DHCP Message Format

The fields in the DHCP message are the following:

• Message Op Code (Op) A 1-byte field that indicates whether the message is a request
(set to 1) or a reply (set to 2).

• Hardware Address Type (Htype). A 1-byte field that indicates the type of hardware being used by the DHCP client. This field uses the same values as the Hardware Type field in the Address Resolution Protocol (ARP) header. For a complete list of ARP Hardware Type values, see http://www.iana.org/assignments/arp-parameters.

• Hardware Address Length (Hlen). A 1-byte field that indicates the number of highorder bytes within the fixed-length Client Hardware Address field that contains the client’s hardware address. For commonly used IEEE 802-based technologies, such as Ethernet and IEEE 802.11, the value of this field is 6.

• Hops. A 1-byte field that indicates how many DHCP relay agents have forwarded the message. The initial value is 0. When a DHCP relay agent forwards a DHCP message on behalf of either a DHCP client or a DHCP server, it increments this field. The maximum number of hops in a DHCP infrastructure is 16. If the value is greater than 16, the receiving DHCP relay agent silently discards the message. DHCP relay agents can also discard DHCP messages if this field exceeds a configurable value. For example, the DHCP Relay Agent component of Routing and Remote Access in Windows Server 2008 uses a default maximum of 4 hops.

• Transaction ID (Xid). A 4-byte field that contains a random number derived by the
DHCP client to group all of the DHCP messages of a given message exchange together, such as all of the messages for a lease acquisition.

• Seconds (Secs). A 2-byte field set by the DHCP client to indicate the number of seconds that have elapsed since the client began the address acquisition process.

• Flags. A 2-byte field that indicates flags that are set by the DHCP client. RFC 2131 defines the high-order bit as the Broadcast flag. A DHCP client uses the broadcast flag to indicate that it can (set to 0) or cannot (set to 1) receive unicast IP datagrams even though it has not been configured with an IP address. Windows Server 2008 and Windows Vista-based DHCP clients set the Broadcast flag to 1 (responses must be broadcast). If the DHCP server has been configured to process this flag, it will send its response as either a unicast (when the Broadcast flag is set to 0) or as a broadcast (when the Broadcast
flag is set to 1).

• Client IP. Address (Ciaddr) A 4-byte field that indicates a DHCP client’s IP address. This field is set by the DHCP client in DHCP messages when it has been successfully configured with the IP address and can respond to ARP requests to defend the use of the address.

• Your IP. Address (Yiaddr). A 4-byte field that indicates the IP address that is being allocated to the DHCP client by the DHCP server.

• Server IP Address (Siaddr). A 4-byte field that indicates the IP address of the DHCP server that is offering an IP address.

• Gateway IP Address (Giaddr). A 4-byte field that indicates an IP address that is assigned to the interface on the initial DHCP relay agent that received the message from the DHCP client. The initial DHCP relay agent is located on the same subnet as the DHCP client that broadcast the DHCP request message (either a DHCPDISCOVER or DHCPREQUEST message). By recording an IP address for the subnet of the DHCP client in this field, the DHCP server can determine the proper scope from which to assign an IP address to the requesting DHCP client.

• Client Hardware Address (Chaddr). A 16-byte field that indicates the hardware address of the DHCP client. To determine how many bytes are used for the hardware address, the DHCP server and relay agent use the value of the Hardware Address Length field. Fo commonly used IEEE 802-based technologies, this field contains the 6-byte media access control (MAC) address of the Ethernet or 802.11 network adapter of the DHCP client and
10 bytes set to 0.

• Server Host Name (Sname). A 64-byte field that indicates a name for the DHCP server.
The DHCP Server service in Windows Server 2008 does not use this field.

• Boot File Name (File). A 128-byte field that indicates the name of the file containing a boot image for a BOOTP client. BOOTP was developed before DHCP to allow a diskless host computer to obtain an IP address configuration, the name of a boot file, and the location of a Trivial File Transfer Protocol (TFTP) server from which the computer loads the boot file. DHCP message exchanges do not use this field.

• Options A variable-length set of fields containing DHCP options.


By default, the DHCP Server service in Windows Server 2008 ignores the Broadcast flag in the Flags field of broadcast-based DHCP messages received by DHCP clients. To configure the DHCP Server service to process the Broadcast flag, create and set theIgnoreBroadcastFlag registry value to 0.

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