Illustrated TCP/IP by Matthew G. Naugle Wiley Computer Publishing, John Wiley & Sons, Inc. ISBN: 0471196568   Pub Date: 11/01/98

Chapter 316
DHCP Operation

First, a client transmits a DHCPDISCOVER message on its local physical subnet. Both the IP destination address and the MAC destination address are set to broadcast. The IP source address is set to 0x00000000 and the MAC source address is set to Chaddr, or the client’s hardware address.

The client may place in this message some options that include an IP address and the lease duration. If the client has placed a “suggested” IP address in the Options field, the client has been previously configured using DHCP and is now restarting and would like to use that address again. If the client was manually configured with an IP address, the client should use the DHCPINFORM message instead of the DHCPREQUEST message. One feature that the client may use is to obtain a specific list of parameters. The client may indicate this by using the “parameter request list,” which indicates to the server which parameters, by tag number, the client is specifically interested in. See RFC 2132 for more information on DHCP options.

This message will be picked up by the routers that implement the BOOTP relay agent and forwarded to other network segments. Again, you can limit the scope (how many routers it can traverse). The Hops field (set to 0 by the client) is incremented (usually by 1) with each router and the administrator of the router sets the maximum hop count. If the received packet has a hop count of 2 and the Max Hops parameter configured in the router is 3, the router will set the Hops field to 3 and forward the packet. If the received packet already has a 3 in the Hops field, the router is not allowed to increment the field to a 4 and it will discard the packet. This is known as the scope (range) of the DHCP packet.

DHCP Operation

Each active server that receives this message may respond with a DHCPOFFER message that includes an IP address in the Yiaddr field of the packet. Not all servers will respond. Some may be preconfigured to not respond to certain requests, and others may not have the binding for that client. It may also appear in various Option fields as well. The server does not have to take the offered IP address off the available list, but it does help when the server does remove this offered IP address from its availability pool. At this time, the server may check for current use of the offered IP address by sending an ICMP ECHO request using the offered IP address. This is configurable.