Skip site navigation (1)Skip section navigation (2)
Date:      Thu, 6 Mar 2008 10:18:18 -0600
From:      David DeSimone <fox@verio.net>
To:        freebsd-net@freebsd.org
Subject:   Re: Path MTU Problem
Message-ID:  <20080306161818.GD15130@verio.net>
In-Reply-To: <000001c87f43$c8075800$37c9010a@Net.ARDS.Corp>
References:  <000001c87f43$c8075800$37c9010a@Net.ARDS.Corp>

Next in thread | Previous in thread | Raw E-Mail | Index | Archive | Help
-----BEGIN PGP SIGNED MESSAGE-----
Hash: SHA1

_lion_2000@mail.ru <_lion_2000@mail.ru> wrote:
>
> Hi, i have a FreeBSD 6.3, fresh install.  It is in the corporate
> network and i can't use any tcp network service on that machine from
> any other, which is behind cisco routers with tunnels.

This is a classic PMTU Discovery problem.  

> Well, not all services, but some, when large packets are sent from
> that box.  After some investigation i found, what router sends ICMP
> Frag packets to that box, but it doesn't reduce packets size and keep
> sending large packets:

Is it possible that you have PF or IPFW filter rules in place that drop
ICMP?  Just because tcpdump shows you the frame arrived at your system,
does not mean that it was "seen" by the kernel.

> here comes icmp frag packets. strange what sometimes tcpdump complains about
> tcp header in icmp packet and sometimes not

The reason for this complaint is that frag_needed packets return a
portion of the original IP frame back to the sender, but the number of
bytes is not sufficient to see the entire TCP header.  However, there is
enough to see the src/dest IP's and src/dest port numbers, as tcpdump
shows you.  But tcpdump cannot decode past the end of the returned
frame, so it shows an error.

- -- 
David DeSimone == Network Admin == fox@verio.net
"This email message is intended for the use of the person to whom
 it has been sent, and may contain information that is confidential
 or legally protected.  If you are not the intended recipient or have
 received this message in error, you are not authorized to copy, dis-
 tribute, or otherwise use this message or its attachments.  Please
 notify the sender immediately by return e-mail and permanently delete
 this message and any attachments.  Verio, Inc. makes no warranty that
 this email is error or virus free.  Thank you."  --Lawyer Bot 6000
-----BEGIN PGP SIGNATURE-----
Version: GnuPG v1.4.1 (GNU/Linux)

iD8DBQFH0BlKFSrKRjX5eCoRAoV2AJ4muSN0vV3HfpxfqKB1S/F+pX7TrACfRiQB
AzsTCoFsun772YGxCxLj8GM=
=BcKe
-----END PGP SIGNATURE-----



Want to link to this message? Use this URL: <http://docs.FreeBSD.org/cgi/mid.cgi?20080306161818.GD15130>