Date: Wed, 4 Jun 2025 17:36:40 +0300 From: Christos Chatzaras <chris@cretaforce.gr> To: questions@freebsd.org Cc: freebsd-net@freebsd.org Subject: Problem with net.inet.tcp.path_mtu_discovery=1 Message-ID: <9728060D-2C02-426B-BACE-F2D2F651A62F@cretaforce.gr>
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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=
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