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

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Date:	Wed, 31 Jul 2013 22:01:39 +0200
From:	Ronny Meeus <ronny.meeus@...il.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	Daniel Borkmann <dborkman@...hat.com>,
	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 5:09 PM, Eric Dumazet <eric.dumazet@...il.com> wrote:
> On Wed, 2013-07-31 at 16:42 +0200, Daniel Borkmann wrote:
>
>> You can use bpfc (git://github.com/borkmann/netsniff-ng.git), it also has
>> an extensive man page. That should probably do it:
>>
>> $ cat foo
>> ld vlant
>> jneq #4094, drop
>> ret #-1
>> drop: ret #0
>>
>> $ bpfc foo
>> { 0x20, 0, 0, 0xfffff02c },
>> { 0x15, 0, 1, 0x00000ffe },
>> { 0x6, 0, 0, 0xffffffff },
>> { 0x6, 0, 0, 0x00000000 },
>

Thanks Daniel, this is very useful information.
I have cloned the repo and compiled the tool myself. It will be very
useful in the future.

> Thanks Daniel.
>
> If the load of this BPF fails (because its an old kernel), then load
> your old filter.
>

I created a small test application after I backported the filter code
to the 3.4 kernel.
I instrumented the kernel with a printk at the moment the
vlan_tx_tag_get call is done to see the actual value of the vlan tag
since it did not work initially.

These are the packets displayed by tcpdump:

tcpdump: WARNING: eth-ntb: no IPv4 address assigned
tcpdump: verbose output suppressed, use -v or -vv for full protocol decode
listening on eth-ntb, link-type EN10MB (Ethernet), capture size 65535 bytes
00:18:49.233283 06:00:00:00:00:80 > f7:00:00:00:ff:ff, ethertype
802.1Q (0x8100), length 64:
        0x0000:  f700 0000 ffff 0600 0000 0080 8100 affe
        0x0010:  08ab 0014 0000 0000 0f00 0001 0096 6000
        0x0020:  0096 0000 0001 0000 000d 0000 0000 0000
        0x0030:  0000 0000 0000 0000 0000 0000 0000 0000

So the Vlan is 0xffe and the priority/CFI field is 0xA.
Apparently the value I need to  use in the filter is 0xaffe to make it
work. Is this normal or is this a bug in the kernel?

This is the filter I used:
{ 0x20, 0, 0, 0xfffff02c }
{ 0x15, 0, 1, 0x0000affe }
{ 0x06, 0, 0, 0x00000800 }
{ 0x06, 0, 0, 0x00000000 }

And this is the trace of the kernel and my application:

[12529.357172] BPF_S_ANC_VLAN_TAG: affe
packets received:          1
[12533.020743] BPF_S_ANC_VLAN_TAG: affe
packets received:          2
[12536.667159] BPF_S_ANC_VLAN_TAG: affe
packets received:          3
[12540.343857] BPF_S_ANC_VLAN_TAG: affe
packets received:          4


Best regards,
Ronny
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