Guide to TCP/IP, Second Edition 1
Guide To TCP/IP, Second Edition
Chapter 8
The Dynamic Host Configuration Protocol (DHCP)
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Objectives
• Understand the basic services DHCP offers to its clients; explain DHCP’s background, history, and origins; discuss DHCP leases; and describe the basic software components that permit DHCP to function
• Understand the specifics of IP address management using DHCP
• Explain the DHCP Discovery, renewal, and release processes
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Objectives (cont.)
• Understand the basic DHCP packet structure and types of DHCP messages in use
• Describe broadcast and unicast addressing, and understand why a DHCP server uses one over the other
• Describe relay agent communications• Discuss Microsoft DHCP scopes and classes
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Introducing DHCP
• Provides a way for client computer to request an IP address
• DHCP delivers the necessary configuration information– IP address
– Subnet mask
– IP Gateways
– Address for DNS servers
– Address for WINS servers
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Introducing DHCP (cont.)
• Administer client IP address assignments and configuration data from a single, centralized server
• Address pool or address scope
• One or more range of IP address
• Exclude an address or range of address
• Address lease
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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
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How DHCP Works (cont.)
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The Role Of Leases
• Length of leases vary
• One to three weeks in length are typical
• One to three days for networks with temps or roving workers
• Four to eight hours are common on ISP networks
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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
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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
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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• [email protected]
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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
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The Standard Address Discovery Process
• DHCP Discovery process uses four packets– DHCP Discover packet– DHCP Offer packet– DHCP Request packet– DHCP Acknowledgment packet
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The Standard Address Discovery Process (cont.)
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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
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The Discover Packet (cont.)
• DHCP options– Option 1: Client’s subnet mask– Option 3: Routers on the client’s 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
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The Discover Packet (cont.)
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The Offer Packet
• DHCP server sends the Offer packet– An IP address is offered– Packet is sent by unicast
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The Offer Packet (cont.)
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The Request Packet
• DHCP Request packet
• DHCP Decline packet
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The Request Packet (cont.)
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The Acknowledgment Packet
• Sent from Server
• Contains configuration options requested by client
• Duplicate IP address test
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The Acknowledgment Packet (cont.)
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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
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The Renewal Process (cont.)
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The Renewal Process (cont.)
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The DHCP Address Release Process
• DHCP Release packet
• Sent over UDP
• DHCP server does not send acknowledgements
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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
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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
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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
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DHCP Packet Structures (cont.)
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DHCP Packet Structures (cont.)
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Broadcast And Unicast in DHCP
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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
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Communications With A DHCP Relay Agent (cont.)
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Communications With A DHCP Relay Agent (cont.)
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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
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Troubleshooting DHCP
• Troubleshoot DHCP with an analyzer
• IPCONFIG utility– ipconfig /release– ipconfig /renew
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Troubleshooting DHCP (cont.)
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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
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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 DHCP’s management, and a superscope when referring to a collection of IP address scopes
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Chapter Summary (cont.)
• 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
• Basic BOOTP and DHCP formats are entirely compatible, so that by configuring a router to forward BOOTP, it also forwards DHCP packets
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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 DHCP’s functions and messages relate to obtaining, renewing, and releasing dynamic address leases, primarily for client machines
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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
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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
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