BAI513 - PROTOCOLS DHCP BAIST – Network Management

Objectives At the end of this presentation, the student will be able to: –Describe the history of DHCP –Describe the DHCP address discovery process –Describe the fields contained in the DHCP header.

History of DHCP DHCP is based on BOOTP and maintains some backward compatibility. The main difference is that BOOTP was designed for manual pre-configuration of the host information in a server database, while DHCP allows for dynamic allocation of network addresses and configurations to newly attached hosts. Additionally, DHCP allows for recovery and reallocation of network addresses through a leasing mechanism.

DHCP Defined The Dynamic Host Configuration Protocol (DHCP) is an Internet protocol for automating the configuration of computers that use TCP/IP. DHCP can be used to automatically assign IP addresses, to deliver TCP/IP stack configuration parameters such as the subnet mask and default router, and to provide other configuration information such as the addresses for printer, time and news servers.

DHCP Components DHCP consists of two components: 1.A protocol for delivering host-specific configuration parameters from a DHCP server to a host 2.A mechanism for allocation of network addresses to hosts.

Allocating IP Addresses DHCP supports 3 mechanisms for allocating IP addresses. –Automatic - assigns a permanent IP address to a client –Dynamic - assigns an IP address to a client for a limited period of time –Manual - a client's IP address is assigned by the network administrator, and DHCP is used simply to convey the assigned address to the client

DHCP Address Discovery The DHCP Address Discovery process actually uses seven packets: –DHCPDISCOVER packet –DHCPOFFER packet –DHCPREQUEST packet –DHCPACK packet –Client (MS W2K WS) then sends 3 ARP packets to validate the IP address assigned by the DHCP Server

The Discover Packet Client to Server When a DHCP client has no IP address (booting for the first time, or after a lease expires), it must broadcast a DHCP Discover Packet onto the network. The DHCP Discover Packet Is Always Sent with a destination Hardware (FF:FF:FF:FF) and IP Broadcast ( ) addresses.

The DHCPDISCOVER Packet 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, 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 NOTE: Parameters found in DHCP Options

The DHCPOFFER Packet Server to Client The DHCP server sends the DHCPOFFER packet (unicast destination address) to offer an IP address to the DHCP client Each responding DHCP server includes an unique server identifier in the option field The DHCPOFFER packet includes the IP address that is offered to the client, and sometimes answers to the requested options in the DHCPDISCOVER packet

The DHCPREQUEST Packet Client to Server Once the DHCPOFFER packet is received, the client can either accept the offer by issuing a DHCPREQUEST (broadcast) packet, or reject the offer by sending a DHCPDECLINE packet Typically, a client only sends a DHCPDECLINE if it received more than one DHCPOFFER

The DHCPACK Packet Server to Client The DHCPACK packet (unicast) includes some answers to the client’s request for information, such as: –The client subnet mask is –The client’s default gateway address is –The client’s DNS server address is

Additional DHCP Packets Additional DHCP packets that may be used include: –DHCPNAK - Server to client indicating client's notion of network address is incorrect (e.g., client has moved to new subnet) or client's lease as expired –DHCPDECLINE - Client to server indicating network address is already in use. –DHCPRELEASE - Client to server relinquishing network address and cancelling remaining lease. –DHCPINFORM - Client to server, asking only for local configuration parameters; client already has externally configured network address.

DHCP Header Structure

DHCP Header Fields 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

DHCP Header Fields Hardware Length Field This 1-byte field indicates the length of the hardware address 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

DHCP Header Fields 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 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

DHCP Header Fields Flags Field 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 Field 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 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

DHCP Header Fields 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

DHCP Header Fields 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

DHCP Options Field DHCP options are used to expand the data that is included in the DHCP packet DHCP Option 53: Message Type - This required option indicates the general purpose of any DHCP message 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)

Summary This presentation covered information that allowed the student to: –Describe the history of DHCP –Describe the DHCP address discovery process –Describe the fields contained in the DHCP header.