DHCP relay - wrong interface for relayed packet

[Paul Thornton]

Hi Alan,

On 07/07/2022 12:51, Alan DeKok wrote:
>    Yup.  Packets using that socket are being sent via vlan4091, because that socket is bound to interface vlan4091.


That makes perfect sense.  Thanks for the steer, I did wonder if it was 
something like that.

I presume that to listen for DHCP packets, we need to operate on a 
per-interface basis to be able to receive broadcasts; so operating with 
one socket - not bound to any specific interface - and listening on that 
isn't going to be an option.


>    The solution is to create a new outbound socket, specifically for DHCP relaying.  It can be bound to "*" for the IP, and then the normal DHCP port.
>
>    There are a few ways to do this... one might be to simply create a "wildcard" DHCP listener, which doesn't have an "interface" configured.  Then somehow update proto_dhcp/dhcpd.c to choose this listener for relayed packets.
>
>    It's likely not hard, it just requires a bit of spelunking in the code.

My C isn't up to fixing this properly but I'm going to attempt a quick 
hack proof-of-concept to see if it does work.

Thanks,

Paul.