pfSense

Hi Jim,

Looking at RFC 2131, this actually looks like a legit configuration. Various excerpts of the RFC seem to encompass this scenario. See below for your quickest reference:
- Sec 1.6, bullet #5:
"A DHCP client must be prepared to receive multiple responses
to a request for configuration parameters. Some installations
may include multiple, overlapping DHCP servers to enhance
reliability and increase performance."
- Sec 3.1.2: "Each server may respond with a DHCPOFFER message [...]"
- Sec 3.1.3:
"The client receives one or more DHCPOFFER messages from one or more
servers. [...] The client chooses one server [...].
The client broadcasts a DHCPREQUEST message
that MUST include the 'server identifier' option to indicate which
server it has selected [...]"
- Sec 3.1.4:
"The servers receive the DHCPREQUEST broadcast from the client.
Those servers not selected by the DHCPREQUEST message use the
message as notification that the client has declined that server's
offer. The server selected in the DHCPREQUEST message commits the
binding for the client to persistent storage and responds with a
DHCPACK message [...]
If the selected server is unable to satisfy the DHCPREQUEST message
[...], the
server SHOULD respond with a DHCPNAK message."

Crucially notice how NAK should be sent by the selected server (in case the request cannot be satisfied), not by a server that was not selected. This is exactly the bug I'm reporting. Hope this clarifies!

We do not control the DHCP daemon on that level, that's up to the ISC DHCP daemon. Take it up with them, but I doubt they would care much about two non-HA servers on the same L2.

Either way, having two non-HA DHCP servers on the same L2 has never been a supported or valid configuration on pfSense, or on many other platforms, and not something we intend on attempting to support.

Hi Jim,

Thanks for your feedback. For future reference, by reading the ISC DHCP manual I found this configuration is fully supported using the shared-network statement [[https://kb.isc.org/docs/isc-dhcp-41-manual-pages-dhcpdconf#reference-declarations]].

Shame it won't make it to pfSense; this is a common scenario for anyone using one pfSense instance as a DHCP relay; the other pfSense instance receives DHCP requests from multiple networks on one interface. As a workaround, I'm patching the generated conf file, and restarting the daemon, taking inspiration by [[https://happy-coder.com/2014/06/27/pfsense-custom-dhcpd-configuration/]]. Hope this helps others in the same situation.