Re: How do I receive vlan tags on an AF_PACKET socket in 3.4 kernel?

[Eric Dumazet <eric.dumazet@gmail.com>]

On Wed, 2013-07-31 at 22:01 +0200, Ronny Meeus wrote:
> On Wed, Jul 31, 2013 at 5:09 PM, Eric Dumazet <eric.dumazet@gmail.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

Right, vlan_tx_tag_get() gets the whole tag, so you want to mask A with
0xfff before the compare (to strip the prio)

ld vlant
and #4095
jneq #4094, drop
ret #-1
drop: ret #0

or something like that.