From owner-freebsd-net@FreeBSD.ORG  Wed Aug  1 22:26:54 2012
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Date: Thu, 2 Aug 2012 00:47:01 +0200
From: Luigi Rizzo <rizzo@iet.unipi.it>
To: John <jwd@freebsd.org>
Message-ID: <20120801224701.GA31112@onelab2.iet.unipi.it>
References: <20120801221240.GA24976@FreeBSD.org>
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Cc: FreeBSD Net <freebsd-net@freebsd.org>
Subject: Re: 0 frame length?
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On Wed, Aug 01, 2012 at 10:12:40PM +0000, John wrote:
> Hi Folks,
> 
>    On a Dell R610 system, I've been tracking down some nework issues
> and ran across this in a tcpdump:
> 
> 0000  84 2b 2b fd be 2e f0 4d  a2 08 c4 13 08 00 45 00   .++....M ......E.
> 0010  00 00 f0 2a 40 00 40 06  00 00 0a 18 09 ee 0a 18   ...*@.@. ........
> 0020  1e 08 58 57 14 02 5e 30  ea dc 61 84 62 b3 80 18   ..XW..^0 ..a.b...
> 0030  08 00 42 10 00 00 01 01  08 0a 1e 67 b2 58 dc 56   ..B..... ...g.X.V
> 0040  81 12 4e 45 54 50 41 43  4b 04 00 00 00 00 00 00   ..NETPAC K.......
> ....  additional packet data
> 
> offset 0x10 should be the frame length - not 0. This only seems to happen
> in packets being sent from this system/interface.
> 
> Corresponding interface:
> 
> bce0: flags=8943<UP,BROADCAST,RUNNING,PROMISC,SIMPLEX,MULTICAST> metric 0 mtu 1500
>         options=c01bb<RXCSUM,TXCSUM,VLAN_MTU,VLAN_HWTAGGING,JUMBO_MTU,VLAN_HWCSUM,TSO4,VLAN_HWTSO,LINKSTATE>
>         ether f0:4d:a2:08:c4:13
>         inet 10.24.9.238 netmask 0xffff0000 broadcast 10.24.255.255
>         media: Ethernet autoselect (1000baseT <full-duplex,flowcontrol,rxpause,txpause>)
>         status: active
> 
> 
> 8.2-RELEASE
> 
> Wireshark reports:
> 
> Bogus IP len (0, less than header length 20)
> 
> 
>    In googling, I've seen comments about lro/tso & the *csum options.
> I have those set to off for the next time the systems get rebooted.

you do not need to reboot to enable/disable the various
offloading options, just "ifconfig bce0 -rxcsum -tso4 -txcsum"
should do the job.

cheers
luigi