PMTUD Black Hole ProblemPlease take a look at this possible code Problem? If not,
please read the message below to find out the original problem!
Is this a code problem in ip_input.c, This code is from FreeBSD 4.8 that I just
installed on my computers.
ip_forward ? It looks to me like case EMSGSIZE can
never be reached. Is this breaking mtu path discovery responses ?
ip_forward(struct mbuf *m, int srcrt, struct sockaddr_in *next_hop)
switch (error) {
case 0: /* forwarded, but need redirect */
/* type, code set above */
break;
case ENETUNREACH: /* shouldn't happen, checked above */
case EHOSTUNREACH:
case ENETDOWN:
case EHOSTDOWN:
default:
type = ICMP_UNREACH;
code = ICMP_UNREACH_HOST;
break;
case EMSGSIZE:
type = ICMP_UNREACH;
code = ICMP_UNREACH_NEEDFRAG;
----- Original Message -----
From: Saraf, Koroush (N-Norman SubSystems)
To: [EMAIL PROTECTED]
Sent: Thursday, June 19, 2003 9:32 AM
Subject: PMTUD Black Hole Problem
Hi All,
I have the following network in the lab
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.
~koroush
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