dhcpd 3.1.1, linux vlans since 2.6.27 and listening on the untagged device

David W. Hankins David_Hankins at isc.org
Mon Nov 10 22:50:46 UTC 2008

On Sat, Nov 08, 2008 at 02:03:22PM +0100, Oliver Wagner wrote:
> Some investigation suggests, however, that this is a problem in dhcpd3 
> which just didn't always show up in 8.04. There is a change regarding the 
> handling of VLAN packets and packet filters in kernel 2.6.27 which makes 
> sure that vlan tagged packets are always presented to filters on the 
> underlying device (eth0 in this case) with the vlan tag removed from the 
> packet data. Previously (depending on the ethernet driver, from what I 
> could understand) the raw data was also presented, but with the vlan tag 
> still present.

Why in the world would they do that?

> If the analysis is correct, this needs to be fixed upstream in dhcpd's 
> linux interfacing code.

If that analysis is correct, I can't imagine a DHCP software scopable
solution.  How can we differentiate between eth0 and vlanx input
except by using very specifically crafted packet filters on those
respective interfaces?

Ash bugud-gul durbatuluk agh burzum-ishi krimpatul.
Why settle for the lesser evil?	 https://secure.isc.org/store/t-shirt/
David W. Hankins	"If you don't do it right the first time,
Software Engineer		     you'll just have to do it again."
Internet Systems Consortium, Inc.		-- Jack T. Hankins

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