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I'm trying to figure out if an IPv4 DHCP Relay Agent needs to care about a rapid-commit (Option 80) included in a DHCP Discover.

My understanding is that the Relay Agent is supposed to simply converts the DHCPDISCOVER broadcast packet into a unicast packet before sending it to the DHCP Server (who should/must care about rapid-commit). Then, converts the received unicast DHCPACK into a broadcast message and send it back to the server's network.

What I don't understand is what behavior change would the Relay Agent have between standard DORA and Rapid-Commit DA?

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It should be a mostly transparent process to the relay agent. The agent gets the broadcast client request, adds it's relay information, and unicasts it on to the server(s). The server uses the relay information to return any message through the agent, who then broadcasts an answer back to the client. The client won't have an address at this point, so it has to be broadcast. At most, the agent may need to recognize the rapid-commit option to know it must be a broadcast reply, but the DHCP options should already indicate a broadcast answer.

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  • I should add, while one could logically unicast the reply at layer-2 as the chaddr should be the MAC, there are various problems receiving unicast IP traffic without IP being configured. Using both L2 and L3 broadcasting has always worked. (which is why the DHCP "B" flag exists.) Also, I've seen way too many broken DHCP clients use the wrong MAC.
    – Ricky
    Apr 16, 2021 at 0:17
  • Hi @Ricky, Would you know of any client OS that supports DHCP IPv4 rapid-commit? This seems to be a mostly IPv6 option (Ubuntu, CentOS, Windows and others seem to support it only on v6)
    – Eric Boivin
    Apr 21, 2021 at 13:34

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