Guide To TCPIP, Second Edition

1
Guide To TCP/IP, Second Edition

• Chapter 8
• The Dynamic Host Configuration Protocol (DHCP)

2
Objectives

• Basic DHCP services
• History and Origins of DHCP
• DHCP leases
• Basic DHCP software components
• IP address management using DHCP
• DHCP Discovery, renewal, and release processes

3
Objectives (cont.)

• DHCP packet structure
• Types of DHCP messages
• Broadcast and unicast addressing
• DHCP server selection
• DHCP relay agents
• Microsoft DHCP scopes and classes
• DHCP troubleshooting utilities

4
Introducing DHCP

• Lets client computer request an IP address
• DHCP delivers the necessary configuration
  information
• IP address
• Subnet mask
• IP Gateways
• Address for DNS servers
• Address for WINS servers

5
Introducing DHCP (cont.)

• Centralized administration of client IP address
  assignments
• Address pool or address scope
• IP address ranges
• Address exclusion
• Address lease

6
How DHCP Works

• Client perspective on DHCP
• Client is configured to Obtain an IP address
  automatically
• At boot-up the client broadcasts DHCP address
  request
• DHCP servers reply offers an address lease
• Client accepts address lease offer
• Server offers an IP address with a lease time
  limit
• Half the lease time the client attempts to renew
  lease

7
How DHCP Works (cont.)
8
The Role Of Leases

• Length of leases vary
• 1 to 3 weeks is typical
• 1 to 3 days for networks with temps or roving
  workers
• 4 to 8 hours are common on ISP networks

9
DHCP Software Elements

• Three elements to DHCP software
• DHCP client
• Built-in DHCP client software in Windows and UNIX
  operating systems
• DHCP Server
• Manages address pools and related configuration
  of Windows and UNIX servers
• DHCP relay agent
• Intercepts address requests
• Repackages requests and unicasts to a DHCP server

10
DHCP Lease Types

• Two types of address leases
• Manual address lease
• Dynamic address lease
• Typical IP addressing schemes
• Servers have fixed IP addresses
• Routers have fixed IP addresses
• Clients use dynamic IP Addresses

11
More About DHCP Leases

• IPCONFIG command supports the /release and /renew
  switches
• How DHCP integrates with DSN
• Server address are advertised using DNS
• DNS is not a dynamic environment
• Client address are resolved when using email
  addresses
• User_at_domain.name

12
Understanding IP Address Management With DHCP

• Booting for the first time or after a lease
  expires
• DHCP Discovery
• Discovery broadcast
• Renewal process
• Rebinding process
• Client must completely release its address if
  rebinding fails
• Servers and clients use PING and ARP as error
  prevention methods

13
The Standard Address Discovery Process

• DHCP Discovery process uses four packets
• DHCP Discover packet
• DHCP Offer packet
• DHCP Request packet
• DHCP Acknowledgment packet

14
The Standard Address Discovery Process (cont.)
15
The Discover Packet

• Client broadcasts a Discover Packet
• Client hardware address
• Source IP address 0.0.0.0
• Destination address 255.255.255.255
• Preferred address
• Message Type value 1
• Client Identifier
• DHCP options

16
The Discover Packet (cont.)

• DHCP options
• Option 1 Clients subnet mask
• Option 3 Routers on the clients subnet
• Option 6 Domain name servers
• Option 15 Domain name
• Option 44 NetBIOS over TCP/IP name servers
• Option 46 NetBIOS over TCP/IP node type
• Option 47 NetBIOS over TCP/IP scope
• Option 57 Maximum DHCP message size
• Option 255 End of options

17
The Discover Packet (cont.)
18
The Offer Packet

• DHCP server sends the Offer packet
• An IP address is offered
• Packet is sent by unicast

19
The Offer Packet (cont.)
20
The Request Packet

• DHCP Request packet
• DHCP Decline packet

21
The Request Packet (cont.)
22
The Acknowledgment Packet

• Sent from Server
• Contains configuration options requested by
  client
• Duplicate IP address test

23
The Acknowledgment Packet (cont.)
24
The Renewal Process

• The Renewal Time (T1)
• Renewal packet is unicast directly to the DHCP
  server
• 0.5 duration_or_lease (i.e., lease time)
• The rebinding Time (T2)
• Broadcast a renewal request to any listening DHCP
  servers
• 0.875 duration_of_lease
• Continues rebinding process until one minute from
  the lease expiration time
• Client releases it address if unsuccessful and
  reinitializes to start DHCP Discovery process

25
The Renewal Process (cont.)
26
The Renewal Process (cont.)
27
The DHCP Address Release Process

• DHCP Release packet
• Sent over UDP
• DHCP server does not send acknowledgements

28
DHCP Packet Structures

• DHCP packet fields
• Operation Code (OPCODE) Field
• DHCP Request (0x01)
• DHCP Reply (0x02)
• Hardware Type Field
• Hardware Length Field
• Hops Field
• Transaction ID Number Field

29
DHCP Packet Structures (cont.)

• DHCP packet fields (cont.)
• Seconds Since Boot Field
• Flags Field
• Client IP Address Field
• Your IP Address Field
• Server IP Address Field
• Gateway IP Address Field

30
DHCP Packet Structures (cont.)

• DHCP packet fields (cont.)
• Client Hardware Address Field
• Server Host Name Field
• Boot File Field
• DHCP Options
• Expand the data that is included in the packet
• DHCP Option 53 Message Type
• Required in all DHCP packets

31
DHCP Packet Structures (cont.)
32
DHCP Packet Structures (cont.)
33
Broadcast And Unicast in DHCP
34
Communications With A DHCP Relay Agent

• Routers do not forward broadcasts
• Relay agents accepts discovery broadcasts and
  unicasts them to the DHCP server
• Relay agent function is usually enabled on a
  router

35
Communications With A DHCP Relay Agent (cont.)
36
Communications With A DHCP Relay Agent (cont.)
37
Microsoft DHCP Scopes And Classes

• Scope is a range of consecutive IP addresses
• Superscope is a group of non-consecutive IP
  address
• A collection of scopes

38
Troubleshooting DHCP

• Troubleshoot DHCP with an analyzer
• IPCONFIG utility
• ipconfig /release
• ipconfig /renew

39
Troubleshooting DHCP (cont.)
40
Chapter 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
• As long as a DHCP server or relay is available on
  the cable segment where an initial DHCP Request
  message is broadcast, the DHCP service makes it
  easy and automatic to include computers on a
  TCP/IP network

41
Chapter Summary (cont.)

• From the administrative side, DHCP makes is easy
  to define and manage pools of IP addresses, which
  Microsoft calls a scope when referring to a set
  of IP addresses under DHCPs management, and a
  superscope when referring to a collection of IP
  address scopes

42
Chapter Summary (cont.)

• DHCPs origins lie in an earlier TCP/IP
  Application layer protocol, called BOOTP, used to
  enable diskless workstations to boot remotely
  across a network
• Basic BOOTP and DHCP formats are entirely
  compatible, so that by configuring a router to
  forward BOOTP, it also forwards DHCP packets

43
Chapter Summary (cont.)

• DHCP supports two types of address allocation
  manual, in which administrators directly manage
  all addresses and dynamic, in which addresses
  are allocated with explicit expiration intervals
  called leases
• Many of DHCPs functions and messages relate to
  obtaining, renewing, and releasing dynamic
  address leases, primarily for client machines

44
Chapter Summary (cont.)

• When a DHCP client starts, it begins the DHCP
  Discovery process, during which the client
  receives an IP address and lease
• 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 not, the client releases its IP address and
  starts the Discovery process over

45
Chapter Summary (cont.)

• DHCP supports a wide variety of message types and
  options, but only Message Type 53 (DHCP Message)
  is mandatory for any given DHCP message
• Because DHCP can ferry a surprisingly large range
  of configuration information (including all kinds
  of network services, such as e-mail and NetBIOS
  over TCP/IP), the protocol makes use of several
  message options
• A protocol analyzer is especially effective when
  diagnosing DHCP difficulties, particularly those
  related to the DHCP boot sequence