Problem with net.inet.tcp.path_mtu_discovery=1

Hello,

I manage some servers hosting websites.

Using the APN "vpn-internet" (this apn provides public IP to my home connection) on a 5G router, I receive a public IP address from the range 149.210.xxx.xxx, but I notice that some websites do not load.

The same problem has been reported by other users who also receive an IP from the same range (149.210.xxx.xxx).

In contrast, when I connect using the APN "internet" (this apn provides CGNAT IP) and get an IP from the range 5.203.136.xxx, all websites load normally.

Eventually, I found that the issue is related to Path MTU Discovery.

When the net.inet.tcp.path_mtu_discovery setting is enabled on the operating system, the problem occurs. If I disable it, then the problem goes away. This setting had been enabled for years on my servers, so something must have changed recently with my ISP's apn "vpn-internet".

Also enabling net.inet.tcp.pmtud_blackhole_detection didn't help.

Does anyone know if PMTUD blackhole detection is broken in FreeBSD?

Kind regards,
Christos Chatzaras