Understand the specifics of IP address management using DHCP
Read and understand basic DHCP message types
Explain and configure basic settings for DHCP services
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4. Introducing DHCP
DHCP is a service that permits network administrators to set up servers to allocate and manage collections of IP addresses for workstations, desktop computers, and other client machines that do not require fixed IP addresses
DHCP can also supply important IP configuration data for clients, including the subnet mask, the local IP gateway (router) address, and even DNS and WINS data, where needed or appropriate
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5. Introducing DHCP
DHCP servers can manage one or more ranges of IP addresses, each of which may be called an address pool (if considered as a range of available addresses from which unused addresses may be allocated), or an address scope (if considered as a range of numeric IP addresses that fall under DHCP’s control)

6. Introducing DHCP
Here’s a brief rundown of how DHCP works, from a client perspective:
When TCP/IP is configured on the client computer, the Obtain an IP address automatically option button is the only necessary set-up element. Everything is automatic
The next time the workstation attempts to access the network (older versions of Windows must be rebooted first), it broadcasts a DHCP address request to the network because it has no IP address, but is now configured as a DHCP client. All DHCP servers present on the same cable segment or broadcast domain receive this request, and send back a message that indicates a willingness to grant an address lease, if an address is available
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7. DHCP’s Origins
The DHCP protocol is an extension of an earlier IP protocol called BOOTP
BOOTP was originally developed to permit diskless workstations to bootstrap from a Programmable Read-Only Memory (PROM) or Erasable PROM (EPROM) on their network interface cards
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8. DHCP Software Elements
Three pieces of software that work together define a complete DHCP networking environment:
DHCP client
DHCP server
DHCP relay agent
Please note that most other DHCP requests—such as lease renewals or surrenders—occur as unicast messages because as soon as a machine obtains an IP address and a default IP gateway address, it is able to communicate directly with the DHCP server and no longer needs an intermediary
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9. DHCP Lease Types
A DHCP server recognizes three types of address leases:
Manual: With a manual lease, the administrator explicitly assigns all IP addresses manually
Automatic: The DHCP server permanently assigns certain IP addresses
Dynamic: The DHCP server assigns addresses for specific periods of time
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10. More About DHCP Leases
Here’s a brief explanation of why dynamic address leasing is the best practice presently used for clients:
Server addresses are advertised using the DNS, which resolves domain names into IP addresses, and vice versa
DNS is not a dynamic environment so all address updates must be entered manually
Client addresses usually come into play only when addresses of the form must be resolved. servers can resolve this information when clients connect so dynamic address resolution works perfectly well for clients
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11. More About DHCP Leases
A typical IP addressing scheme on a network might look like this:
Servers have fixed IP addresses because their DNS entries must stay consistent
Routers (or IP gateways, be they routers or other machines) have fixed IP addresses because their addresses are key parts of any subnet’s IP configuration
Clients use dynamic IP addresses because access through POP3, SMTP, and IMAP4 avoids the need for fixed addresses (resolution occurs each time a message transfer is made)
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12. Understanding IP Address Management with DHCP
When a DHCP client has no IP address (booting for the first time, or after a lease expires), it must broadcast a request for an IP address to obtain one—this process is called DHCP Discovery
DHCP servers that can hear this discovery broadcast offer an IP address to a client for a specific amount of time (the lease time)
The default DHCP lease time varies according to which server is used (eight days for Windows and 36 hours for Windows NT 4.0, for example)
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13. Understanding IP Address Management with DHCP
In the middle of the lease time, the client starts a renewal process to determine if it can keep the address past the lease time
If the client cannot renew the address from that DHCP server within the stipulated lease period, that client must begin the more desperate process of renewing the address from another DHCP server
This is called the rebinding process
If rebinding fails, a client must completely release its address
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14. Understanding IP Address Management with DHCP
The DHCP Discovery process relies on the initial DHCP broadcast
Naturally, routers do not forward these discovery broadcasts so the entire discovery process is a local process
There must be a DHCP server on the local network segment
Because it is impractical to place a DHCP server on every network segment, the DHCP specification includes the relay agent process to help route the DHCP discovery broadcasts to another network segment
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15. The Standard Address Discovery Process
When a DHCP client boots up, it performs the Standard Address Discovery process before it can communicate on the network
After the process completes successfully, the DHCP client tests the IP address using a duplicate IP address ARP broadcast
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16. The Standard Address Discovery Process
The DHCP Discovery process actually uses four packets:
DHCP Discover packet
DHCP Offer packet
DHCP Request packet
DHCP Acknowledge packet
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17. The Discover Packet
During the DHCP Discovery process, the client broadcasts a Discover packet that identifies the client’s hardware address
If the DHCP client was on the network before, the client also defines a preferred address—typically the client prefers the last address it used
In the DHCP Discover packet shown in Figure 8-3, the Message Type value is one—this indicates that this packet is a DHCP Discover packet
The Client Identifier field value is based on the client’s hardware address
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18. DHCP Discover Packet Is Always Sent as a Hardware and IP Broadcast
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19. The Offer Packet
The DHCP server sends the Offer packet to offer an IP address to the DHCP client
The Offer packet includes the IP address that is offered to the client, and sometimes answers to the requested options in the DHCP Discover packet
Note in the IP address field that the DHCP server offers to the client
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20. DHCP Offer Packet Includes the Suggested IP Address for the DHCP Client
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21. The Request Packet
Once the Offer packet is received, the client can either accept the offer by issuing a DHCP Request packet, or reject the offer by sending a DHCP Decline packet
Typically, a client only sends a Decline if it received more than one Offer
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22. DHCP Client May List Additional Configuration Parameters in the DHCP Request Packet
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23. The Acknowledgement Packet
The Acknowledgement packet is sent from the server to the client to indicate the completion of the four- packet DHCP Discovery process
This response contains answers to any options to which the DHCP server replies
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24. The Acknowledgement Packet
The Acknowledgement packet shown in Figure 8-6 includes some answers to the client’s request for information, as listed below:
The client subnet mask is
The client’s default gateway address is
The client’s DNS server address is
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25. The Address Renewal Process
When a DHCP client receives an address from a DHCP server, the client also receives a lease time and notes the time that the address was received
The Renewal Time (T1)
T1 is defined as the time that the client tries to renew its network address by contacting the DHCP server that sent the original address to the client
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26. The Address Renewal Process
The Rebinding Time (T2)
T2 is defined as the time that the client begins to broadcast a renewal request hoping that another DHCP server can extend the lease time
The DHCP client continues to retry the rebinding process until one minute from the lease expiration time
If the client is unsuccessful in renewing the lease, it must give up the address at the expiration of the lease time, and reinitialize
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27. DHCP Timeline Includes the Lease Time (LT), Renewal Time (T1), and Rebinding Time (T2)
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28. DHCP Client Begins Advertising an Address of 0
DHCP Client Begins Advertising an Address of When It Gives Up Its IP Address
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29. The DHCP Address Release Process
Although not required by the specification, the client should release its address by sending a DHCP Release packet to the server (called the release process)
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30. DHCP Packet Structures
Figure 8-9 shows the standard DHCP packet structure
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31. DHCP Packet Structures
Operation Code Field
This 1-byte field indicates whether this packet is a DHCP request (0x01) or DHCP reply (0x02)
Hardware Type Field
This 1-byte field identifies the hardware address type, and matches the values assigned for ARP hardware type definitions
Hardware Length Field
This 1-byte field indicates the length of the hardware address
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32. DHCP Packet Structures
Hops Field
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
Transaction ID Number Field
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
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33. DHCP Packet Structures
Seconds Since Boot Field
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
Flags Field
The first bit 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
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34. DHCP Packet Structures
Client IP Address Field
The DHCP client fills in this 4-byte field with its IP address after it is assigned and bound to the IP stack
Your IP Address Field
This 4-byte field contains the address being offered by the DHCP server
Server IP Address Field
This 4-byte field contains the IP address of the DHCP server to use in the boot process
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35. DHCP Packet Structures
Gateway IP Address Field
This 4-byte field contains the address of the DHCP relay agent, if one is used
Client Hardware Address Field
This 16-byte field contains the hardware address of the client
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36. DHCP Packet Structures
Server Host Name Field
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 File Field
This field contains an optional boot filename or null-terminated string
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37. DHCP Options Fields
DHCP options are used to expand the data that is included in the DHCP packet
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38. DHCP Options
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39. DHCP Options
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40. DHCP Option 53: Message Type
This required option indicates the general purpose of any DHCP message
As you may recall, the DHCP boot sequence uses the following message types:
DHCP Message Type 1: Discover (client to server)
DHCP Message Type 2: Offer (server to client)
DHCP Message Type 3: Request (client to server)
DHCP Message Type 4: ACK (server to client)
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41. Broadcast and Unicast in DHCP
As you examine DHCP communications, you will note they use a strange mix of broadcast and unicast addressing
DHCP clients must use broadcast until obtaining IP addresses through a successful completion of the Discovery, Offer, Request, and Acknowledgement processes
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42. DHCP Relay Agents
The relay agent function is typically loaded on a router connected to the segment containing DHCP clients
This relay agent device is configured with the address of the DHCP server, and can communicate unicast directly with that server
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43. DHCP Relay Agents
Figure 8-11 shows the communication sequence on a network that supports a DHCP relay agent
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44. Microsoft DHCP Scopes and Classes
Microsoft uses the term scope to define a set of IP addresses that the Windows DHCP server can assign to clients
Microsoft refers to a collection of multiple individual scopes as a superscope
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45. The Future of DHCP
As IPv6 development and deployment move forward, DHCP’s role changes significantly
One of the great advantages of IPv6 is autoconfiguration—IPv6 hosts can create local IP addresses using their hardware addresses and the Neighbor Discovery process
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46. Troubleshooting DHCP
The best way to troubleshoot DHCP is with an analyzer— compare a problematic boot up sequence with the standard four-packet boot up sequence
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47. Troubleshooting DHCP
You can release and/or renew an IP address using the following commands:
ipconfig/release - Releases the IP address for the specified adapter
ipconfig/renew – Renews the IP address for the specified adapter
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48. Summary
DHCP provides a way for computers to obtain usable, unique IP addresses and necessary TCP/IP configurations even when no IP addresses were assigned to those machines
From the administrative side, DHCP makes is easy to define and manage pools of IP addresses, which Microsoft calls a scope in the singular and a superscope in the plural
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49. Summary
DHCP’s origins lie in an earlier TCP/IP Application layer protocol, called BOOTP, used to enable diskless workstations to boot remotely across a network
DHCP supports three types of address allocation: manual, in which administrators directly manage all addresses; automatic, in which relatively static address assignments may be made under DHCP’s control; and dynamic, in which addresses are allocated with explicit expiration intervals called leases
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50. Summary
DHCP supports a wide variety of message types and options, but only Message Type 53 (DHCP Message) is mandatory for any given DHCP message
A protocol analyzer is especially effective when diagnosing DHCP difficulties, particularly those related to the DHCP boot sequence, because it can display the sequence of messages that occurs on a network, which may then be compared to the usual, prescribed sequence
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