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SubjectRe: ppp mru/mtu and ip masquerading
On Sun, 28 Feb 1999, Alan Cox wrote:

> > David Miller asked me for some tcpdumps when I first reported it on
> > linux-kernel over a year ago, which I sent, but he was unable to find the
> > source of the problem. Toggling ip_no_pmtu_disc has no effect. Sort of
> > embarrassing that Linux networking has held this bug for so long.
> The Linux networking traces I've seen with MTU discovery all show it working
> correctly and faults at remote sites. I have yet to see a single case where
> the problem is the Linux end with 2.0.x

The reason I was so suspicious of the problem being at our (Linux) end, is
because you can reach the site no problem from the masq/firewall box; just
not from the clients. I was also suspicious because such a large number of
sites have this problem (though I've since learned not to underestimate the
potential for mass ignorance. ;) The masq box and the clients are both
reaching the outside through the same interface and MTU (obviously). The
problem only occurs with the client. I could be mistaken, but IIRC the
dumps showed the masq'd client's SYN go out and hit the target, and the
target response come back and hit the firewall. I don't know enough about
tcpdump output to see why the response wasn't demasq'd and returned to the
client. I'm no longer using a modem for Internet access, so I can't test
it anymore.

B. James Phillippe .
Software Engineer, WGT Inc. .

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