DHCPv6 Relay Agent Echo Request Option
Extracted elements (11)
DHCPv4 servers generally echo relay agent options verbatim back to the relay agent, but DHCPv6 does not require this. The ERO option was introduced to allow relay agents to explicitly request echoing of specific options they need, even when the server does not recognize them.
The relay agent uses OPTION_ORO to request non-relay-agent options in the Relay Forward message, while OPTION_ERO is specifically for requesting relay agent options to be echoed back; these are distinct mechanisms.
A relay agent MAY include an Echo Request option in a Relay Forward message to inform the server about options the relay agent wants echoed back.
If an option requested in the ERO is already present in the Relay-Reply, the server MUST ignore that option and continue processing remaining ERO options.
If an option requested in the ERO was present in the Relay-Forward and not already in the Relay-Reply, the server MUST copy the option verbatim from the Relay-Forward to the Relay-Reply, even if the server does not otherwise recognize that option.
If the relay agent takes different actions based on whether an option is echoed back or not, the relay agent SHOULD NOT include such an option in the Echo Request option.
When a server creates a Relay-Reply, it SHOULD perform ERO processing after processing the ORO and other options processing.
The Relay Agent Echo Request option (OPTION_ERO, code 43) allows a DHCPv6 relay agent to request that the server echo back a specified list of relay agent options in the Relay-Reply message.
IANA assigned DHCPv6 option code 43 for the OPTION_ERO (Relay Agent Echo Request) option.
The Echo Request option is only exchanged between relay agents and DHCPv6 servers; RFC 3315 section 21.1 covers securing DHCPv6 messages between servers and relay agents, and section 23 provides general DHCPv6 security considerations.
OPTION_ERO format: 2-octet option-code field (value 43), 2-octet option-len field (value = 2 * number of requested options), followed by a list of 2-octet requested-option-code-n fields, one per requested option.