Date: Thu, 12 Aug 2004 17:47:10 +0300 From: Ruslan Ermilov <ru@freebsd.org> To: Robert Watson <rwatson@freebsd.org> Cc: FreeBSD Current <freebsd-current@freebsd.org> Subject: Re: bad tcp cksum on outgoing packets Message-ID: <20040812144710.GA72404@ip.net.ua> In-Reply-To: <Pine.NEB.3.96L.1040812104300.38103G-100000@fledge.watson.org> References: <20040812130917.GC24142@ip.net.ua> <Pine.NEB.3.96L.1040812104300.38103G-100000@fledge.watson.org>
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--J2SCkAp4GZ/dPZZf Content-Type: text/plain; charset=us-ascii Content-Disposition: inline Content-Transfer-Encoding: quoted-printable On Thu, Aug 12, 2004 at 10:44:12AM -0400, Robert Watson wrote: >=20 > On Thu, 12 Aug 2004, Ruslan Ermilov wrote: >=20 > > On Thu, Aug 12, 2004 at 12:50:58PM +0100, Chris Stenton wrote: > > > I have just been doing some debugging on my 5.2.1 box and noticed that > > > outgoing tcp packets on the box are coming up with bad checksums on > > > tcpdump. I am using the nge interface. > > >=20 > > > Here is a sample output. > > >=20 > > > 12:44:29.458021 0:4:e2:10:60:83 0:c:6e:4e:a0:cc ip 82: > > > hawk.gnome.co.uk.ssh > kite.gnome.co.uk.2167: P [bad tcp cksum 9420!] > > > 2071:2099(28) ack 753 win 65535 (DF) [tos 0x10] (ttl 64, id 35623, l= en > > > 68, bad cksum 0!) > > >=20 > > > 12:44:29.642088 0:c:6e:4e:a0:cc 0:4:e2:10:60:83 ip 60: > > > kite.gnome.co.uk.2167 > hawk.gnome.co.uk.ssh: . [tcp sum ok] 753:753(= 0) > > > ack 2099 win 64956 (DF) (ttl 128, id 44852, len 40) > > >=20 > > >=20 > > > Any ideas whats going on as the packet does not seem to be resent? > > >=20 > > You don't have hardware checksums enabled, do you? I barely > > recall they are incompatible with bpf(4). >=20 > They're not incompatible per se, but if you're sniffing outgoing packets > and the network interface is calculating the checksum on send, BPF will > see a version of the packet before the checksum is calculated. If tcpdump > later attempts to verify the checksum, it still won't be calculated in the > copy it sees, and will whine. It was unclear to me in the above e-mail if > this was a tcpdump of packets on the wire (say, the receiver), or on the > sender before they hit the wire. >=20 That's what I meant exactly. Thanks for clarifying my thoughts. ;) Cheers, --=20 Ruslan Ermilov ru@FreeBSD.org FreeBSD committer --J2SCkAp4GZ/dPZZf Content-Type: application/pgp-signature Content-Disposition: inline -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.2.4 (FreeBSD) iD8DBQFBG4LuqRfpzJluFF4RAn3aAJ9oicqVISg6gPjUf/3SIYDAcJ+MVQCfcczt ugfTVw+VylL/3OYDKTbjHIQ= =Ehy7 -----END PGP SIGNATURE----- --J2SCkAp4GZ/dPZZf--
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