DHCP Messages

DHCP clients and DHCP servers communicate by exchanging DHCP messages. There are eight types of DHCP messages, all of which are sent as User Datagram Protocol (UDP) messages. DHCP clients in the process of obtaining an IP address configuration use broadcast DHCP messages, sent to the limited broadcast IP address 255.255.255.255. DHCP clients with an IP address and a valid lease use unicast DHCP messages. DHCP clients listen on UDP port 68. DHCP servers and DHCP relay agents listen on UDP port 67.

The eight DHCP message types are the following:

• DHCPDISCOVER Sent by a DHCP client to locate a DHCP server.

• DHCPOFFER Sent by a DHCP server to a DHCP client in response to the DHCPDISCOVER message, containing an offered IP address and other configuration settings.

• DHCPREQUEST Sent by the DHCP client to DHCP servers to request an offered IP address and other configuration settings from a specified DHCP server while implicitly declining offers from other servers, or to confirm the validity of previously allocated addresses (for example, after a restart or to extend an existing DHCP lease).

• DHCPACK Sent by a DHCP server to a DHCP client in response to a DHCPREQUEST message to confirm an IP address and provide the client with those configuration parameters that the client has requested and the server has been configured to provide.

• DHCPNAK Sent by a DHCP server to a DHCP client denying the clients DHCPREQUEST. This might occur if the requested address is incorrect because the client has moved to a new subnet or because the DHCP client’s lease has expired and cannot be renewed.

• DHCPDECLINE Sent by a DHCP client to a DHCP server, informing the server that the offered IP address is unusable because it is in use by another computer.

• DHCPRELEASE Sent by a DHCP client to a DHCP server, relinquishing an IP address and canceling the remaining lease.

• DHCPINFORM Sent from a DHCP client to a DHCP server, requesting additional configuration settings; the client already has a configured IP address. This message type is also used for rogue DHCP server detection in Windows Server 2008.

DHCP messages, options, and protocol operation are defined in RFCs 2131 and 2132.

Source of Information : Microsoft Press Windows Server 2008 TCP IP Protocols and Services

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