DHCP Messages
DHCP uses eight standard message types, which are identified by an option type number in the DHCP message. For example, when a client wants to find a DHCP server, it broadcasts a DHCPDISCOVER message on its local physical subnetwork. If there is no DHCP server on its subnet and if DHCP Helper or DHCP Relay is configured properly, the message is forwarded to DHCP servers on a different physical subnet. Otherwise, the message will go no further than the subnet on which it originated. One or more DHCP servers will respond with a DHCPOFFER message that contains an available network address and other configuration parameters.
When the client needs an IP address, it sends a DHCPREQUEST to one or more servers. Of course if the client is requesting an IP address, it doesn’t have one yet, so RFC 2131 requires that the broadcast message the client sends out have a source address of 0 in its IP header.
When a client requests configuration parameters from a server, it might receive responses from more than one server. Once a client has received its IP address, it is said that the client has at least an IP address and possibly other configuration parameters bound to it. DHCP servers manage such binding of configuration parameters to clients.
The following table lists the DHCP messages.
DHCP Message Description
DHCPDISCOVER Client broadcast to find available DHCP servers.
DHCPOFFER Server response to client’s DHCPDISCOVER, offering configuration parameters.
DHCPREQUEST Client message to one or more servers to do any of the following: Request parameters from one server and implicitly decline offers from other servers. Confirm that a previously allocated address is correct after, for example, a system reboot. Extend the lease of a network address.
DHCPACK Server to client acknowledgment message containing configuration parameters, including a confirmed network address.
DHCPNAK Server to client negative acknowledgment indicating the client’s understanding of the network address is incorrect (for example, if the client has moved to a new subnet), or a client’s lease has expired.
DHCPDECLINE Client to server message indicating the network address is already being used.
DHCPRELEASE Client to server message giving up the user of the network address and canceling the remaining time on the lease.
DHCPINFORM Client to server message requesting only local configuration parameters; client has an externally configured network address.

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