Dropping reply received on <downstream interface> when using -iu/-id

[Brian J. Murrell]

I want to use the -iu/-id options with dhcrelay to prevent relays from
relaying (and thus duplicating) requests coming from the network that
the DHCP server is on[1]:

# /usr/sbin/dhcrelay -4 -d -pf /var/run/dhcrelay4.pid -id br-guest -iu br-lan 10.75.22.247

When I do this, DHCP Discover requests coming from clients on the br-
guest interface have their Relay agent IP address set to the address of
the br-guest interface.  When the DHCP server sends back an DHCP Offer
it sends it to the IP address of the br-guest interface on the relay
machine.  dhcrelay then proceeds to drop that reply:

Dropping reply received on br-guest

Is there something I am misunderstanding about how -iu/-id are supposed
to work?

Cheers,
b.

[1] I'm surprised this is not implicit to be honest.  The relay knows
which network the server is on.  Why is it relaying requests that come
from that network to that network?
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