I have the following network in the lap
WINXP <---mtu1500--->FREEBSD4.4<---mtu1280(gif tunnel--->FREEBSD4.4<---mtu1500--->WINXP
The BSD computers are setup as gateway routers. As you might see from diagram above,
the MTU of the link between the two BSD computers is 1280Bytes because of a tunnel.
When I try to transfer a file between the XP endpoints, the PMTUD goes off by sending
a 1514B packet to the other end with the Don't fragment bit set. However this packet
never generates the ICMP unreachable message back to the XP box during Path MTU
discovery. So that's why I have concluded that the BSD router is the black hole.
Now I would like to know how to make my BSD router to participate in the MTU
discovery. Changing the MTU of the windows computers is not an option for me. Also
please note that the BSD computers only have one NIC interface, and the network is
logically sperate but physically all connected to a switch, so playing with the MTU of
the interface connecting the BSD computers is not an option either.
I hope that this is a known problem and easy to fix and thanks for taking the time to
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