DHCP Option 81, also known as the Client Fully Qualified Domain Name (FQDN) option, is used to specify the client's FQDN in DHCP communications. This option allows the client to inform the DHCP server about its FQDN and the desired level of DNS update service.

Fields in DHCP Option 81

The Client FQDN option includes the following fields:

• Code: Specifies the code for this option (81).

• Len: Specifies the length of this option (minimum of 4).

• Flags: Specifies the type of service: 0: Client will register the "A" (Host) record. 1: Client wants DHCP to register the "A" (Host) record. 3: DHCP will register the "A" (Host) record regardless of the client's request.

• RCODE1: Specifies a response code the server is sending to the client.

• RCODE2: Specifies an additional delineation of RCODE1.

• Domain Name: Specifies the FQDN of the client.

How DHCP Option 81 Works

When a DHCP client sends a DHCPREQUEST packet, it can include Option 81 to specify its FQDN and the desired DNS update behavior. The DHCP server processes this option and determines how to handle the DNS updates based on the client's request and the server's configuration.

Example Interaction

1. Client Request: The client initiates a DHCP request message (DHCPREQUEST) to the server, including Option 81.

2. Server Acknowledgment: The server returns a DHCP acknowledgment message (DHCPACK) to the client, granting an IP address lease and including Option 81.

3. DNS Update: Depending on the configuration, the client or the server will register the DNS A and PTR records.