Re: [PATCH net 2/2] net/af_packet: fix tx skb network header on SOCK_RAW sockets over VLAN device

[Hervé Boisse]

On Thu, Jan 12, 2023 at 04:47:38PM +0100, Paolo Abeni wrote:
> I understand, thanks. Still is not clear why the user-space application
> would attach to dummy0.832 instead of dummy0.
> 
> With your patch the filter will match, but the dhcp packet will reach
> the wire untagged, so the app will behave exactly as it would do
> if/when attached to dummy0.
> 
> To me it looks like the dhcp client has a bad configuration (wrong
> interface) and these patches address the issue in the wrong place
> (inside the kernel).

No, the packet will actually reach the wire as a properly tagged 802.1Q frame.
For devices that do not support VLAN offloading (such as dummy but also the network card I am using), the kernel adds the tag itself in software before transmitting the packet to the real device. 

You can verify this with a capture using tcpdump/wireshark on dummy0 versus dummy0.832.
That's why dhclient has to send its packets over dummy0.832 and not dummy0.

The same will happen on a real device. I checked on real hardware, with two boxes and their network cards connected through a cable.
If dhclient is started directly on the first box real device (eth0), the frame is received untagged by the second box, as intended.
But, if dhclient is started on top of the VLAN device (eth0.832), the second box receives a properly tagged frame.

Hervé