[En-Nut-Discussion] DHCP on multiple interfaces?

[Bernd Walter]

On Fri, Dec 11, 2009 at 03:24:31PM +0100, Harald Kipp wrote:
> Thiago A. Corrêa wrote:
> 
> > About the reply, I think the request is broadcasted, but the reply is
> > targeted, therefore, even if it's not one of the fields, it's possible
> > to determine from which interface it came from (and examine the mac
> > address field from the ethernet frame).
> 
> What do you mean by "the reply is targeted". You mean that the offer is
> a unicast packet? How can this be?

The anser is directly send to the client MAC.
It is unicast at Ethernet Layer, but not at IP.
Maybe not every server is doing this, but some do for sure.
Under UNIX systems you need raw Socket access to inject such special
packets.
This is the same for the client - you send to 255.255.255.255, but
also on a special interface, which is only possible using raw sockets.
Normaly you also need to setup a hop-count of 1 to avoid that the
requests is forwarded to other networks, but forwarding broadcast
packets is considered dangerous today, so most routers won't do this
by default.

DHCP is very special, because it has to work before IP is setup.

> DHCP is UDP and the destination of the reply still doesn't have an IP
> address. IMHO, the offer reply needs to be a broadcast as well.

-- 
B.Walter <bernd at bwct.de> http://www.bwct.de
Modbus/TCP Ethernet I/O Baugruppen, ARM basierte FreeBSD Rechner uvm.