PMTUD Black Hole Problem

Koroush Saraf koroush.saraf at
Thu Jun 19 09:37:14 PDT 2003

Hi All,

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 respond.

More information about the freebsd-questions mailing list