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Date:	Fri, 17 Oct 2014 17:48:50 +0200
From:	Daniel Borkmann <>
To:	Lukas Tribus <>
CC:	"" <>,
	John Fastabend <>,
	Michał Mirosław <>,
	Jiri Pirko <>,
	Ben Hutchings <>,
	Atzm Watanabe <>,
	Patrick McHardy <>,
	Jesse Gross <>,
	Michael Richardson <>,
	Ani Sinha <>
Subject: Re: tcpdump's capture filter: "vlan" doesn't match

On 10/17/2014 01:25 AM, Lukas Tribus wrote:
>>> Isn't disabling rx-vlan-offloading supposed to remedy those problems?
>> There were some discussions on this in the past e.g. [1]. We have
>> SKF_AD_VLAN_TAG and SKF_AD_VLAN_TAG_PRESENT for the BPF filter on
>> this, but libpcap is currently not making use of any of them.
>> [1]
> Thanks for the link. I see the situation is unfortunate and although those
> new BPF filters in the kernel may fix the actual filtering problem, one
> thing seems to remain impossible: disabling all this kernel magic and
> passing the frame as-is to libpcap without interception (avoiding any
> kind of artificial header reconstruction).
> How is the situation with netsniff-ng anyway? Does it use vlan BPF filter
> in the kernel?

So in netsniff-ng we don't do obscure header reconstruction as it
hurts performance and it can result in incorrect reconstruction cases.

You however can define a bpf_asm program (e.g. tools/net/ in kernel
tree) and use instruction overloading for the vlan case from there.
Thus, you're not tied to the libpcap compiler which misses this.

For more details, I refer you to Documentation/networking/filter.txt .
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