Re: Receiving raw packets (incl. VLAN tags) on raw sockets

On 14/06/17 16:41, Eric W. Biederman wrote:

That does not work.  That is is just the software fallback for when
the device driver does not have a special case the processing
vlan tagged packets.

There was a major inconsistency that for a long time the hardware
network drivers were stripping tags and the software ones were not.

The code you are playing with is the fix for the rare slow path
that does not happen to strip the tags.  Disabling the rare slow path
might temporarily solve your symptoms but it will be much more painful
when you are entrenched in your ways and discover that high performance
hardware behaves differently than your software device.

Thanks for your reply! Actually, I was referring to COTS hardware that incorporates offloading features. But, when it comes to (security) monitoring, offloading is usually disabled [1,2] to process the packets as seen on the wire. Thus the "slow path" would be the default path for most monitoring applications. That is, what makes this situation kind of weird. After turning off the NIC's VLAN offloading, it took me some time to realize that now the kernel strips off VLAN tags. If someone decides that VLAN offloading is not needed, I think the kernel should not enforce it.

Jan

[1] https://redmine.openinfosecfoundation.org/projects/suricata/wiki/File_Extraction#NIC-offloading
[2] https://www.bro.org/documentation/faq.html#capture-loss-without-dropped-packets