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Date:	Mon, 17 Dec 2012 09:50:17 -0000
From:	"David Laight" <David.Laight@...LAB.COM>
To:	"Daniel Borkmann" <danborkmann@...earbox.net>,
	"Ani Sinha" <ani@...stanetworks.com>
Cc:	"Michael Richardson" <mcr@...delman.ca>, <netdev@...r.kernel.org>,
	<tcpdump-workers@...ts.tcpdump.org>,
	"Francesco Ruggeri" <fruggeri@...stanetworks.com>
Subject: RE: [tcpdump-workers] vlan tagged packets and libpcap breakage

> > I do agree that instead of a /proc entry, we should check for a kenrel
> > version >= X where X is the upstream version that first started
> > supporting all the features needed by libpcap for vlan filtering. This
> > is not a compile time check but a run time one. Does anyone see any
> > issues with this? Is there any long term implications of this, like if
> > you backport patches to an older long term supported kernel? Are there
> > other better ways to do this, like may be returning feature bits from
> > an ioctl call? This is something we need to deal with on a continuous
> > basis as we keep supporting newer AUX fields and libpcap and other
> > user land code needs to make use of it. At the same time, they need to
> > handle backward compatibility issues with older kernels.
> 
> As Eric mentioned earlier, for now there seems not to be a reliable
> way to get to know which ops are present and which not. It's not
> really nice, but if you want to make use of those new (ANC*) features,
> probably checking kernel version might be the only way if I'm not
> missing something. Now net-next is closed, but if it reopens, I'll
> submit a version 2 of my patch where you've been CC'd to. If it gets
> in, then at least it's for sure that since kernel <xyz> this kind of
> feature test is present.

How are you going to tell whether a feature is present in a non-Linux
kernel ?

Testing kernel versions is somewhat suboptimal as support
could be patched into a much older kernel (maybe not for
this but ...)

	David



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