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

Chapter 317
DHCP Responses

Responses from the server can be addresses in a few different ways:

•  If the Giaddr field of the received client packet is set to 0 and the Ciaddr field is set to a non-0, then responses are to be sent as unicast to the client using the yiaddr as the destination IP address and the chaddr as the destination MAC address. (Giaddr field set to 0 indicates the client is on the local subnet).

•  If the Giaddr field is set to a non-0 value, then the response is sent to the BOOTP Relay agent using the IP address indicated by the Giaddr field.

•  If the broadcast bit was set, then the server broadcasts (IP and MAC addresses) its responses to the client. Using the B bit allows the client to indicate to a potential server that it cannot receive unicast IP datagrams before its TCP/IP configuration has been set.

If the server receives a DHCPREQUEST messages with an invalid “requested IP address,” the server should respond to the client with a DHCPNAK message and report this error in a log.

The client may receive one or more offers from different servers. The client will select one of the servers from the responses that closely match its original request parameters. If the client does not receive a response to its DHCPREQUEST, it will time-out, and retransmit a DHCPREQUEST message.

To respond to a DHCPOFFER, the client transmits a DHCPREQUEST. In the Options field of this message is the server identifier (the server’s IP address) indicating which server the client has selected. All other servers will partially ignore this message. However, those declined servers do use this message to indicate that the client will not be using their services, and this releases the offered IP address back to the available pool.

The selected server commits this binding to a place in memory where it will be stored. (A binding is a key that is used to look up information. For example, the preceding entry could be an IP-address-to-client-hardware address.) The server will respond to the client with a DHCPACK message containing all the client’s configuration parameters. This binding is indicated as the client identifier, and the assigned IP address.

When the client receives the DHCPACK, it will perform some final checks, such as ARPing for the newly assigned IP address to ensure that no one else is assigned to this address. If there are any inconsistencies, the client will send a DHCPDECLINE message to the server and after waiting 10 seconds (it should wait at least 10 seconds), it will restart the configuration process. Also, if the server transmitted a DHCPNAK message to the client, the client will restart (after 10 seconds).

DHCP Responses