netdev - [RFC] VLAN aux info for AF_PACKET available only with ETH_P

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Message-ID: <570FB48E.2060108@fri.uniza.sk>
Date:	Thu, 14 Apr 2016 17:17:34 +0200
From:	Peter Palúch <Peter.Paluch@....uniza.sk>
To:	netdev@...r.kernel.org
Subject: [RFC] VLAN aux info for AF_PACKET available only with ETH_P_ALL

Greetings,

When using AF_PACKET sockets with PACKET_AUXDATA socket option to access 
the VLAN TCI information of received frames, I have noticed that the 
VLAN information in struct tpacket_auxdata, namely,

- tp_vlan_tci
- tp_vlan_tpid
- TP_STATUS_VLAN_VALID and TP_STATUS_VLAN_TPID_VALID flags

is filled in only when the socket is bound to htons (ETH_P_ALL). If the 
socket is bound to any specific protocol, the VLAN information fields in 
struct tpacket_auxdata are set to 0 even if the datagram of the specific 
protocol was received in an 802.1Q-tagged frame.

As the VLAN tag is being stripped off the frame soon in the receive 
path, using PACKET_AUXDATA is the only way for an application over an 
AF_PACKET socket to know what VLAN did a particular frame arrive in; 
yet, the current behavior forces the application to listen to all 
received traffic to get the actual VLAN info.

Is this behavior intentional, or is this lack of VLAN info a bug? I am 
running vanilla Linux kernel v4.4.6.

Thanks!

Best regards,
Peter