We have run into an issue with some client DHCP Discover requests not being answered with DHCP Offer. We are using DHCP server built in our RUTX50 router. After some debugging we narrowed down the issues to only happen when client requests the Offer to be sent over broadcast (flags 0x8000). Sending the same discover request with flags 0x0000 (requests the dhcp offer to be sent via unicast) works without issues.
with tcpdump on the router we were able to verify that 1) both DHCP discover requests arrive to the router and 2) only the dhcp offer over unicast leaves the router
This issue has started happening after someone briefly configured a wifi connection to be used as WAN, however our issue persists after this was undone later.
Given the circumstances this makes me suspect it’s an iptables configuration issue, somehow dropping multicast packets destined to lan. I have looked at iptables-save output, but I was unable to see anything that would confirm this suspicion, but I am not entirely proficient in reading these rules so I may have missed something.
I have also found a thread on openwrt forum where someone ran into a similar issue in 2019 caused by mwan (a likely suspect given the circumstances when our issues started to arise) Certain devices ignore DHCPOFFER - #25 by mcmlxxvi - Installing and Using OpenWrt - OpenWrt Forum, but that was caused by them using a deprecated option. Again I didn’t see anything suspicious in /etc/config/mwan3.
(I have tried to upload output of iptables-save and contents of etc/config/mwan3 for completeness but apparently as a new user I cannot add attachments)
At this point I’m unsure how to continue. Any ideas would be welcome.