PMTU and Broken Servers

Curtis Maurand curtis at
Mon May 12 14:15:09 UTC 2003

I've had the problem before.  Not all routers handle PMTU correctly.


On Thu, 8 May 2003, Leo Bicknell wrote:

> I've recently had the pleasure of troubleshooting a problem I don't
> normally have to deal with, and the results don't quite make sense
> to me.  I'm hoping someone can enlighten me as to what is going on.
> A diagram:
> server---internet---fw---tunnelbox1----tunnelbox2----user
> The tunnel between the tunnelboxes is a lower (1480) MTU.  Originally
> the user couldn't access some servers, turns out the firewall was
> filtering ICMP Can't Fragment messages, preventing PMTU from working
> in the server->user direction (tunnelbox1 would generate Can't
> Fragement, firewall would filter).
> That's been corrected.  Going to a server I control I see good PMTU
> in both directions between the server and the user.  However, there
> are still a number of web servers for popular sites that behave
> just like the firewall was still filtering Can't Fragments.  The
> theory is that the servers are behind a firewall/load balancer that
> is filtering them on the server side -- but I find it slightly
> (emphasis on the slightly) that someone would turn on PMTU discovery,
> and then filter it out right in front of the boxes where they turned
> it on.  Also, it seems to me most DSL users are behind PPPoE links
> with lower MTU, and should get hit by the same problem.
> The temporary hack is to have tunnelbox1 clear the DF bit on all
> incoming packets, which just causes the packets to get fragmented
> going down the tunnel.  A minor performance hit, but it works.
> This is a new problem to me, but I'm sure people have run into it
> before.  Are the servers really that broken (PMTU enabled, ICMP
> Can't Fragement filtered)?  Does the head end box of DSL services
> generally do something to work around this (ie, clear the DF bit)?
> Am I just being an idiot and missing something obvious?

Curtis Maurand
mailto:curtis at

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