From owner-freebsd-current@FreeBSD.ORG Fri Aug 20 22:20:34 2004 Return-Path: Delivered-To: freebsd-current@freebsd.org Received: from mx1.FreeBSD.org (mx1.freebsd.org [216.136.204.125]) by hub.freebsd.org (Postfix) with ESMTP id 4446416A4CE for ; Fri, 20 Aug 2004 22:20:34 +0000 (GMT) Received: from tigra.ip.net.ua (tigra.ip.net.ua [82.193.96.10]) by mx1.FreeBSD.org (Postfix) with ESMTP id 812DD43D3F for ; Fri, 20 Aug 2004 22:20:33 +0000 (GMT) (envelope-from ru@ip.net.ua) Received: from heffalump.ip.net.ua (heffalump.ip.net.ua [82.193.96.213]) by tigra.ip.net.ua (8.12.11/8.12.11) with ESMTP id i7KMKPjw060835 (version=TLSv1/SSLv3 cipher=DHE-RSA-AES256-SHA bits=256 verify=NO); Sat, 21 Aug 2004 01:20:26 +0300 (EEST) (envelope-from ru@ip.net.ua) Received: (from ru@localhost) by heffalump.ip.net.ua (8.13.1/8.13.1) id i7KMKQGu089949; Sat, 21 Aug 2004 01:20:26 +0300 (EEST) (envelope-from ru) Date: Sat, 21 Aug 2004 01:20:26 +0300 From: Ruslan Ermilov To: Maxim Sobolev Message-ID: <20040820222026.GC73564@ip.net.ua> References: <20040820190525.GA21626@isc.org> <16678.21478.381551.833252@grasshopper.cs.duke.edu> <41265A06.3030800@portaone.com> Mime-Version: 1.0 Content-Type: multipart/signed; micalg=pgp-sha1; protocol="application/pgp-signature"; boundary="Pk6IbRAofICFmK5e" Content-Disposition: inline In-Reply-To: <41265A06.3030800@portaone.com> User-Agent: Mutt/1.5.6i X-Virus-Scanned: by amavisd-new cc: "David W. Hankins" cc: Andrew Gallatin cc: current@FreeBSD.org Subject: Re: on amd64 tcp4 cksums are bad (FYI) X-BeenThere: freebsd-current@freebsd.org X-Mailman-Version: 2.1.1 Precedence: list List-Id: Discussions about the use of FreeBSD-current List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Fri, 20 Aug 2004 22:20:34 -0000 --Pk6IbRAofICFmK5e Content-Type: text/plain; charset=us-ascii Content-Disposition: inline Content-Transfer-Encoding: quoted-printable On Fri, Aug 20, 2004 at 11:07:34PM +0300, Maxim Sobolev wrote: > Andrew Gallatin wrote: >=20 > >David W. Hankins writes: > > >=20 > > > This is as observed via tcpdump on [client], which is what is produci= ng > > > the bad checksums. Obviously it doesn't cause a problem since no one > > > listens to TCP checksums, but it's interesting. I only noticed it > > > because I was tcpdump'ing for completely unrelated reasons, and it=20 > > caught > > > my eye. > > > ><...> > > > > > Client machine is amd64, running 64-bit mode 5-current fresh as of > > > yesterday. Network interface is e1000, so fxp. Server is also freeb= sd > > > >e1000 is actually em. =20 > > > >You're almost certainly using a driver which offloads transmit > >checksums. (both fxp and em do) Since BPF sniffs the packet before it > >leaves the host, the checksum has not yet been calculated, so it looks > >bad. >=20 > Is it possible to detect this situation and flag tcpdump somehow, so=20 > that it don't trust checksum? With the widespread adoption of GigE=20 > cards, this "problem" is likely to be more and more common. >=20 It's easy to detect using the m_pkthdr.csum_flags. It shouldn't be impossible to make a writable mbuf chain copy, and call in_delayed_cksum() on a copy, before calling bpf_mtap(). Cheers, --=20 Ruslan Ermilov ru@FreeBSD.org FreeBSD committer --Pk6IbRAofICFmK5e Content-Type: application/pgp-signature Content-Disposition: inline -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.2.5 (FreeBSD) iD8DBQFBJnkqqRfpzJluFF4RArMsAJ99I8i5h7n6l1tGvQwuLTuxiamv5gCeIFBD IUCQvcxLFl2ATHVRKH5m6jo= =b66l -----END PGP SIGNATURE----- --Pk6IbRAofICFmK5e--