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DHCP uses eight standard message types identified by an option type number in the
DHCP message. For example, when a client wants to find a DHCP server, it broadcasts
DHCPDISCOVER messages on its local physical subnetwork.
If there’s no DHCP server on its subnet and if the 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 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. 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’s 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.
DHCP Message
Description
DHCPDISCOVER
Client broadcast to find available DHCP
servers.
DHCOPOFFER
Server responds 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.
