dhclient VLAN 0 decapsulation

From: Franco Fichtner <franco_at_opnsense.org>
Date: Thu, 03 Feb 2022 13:40:21 UTC
Hi all,

We've been working on VLAN 0 decapsulation for dhclient last year and would like
a wider audience for the proposed change at https://reviews.freebsd.org/D31515 <https://reviews.freebsd.org/D31515>

Basically what is being done is extend the BPF read filter program to parse an optional
VLAN tag if found and skipped over when the priority is zero. The write side does not
require any changes as the server can read the packet without the priority tag inserted.

The code has been deployed in two OPNsense releases and is being used out in
the wild as some ISPs send VLAN-0 wrapped responses to DHCP requests which
will not be seen by the client without said patch.

All functional and technical feedback is welcome.