netdev - Re: Q: How to disable vlan strip in Intel igb driver ?

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Date:	Sun, 1 May 2016 22:51:08 +0200
From:	Peter Palúch <Peter.Paluch@....uniza.sk>
To:	Ran Shalit <ranshalit@...il.com>
Cc:	netdev@...r.kernel.org, alexander.duyck@...il.com
Subject: Re: Q: How to disable vlan strip in Intel igb driver ?

Hi Ran,

> Alex,
> I don't see rx-vlan-offload option in my ethtool. strange, maybe it is
> not available in all ethtool versions ?

According to my manpage for ethtool v4.5, the relevant -K option is 
"rxvlan".

> Hi Peter,
> Yes, I'm using AF_PACKET (I can't validate it now for 100%, but I
> quite sure about it).

Okay - but I assume that you are writing a userspace application, not a 
kernelspace driver or module, right?

> I'm accessing the ethernet header, and it always gives me non extended
> ethernet header (without vlan information).

It seems that this behavior has been around for a long time, and is 
intentional. See the following thread for more information:

http://www.spinics.net/lists/netdev/msg244668.html

The only legit way of accessing VLAN tag info on an AF_PACKET socket I 
know about is by setting the PACKET_AUXDATA socket option, and reading 
the auxiliary data as a struct tpacket_auxdata using recvmsg().

In addition, however, there seems to be a bug in the kernel in that the 
tp_vlan_tci member of the struct tpacket_auxdata is filled in only if 
the AF_PACKET socket is bound to htons(ETH_P_ALL). If the socket is 
bound to any other specific protocol, the tp_vlan_tci is set to 0. I 
have raised this issue recently in the following thread(s) but have not 
received a response yet:

http://www.spinics.net/lists/netdev/msg372830.html
http://www.spinics.net/lists/netdev/msg373112.html

> I can see the vlan tag in vlan_tci field in sk_buff, but this is not
> exactly what I need, I need the header AS-IS with the original
> (extended) ethernet header.

How are you accessing the vlan_tci field from your software?

In any case, with AF_PACKET sockets, I am afraid you don't have much 
choice. Even libpcap reconstructs VLAN-tagged frames by reading the VLAN 
tag from an auxiliary structure and then re-inserting it into the frame, 
because the frame delivered through an AF_PACKET socket does not have 
the VLAN tag present anymore.

Best regards,
Peter