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Support dhcrelay on interfaces with zero MAC address

Open Qingtao Cao requested to merge qingtao/dhcp:152-dhcrelay-support-ARPHRD_NONE-interfaces into master

The DHCP relay is designed to use interfaces with L2 headers. Add following support so that it can work on interfaces without L2 headers:

  1. Handle sa_family == ARPHRD_NONE in get_hw_addr();

  2. Use the Linux cooked socket instead of raw socket so that a copy of L4 / protocol handled packet can be reinjected to the PF_PACKET taps (this is critical when the DHCP relay doesn't understand the L2 packets captured, for example, when they are encrypted IPSec packets);

  3. Avoid assembling / decoding L2 headers when sending / receiving L2 packets

  4. Adjust the BPF filter code to skip over analysing L2 headers

Signed-off-by: Qingtao Cao qingtao.cao@digi.com

Closes #152