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 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'

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.