Broken PMTUD in FreeBSD?

[Phil Dibowitz <webmaster () ipom com>]

[Note: I accidentally posted this last week from the wrong email address. It's 
probably sitting in queue somewhere - but since it hasn't come through yet, 
I'm sending it from the correct address, I'm sorry if you get this twice]

[Note2: Dave A., since I haven't heard back from you, I'm assuming this is OK 
to post.]

Bugtraqers,

BUG OVERVIEW
I believe there is a bug in the PMTUD (Path MTU Discovery) implementation in 
FreeBSD. According to RFC 1191, when using PMTUD all TCP datagrams must have 
the Don't Fragment (DF) bit set. It seems that FreeBSD does not fully obey 
this rule. On "SYN ACK" packets, the DF bit is not set. It is set on all other 
packets though (including SYN packets). The details are below - I have been 
unable to find any reason for this behavior, but if someone can explain a 
reason for this other than it being a bug, wonderful, you're smarter than I am! =)

NOTIFICATION
My friend Richard van den Berg, who originally found the bug, posted to the 
FreeBSD mailing list on April 21, 2002. The post can be found here:
http://docs.freebsd.org/cgi/getmsg.cgi?fetch=9182+0+archive/2002/freebsd-net/20020428.freebsd-net

That's a month and a half of notice. We received no response either on the 
list or in person.

SEVERITY
I don't consider this a big security hole, but it is a bug. It could be used 
to do TCP fingerprinting, and it also breaks a standard (which makes 
troubleshooting PTMUD Blackholes a little more difficult, something Richard 
and I do as part of the MSS Initiative[1]).

DETAILS
I have made available packet sniffer logs of both sides of a test at the 
following locations.
http://home.earthlink.net/~jaymzh666/mss/snoop-log-solaris-to-bsd.gz
http://home.earthlink.net/~jaymzh666/mss/tcpdump-log-bsd-to-solaris.gz

The test systems were as follows:
$ uname -a
SunOS mort 5.9 s81_57 sun4u sparc SUNW,Sun-Blade-100
$ uname -a
FreeBSD trantor.xs4all.nl 5.0-CURRENT FreeBSD 5.0-CURRENT #6: Mon Apr 15
20:16:39 MET DST 2002
paulz () trantor xs4all nl:/usr/obj/usr/source/src/sys/trantor i386

If I can provide any more information, or if you have any light to shed on 
this topic, please feel free to let me know.

[1] MSS Initiative: http://home.earthlink.net/~jaymzh666/mss/

Sincerely,
Phil Dibowitz
--
Insanity Palace of Metallica
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