From owner-freebsd-questions@FreeBSD.ORG Wed Mar 17 03:32:40 2004 Return-Path: Delivered-To: freebsd-questions@freebsd.org Received: from mx1.FreeBSD.org (mx1.freebsd.org [216.136.204.125]) by hub.freebsd.org (Postfix) with ESMTP id 1DF0416A4CE for ; Wed, 17 Mar 2004 03:32:40 -0800 (PST) Received: from smtp.infracaninophile.co.uk (ns0.infracaninophile.co.uk [81.2.69.218]) by mx1.FreeBSD.org (Postfix) with ESMTP id 9E34643D1D for ; Wed, 17 Mar 2004 03:32:38 -0800 (PST) (envelope-from m.seaman@infracaninophile.co.uk) Received: from happy-idiot-talk.infracaninophile.co.uk (localhost [IPv6:::1]) i2HBWXFQ027947 (version=TLSv1/SSLv3 cipher=DHE-RSA-AES256-SHA bits=256 verify=NO); Wed, 17 Mar 2004 11:32:33 GMT (envelope-from matthew@happy-idiot-talk.infracaninophile.co.uk) Received: (from matthew@localhost)id i2HBWX6g027946; Wed, 17 Mar 2004 11:32:33 GMT (envelope-from matthew) Date: Wed, 17 Mar 2004 11:32:33 +0000 From: Matthew Seaman To: Sherwood Botsford Message-ID: <20040317113233.GA26606@happy-idiot-talk.infracaninophile.co.uk> Mail-Followup-To: Matthew Seaman , Sherwood Botsford , freebsd-questions@freebsd.org References: <200403162043.30590.sbotsford@sjsa.ab.ca> Mime-Version: 1.0 Content-Type: multipart/signed; micalg=pgp-sha1; protocol="application/pgp-signature"; boundary="CE+1k2dSO48ffgeK" Content-Disposition: inline In-Reply-To: <200403162043.30590.sbotsford@sjsa.ab.ca> User-Agent: Mutt/1.5.6i X-Spam-Status: No, hits=-4.8 required=5.0 tests=AWL,BAYES_00 autolearn=ham version=2.63 X-Spam-Checker-Version: SpamAssassin 2.63 (2004-01-11) on happy-idiot-talk.infracaninophile.co.uk X-Virus-Scanned: clamd / ClamAV version devel-20040304, clamav-milter version 0.67j cc: freebsd-questions@freebsd.org Subject: Re: tcpdump check sum errors. X-BeenThere: freebsd-questions@freebsd.org X-Mailman-Version: 2.1.1 Precedence: list List-Id: User questions List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Wed, 17 Mar 2004 11:32:40 -0000 --CE+1k2dSO48ffgeK Content-Type: text/plain; charset=us-ascii Content-Disposition: inline Content-Transfer-Encoding: quoted-printable On Tue, Mar 16, 2004 at 08:43:30PM -0700, Sherwood Botsford wrote: >=20 > I'm running mail services (exim, fetchmail, popper) on FreeBSD 4.5 >=20 > In a session trying to track down a problem related to a bad=20 > cable, I found that=20 > tcpdump -i xl0 -v -v -v host pop.incentre.net=20 >=20 > has lots of lines like this: >=20 > 20:20:11.166767 postie.sjsa.internal.net.1126 >=20 > bach.ccinet.ab.ca.pop3: F [bad tcp cksum 4f5e!]=20 > 61:61(0) ack 2075 win 33304 =20 > (DF) (ttl 64, id 57145, len 52, bad cksum 0!) >=20 > But netstat -I xl0 -w 10 during the same interval when the=20 > above were coming down at 5-10 per second showed: >=20 > input (xl0) output > packets errs bytes packets errs bytes colls > 32 0 4019 28 0 4573 0 > 24 0 3512 22 0 4934 0 > 444 0 30070 433 0 227132 0 >=20 > Explanations? (I also get them on the local network; > it's not just this destination host.) So you're asking why all of those corrupted packets you can see on the wire don't appear on the error counters in the netstat -w output? Good question. About the only explanation I can think of is that the packets are so mangled that they get dropped very early on. You've probably replaced the broken cable by now, but for future reference you could try adding '-d' to your netstat command line, to count the number of dropped packets. You might try asking again on the freebsd-net@freebsd.org list, where you're more likely to find people with a great deal of familiarity with the internals of the networking code. Cheers, Matthew --=20 Dr Matthew J Seaman MA, D.Phil. 26 The Paddocks Savill Way PGP: http://www.infracaninophile.co.uk/pgpkey Marlow Tel: +44 1628 476614 Bucks., SL7 1TH UK --CE+1k2dSO48ffgeK Content-Type: application/pgp-signature Content-Disposition: inline -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.2.4 (FreeBSD) iD8DBQFAWDdQdtESqEQa7a0RAkbYAJ0fa4bWRqQY9N2V/MlDYlHA2WYitgCeP1e/ TY+7GYdwXtY/wyD3ayfkyng= =7znL -----END PGP SIGNATURE----- --CE+1k2dSO48ffgeK--