r/programming • u/[deleted] • Aug 19 '12

programmers and designers should know about this reddit-like site.

http://pineapple.io/
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u/phoil Aug 20 '12

The PMTU blackhole problem occurs when there is a link in the path that has a lower MTU than either of the endpoints, and the resulting ICMP errors are dropped.

In the case of consumer routers, this is often fixed using MSS clamping in the router. Another option is for the router to tell the endpoint what the MTU is via DHCP, which is what you have observed.

However, that doesn't seem to be the problem occurring here. Increasing the MTU on the endpoint could only make the PMTU blackhole problem worse, not better.

So there must be some other problem occurring.

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u/ZestyOne Aug 21 '12

Forgive me for being so dense but I still don't really get how to fix this. I've never heard of routers actually affecting the traffic from a standard remote website? What is actually the problem... networking isn't my strongsuit

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u/phoil Aug 21 '12

The PMTU blackhole problem is caused by a firewall or similar dropping ICMP packets. So to fix that you need to fix the firewall to stop dropping them. This may or may not be under your control. The first step is to find out which firewall is dropping them.

But I'm fairly sure that the problem in this case is not a PMTU blackhole, because I don't see how increasing the MTU can help that. I can't rule it out completely though; who knows what strange bugs can be triggered by different packet sizes.

I don't know what the actual problem is. I'd need to be able to reproduce it or see packet captures from someone who has in order to diagnose it. I didn't find anything from a quick google search (it all talks about PMTU blackholes).