Re: Linux, tcpdump and vlan

[Stephen Hemminger <shemminger@linux-foundation.org>]

On Thu, 19 Jul 2007 15:28:46 +0200
Krzysztof Halasa <khc@pm.waw.pl> wrote:

> Patrick McHardy <kaber@trash.net> writes:
> 
> > Your suggestion of disabling VLAN acceleration in promiscous
> > mode sounds like a reasonable solution until then ..
> 
> From a user perspective:
> 
> I'm not sure promiscous mode is related to the problem.
> Tcpdump without promiscous mode makes perfect sense.
> 
> I don't know very well VLAN code internals, but I think
> the VLAN # is used for looking up the interface, so
> presenting the "original" packet on the trunk device
> would IMHO involve some skb cloning, and perhaps some
> ethtool option could probably control that.
> 
> Not sure about untagged frames vs. tagged frames with
> the default VLAN id - can the hardware at all differentiate
> between them?
> 
> 
> Or, perhaps it should be left (almost) as is - with "software"
> VLANs the traffic always goes through the master interface,
> but with "accelerated" mode it only goes through logical
> interfaces and doesn't show up on master? Probably with
> exception of invalid VLANs, which could be injected back to
> master (because no logical device exists)?


I don't claim to be a VLAN expert but there are really three cases
for handling tagged frames

1) non-accelerated device 
    * all frames show in promiscious mode
    * tag is part of the frame that shows up
       in tcpdump, and then gets stripped by the 8021q module.
2) rx tag stripping device
     * all frames show in promiscious mode
     * tag is in skb but NOT passed to tcpdump
3) rx vlan acceleration
     * only frames that for vlan's that are registered show up
        in promisicous mode
     * tag is in skb but NOT passed to tcpdump

Unfortunately, the tag is lost as part of the VLAN acceleration process
so it is not a simple matter of changing code in AF_PACKET receive
to restore the tag.
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