Re: [gentoo-bsd] G/FBSD TCP Troubles

[Nathan Smith]

Alex Unleashed wrote:
> On 3/19/07, *Nathan Smith* <ndansmith@×××××.com
> <mailto:ndansmith@×××××.com>> wrote:
>
>
>     So anyhow, I am making due, but it is a frustrating problem to say the
>     least.  Any suggestions are greatly appreciated.  :-)
>
>
> I would start capturing a reasonable amount of network traffic showing
> normal and error conditions, if at all possible on both ends to confirm
> there's nothing wrong with some machine within the path. FWIW, I recall
> hearing people having problems with FreeBSD's SACK in the past, though
> that should be fixed by now. Anyway, you have good hints on how to
> approach a closer analysis in this mail thread:
> http://lists.freebsd.org/pipermail/freebsd-net/2006-December/012830.html
>
> Alex

I've attached the highlights of what I've been able to tell with tcpdump
as of yet.  These packets were captured by a Gentoo Linux machine
running apache (all of the requests are HTTP).  The transmitting
computer in each case is the same machine (IBM Thinkpad Z60t - Pentium
M), but either the OS, or some feature of Gentoo/FreeBSD is altered for
each sample.  These two machines were at the same location with no
firewall and few routers separating them.

There is a little annotation in the file, but in essence the Linux box
replied to packets which were sent from non-Gentoo/FreeBSD systems
(Linux and FreeSBIE), and did not reply to the Gentoo/FreeBSD system.  I
varied the tcp window size, and I also tried turning rfc1323 on and off.
 I've also altered the mtu and max-mss with no success.  The common
denominator in the packets which failed to get a response is that they
have bad checksums.  If you compare the first packet listed (G/FBSD) and
the packet from the FreeSBIE liveCD, you will see that they are
identical in every pertinent way except that the packets coming from the
G/FBSD machine were mangled.

Apparently these packets are objectionable to some routers and not
others.  Depending on where I am, I can connect to some sites and not
others (and never gentoo-sunrise.org :-( ).  I set up a packet capture
on my home machine and send HTTP requests from the G/FBSD box at work,
and the packets never arrived.

Now it is just a matter of finding out _why_ they packets are getting
messed up.  It is apparently not a TCP window scaling problem (toggling
that has no effect), and it is not a FreeBSD proble, (FreeSBIE on the
same hardware works great), it is apparently a breakage in Gentoo/FBSD.

-- 
Nathan Smith
ndansmith@×××××.com