IPv6 Path MTU Discovery automatically sets MTU to 1280 regardless of interface

[jeremy.duncan]

Hello, I have OPNsense 21.7.7-amd64 and OPNsense 21.7.8-amd64 - as both behave the same.  When doing a path trace on my IPv6 network I see the opnsense firewall sending packet too big messages for 1280 to all hosts on the network (for packets higher than 1280) even though the host is set to 1450, interface is set to 1500, and the RA MTU option is set to 1450.  Here's a path trace:

tracepath google.com
 1?: [LOCALHOST]                        0.040ms pmtu 1450
 1:  2001:470:e5bf:1001:cafe:dead:beef:1                   0.910ms
 1:  2001:470:e5bf:1001:cafe:dead:beef:1                   0.533ms
 2:  2001:470:e5bf:3000::2                                 1.178ms
 3:  2001:470:e5bf:3000::2                                 1.229ms pmtu 1280
 3:  tunnel161881.tunnel.tserv13.ash1.ipv6.he.net          8.896ms
 4:  10ge2-2.core1.ash1.he.net                             7.659ms
 5:  pr61.iad07.net.google.com                            10.415ms
 6:  no reply

It's also in packet #9 in the attached PCAP.  This behavior is wrong and violates the RFC specs. Anyone have any idea what the issue is?

[jeremy.duncan]

OK, I think I figured it out looking at the tunnel interface MTU on the firewall. BY DEFAULT it sets to 1280 unless you set it to match the MTU on the other end of the tunnel - 1480 per HE.  When I set to 1480, it no longer sent PMTU for 1280, but for 1480 like it's supposed to. Not at all intuitive...
tracepath google.com
 1?: [LOCALHOST]                        0.029ms pmtu 1500
 1:  2001:470:e073:101::2                                  0.392ms
 1:  2001:470:e073:101::2                                  0.407ms
 2:  2001:470:e073:101::2                                  0.425ms pmtu 1480
 2:  tunnel202636.tunnel.tserv13.ash1.ipv6.he.net         29.177ms
 3:  10ge2-2.core1.ash1.he.net                            13.809ms
 4:  pr61.iad07.net.google.com                            12.468ms

 tracepath google.com
 1?: [LOCALHOST]                        0.033ms pmtu 1400
 1:  2001:470:e5bf:1001:cafe:dead:beef:1                   8.834ms
 1:  2001:470:e5bf:1001:cafe:dead:beef:1                   0.516ms
 2:  2001:470:e5bf:3000::2                                 1.576ms
 3:  tunnel161881.tunnel.tserv13.ash1.ipv6.he.net          7.791ms
 4:  10ge2-2.core1.ash1.he.net                             7.385ms
 5:  pr61.iad07.net.google.com                             7.862ms