netdev - Re: How do I receive vlan tags on an AF

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Date:	Wed, 31 Jul 2013 16:36:41 +0200
From:	Ronny Meeus <ronny.meeus@...il.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	netdev <netdev@...r.kernel.org>
Subject: Re: How do I receive vlan tags on an AF_PACKET socket in 3.4 kernel?

On Wed, Jul 31, 2013 at 4:16 PM, Eric Dumazet <eric.dumazet@...il.com> wrote:
> On Wed, 2013-07-31 at 14:51 +0200, Ronny Meeus wrote:
>
>> Thanks for the feedback. High level it is almost clear.
>>
>> At implementation level I do not understand how it is supposed to work.
>> If I use tcpdump to generate a filter for example on vlan 4094 I see
>> no reference at all to the newly added instructions to get the VLAN.
>>
>> ~ # tcpdump -i eth-ntb vlan 4094 -d
>> tcpdump: WARNING: eth-ntb: no IPv4 address assigned
>> (000) ldh      [12]
>> (001) jeq      #0x8100          jt 3    jf 2
>> (002) jeq      #0x9100          jt 3    jf 7
>> (003) ldh      [14]
>> (004) and      #0xfff
>> (005) jeq      #0xffe           jt 6    jf 7
>> (006) ret      #65535
>> (007) ret      #0
>>
>> To me it looks like to code above is just checking the bytes in the
>> raw Ethernet packet at offset 12 and 14.
>> Since the command above seems to work it looks to me that the
>> filtering is done in the tcpdump application instead of in the kernel.
>>
>> If I use the strace command while starting tcpdump I see that the
>> SO_ATTACH_FILTER sockopt is passed to the kernel:
>>
>> <snip>
>> setsockopt(3, SOL_SOCKET, SO_ATTACH_FILTER, "\0\1\0\0\20\f\366\340", 8) = 0
>> fcntl64(3, F_GETFL)                     = 0x2 (flags O_RDWR)
>> fcntl64(3, F_SETFL, O_RDWR|O_NONBLOCK)  = 0
>> recvfrom(3, 0x7f6f6630, 1, 32, 0, 0)    = -1 EAGAIN (Resource
>> temporarily unavailable)
>> fcntl64(3, F_SETFL, O_RDWR)             = 0
>> setsockopt(3, SOL_SOCKET, SO_ATTACH_FILTER, "\0\10\0\0\20>\210@", 8) = 0
>> <snip>
>>
>> So I'm confused. I would expect to see some commands to read access
>> the VLAN field in the additional data and compare it to the VLAN
>> (4094) I want to filter.
>>
>
> I assumed from you initial mail you were using a BPF filter, not
> libpcap, which presumably doesnt use these new 'instructions'
>

I used the tcpdump tool to generate the filter I need to use in my application.

> Adapting the BPF filter generated by libpcap is a matter of adding 3 or
> 4 instructions. In your case 2 instructions actually
>
> One to load tag id into A
> One to compare A against immediate value 4094 and conditional jump.
>

Can you give an real example of a filter that passes all packets that
have a VLAN 4094 attached and drops all others?
Thanks
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