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I would research this from the hyper-v side and not OPNsense.

Interfaces-->Overview and check each interface (WAN and LAN).

Have you tried replacing the ethernet cables on both interfaces.

What hardware are you running OPNsense on?

Are the WAN and/or LAN interfaces logging errors?

Have you replaced the WAN ethernet cable to see if that helps?

Make sure hardware offloading is disabled. It's the default, but could cause your flapping.

Not much to go on here....what hardware are you running OPNSense on?

I managed to resolve the problem. It turned out to be my core ethernet switch mishandling multicast traffic.

Realtek NICs are garbage, especially when running FreeBSD (which is OPNSense's underlying OS) because the drivers are poorly written. Find some better hardware to solve your problem.

Bumping this thread because I have not made any progress toward fixing IPv6.

Digging a bit deeper, it appears dhcpd runs chroot, so the missing file is actually found at /var/dhcpd/etc. So that isn't the cause of the problem...

IPv6 has been working fine for a few months. After updating to the latest OPNSense release, none of my clients can obtain IPv6 addresses. The configuration looks correct and OPNSense is getting a prefix from my provider (Verizon FIOS). DHCPv6 is running, but shows no leases.

My LAN is set to track interface mode and ifconfig shows the LAN interface is being assigned IPv6 addresses. One possible issue is the file /etc/dhcpdv6.conf does not exist. I tried changing the LAN IPv6 config (IPv6 Prefix ID) to see if the file might be regenerated, but no luck.

Suggestions on how to debug this?

What size prefix are you getting on the WAN?

I think you need to figure out what is common when connected with wifi and ethernet. It sounds like a bad cable somewhere...perhaps the connection between your switches.

Realtek NICs are not very good, especially when running FreeBSD. Replace with a decent Intel NIC and your problems should go away.

Or shorten the DHCP lease times so as to recover inactive IPs.