20. Relay Agent Behavior

The relay agent MAY be configured to use a list of destination addresses, which MAY include unicast addresses, the All_DHCP_Servers multicast address, or other addresses selected by the network administrator. If the relay agent has not been explicitly configured, it MUST use the All_DHCP_Servers multicast address as the default.

If the relay agent relays messages to the All_DHCP_Servers multicast address or other multicast addresses, it sets the Hop Limit field to 32.

20.1. Relaying a Client Message or a Relay-forward Message

A relay agent relays both messages from clients and Relay-forward messages from other relay agents. When a relay agent receives a valid message to be relayed, it constructs a new Relay-forward message. The relay agent copies the source address from the header of the IP datagram in which the message was received to the peer-address field of the Relay-forward message. The relay agent copies the received DHCP message (excluding any IP or UDP headers) into a Relay Message option in the new message. The relay agent adds to the Relay-forward message any other options it is configured to include.

20.1.1. Relaying a Message from a Client

If the relay agent received the message to be relayed from a client, the relay agent places a global or site-scoped address with a prefix assigned to the link on which the client should be assigned an address in the link-address field. This address will be used by the server to determine the link from which the client should be assigned an address and other configuration information. The hop-count in the Relay-forward message is set to 0.

If the relay agent cannot use the address in the link-address field to identify the interface through which the response to the client will be relayed, the relay agent MUST include an Interface-id option (see section 22.18) in the Relay-forward message. The server will include the Interface-id option in its Relay-reply message. The relay agent fills in the link-address field as described in the previous paragraph regardless of whether the relay agent includes an Interface-id option in the Relay-forward message.

20.1.2. Relaying a Message from a Relay Agent

If the message received by the relay agent is a Relay-forward message and the hop-count in the message is greater than or equal to HOP_COUNT_LIMIT, the relay agent discards the received message.

The relay agent copies the source address from the IP datagram in which the message was received from the client into the peer-address field in the Relay-forward message and sets the hop-count field to the value of the hop-count field in the received message incremented by 1.

If the source address from the IP datagram header of the received message is a global or site-local address (and the device on which the relay agent is running belongs to only one site), the relay agent sets the link-address field to 0; otherwise the relay agent sets the link-address field to a global or site-local address assigned to the interface on which the message was received, or includes an Interface-ID option to identify the interface on which the message was received.

20.2. Relaying a Relay-reply Message

The relay agent processes any options included in the Relay-reply message in addition to the Relay Message option, and then discards those options.

The relay agent extracts the message from the Relay Message option and relays it to the address contained in the peer-address field of the Relay-reply message.

If the Relay-reply message includes an Interface-id option, the relay agent relays the message from the server to the client on the link identified by the Interface-id option. Otherwise, if the link-address field is not set to zero, the relay agent relays the message on the link identified by the link-address field.

20.3. Construction of Relay-reply Messages

A server uses a Relay-reply message to return a response to a client if the original message from the client was relayed to the server in a Relay-forward message or to send a Reconfigure message to a client if the server does not have an address it can use to send the message directly to the client.

A response to the client MUST be relayed through the same relay agents as the original client message. The server causes this to happen by creating a Relay-reply message that includes a Relay Message option containing the message for the next relay agent in the return path to the client. The contained Relay-reply message contains another Relay Message option to be sent to the next relay agent, and so on. The server must record the contents of the peer-address fields in the received message so it can construct the appropriate Relay-reply message carrying the response from the server.

For example, if client C sent a message that was relayed by relay agent A to relay agent B and then to the server, the server would send the following Relay-Reply message to relay agent B:

msg-type:       RELAY-REPLY
hop-count:      1
link-address:   0
peer-address:   A
Relay Message option, containing:
  msg-type:     RELAY-REPLY
  hop-count:    0
  link-address: address from link to which C is attached
  peer-address: C
  Relay Message option: &lt;response from server>

When sending a Reconfigure message to a client through a relay agent, the server creates a Relay-reply message that includes a Relay Message option containing the Reconfigure message for the next relay agent in the return path to the client. The server sets the peer-address field in the Relay-reply message header to the address of the client, and sets the link-address field as required by the relay agent to relay the Reconfigure message to the client. The server obtains the addresses of the client and the relay agent through prior interaction with the client or through some external mechanism.