Ethereal-users: Re: [Ethereal-users] problem with vlan tags

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From: Guy Harris <gharris@xxxxxxxxx>
Date: Tue, 22 Feb 2005 09:36:50 -0800
Alexander Sirotkin wrote:


I'm having a weird problem trying to sniff some traffic with VLAN tag.
My Linux box (2.4.20) is configured to support vlans (add vlan tag to
outgoing packets and remove vlan tag from incoming packets). When I
capture packets on the vlan enabled interface (eth0.42, with 42 being vlan id, for instance) all packets appear normal and without vlan tag. When I capture packets on the eth0 interface, outgoing packets look OK (including vlan tag) but icoming packets show some weird values in ethertype field, yet OS seem 
to process them OK.


	http://wiki.ethereal.com/CaptureSetup_2fVLAN
discusses capturing packets with VLAN tags on various platforms, although it currently has a placeholder for Linux - you might want to update the Linux section to give the correct information on Linux (is the "ethN", or whatever, vs. "ethN.TAG" scheme used on all current versions of the kernel that include VLAN support?).
I assume the weird values in the Ethertype field aren't 0x8100; what are the values? Perhaps this is yet another example of parts of the Linux networking stack manipulating skbuffs in place without any copy-on-write happening, so that the skbuff handed to the PF_PACKET socket no longer contains the packet bytes received from the network.