Guide to TCP/IP, Third Edition Chapter 8: The Dynamic Host Configuration Protocol

IP Addressing and Related Topics2 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 Topics3 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 Topics4 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 Topics5 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 Topics6 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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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 Topics9 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 Topics10 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 Topics11 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 addresses of the form must be resolved

IP Addressing and Related Topics12 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 Topics13 DHCP Address Discovery Uses four packets –DHCP Discover packet –DHCP Offer packet –DHCP Request packet –DHCP Acknowledgment packet

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IP Addressing and Related Topics15 The Discover Packet During DHCP Discovery process –Client broadcasts Discover packet that identifies client’s hardware address –Header contains Source IP address Preferred address –Typically the last address the client used

IP Addressing and Related Topics16 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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IP Addressing and Related Topics18 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 Topics19 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 Topics20 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 Topics21 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 Topics22 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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IP Addressing and Related Topics25 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 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 (0x ) and ends with the “End” identifier (0xFF) DHCP Option 53: Message Type - This required option indicates the general purpose of any DHCP message

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IP Addressing and Related Topics36 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)

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IP Addressing and Related Topics38 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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IP Addressing and Related Topics40 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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IP Addressing and Related Topics43 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 Topics44 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 Topics45 Setting up a Simple DHCP Server (continued) Field values in the Dynamic Pool Settings –DNS –Domain Name –Lease Time (60 – sec) –Status

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IP Addressing and Related Topics47 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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IP Addressing and Related Topics49 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 Topics50 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 Topics51 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