DHCP Relay + PPP (your mail)

Simon Hobson dhcp at thehobsons.co.uk
Wed May 24 07:05:14 UTC 2006


>I didn't understand the part I have pasted below.
>- you just need to be able to receive a unicast packet to the
>When I captured packets using ethereal there it is shown DHCP OFFER
>is sent as a unicast packet to GIAddr

Yes, that is what you would see.

>so there seems already support to recieve a unicast packet to

But not on a PPP interface

>	When I start dhcrelay I see a message like this....
>	./dhcrelay -d -i eth0 -i ppp0
>	Copyright 2004 Internet Systems Consortium.
>	All rights reserved.
>	For info, please visit  <http://www.isc.org/sw/dhcp/>
>	ppp0 unknown hardware address type 514
>	Listening on LPF/ppp0/
>	Sending on   LPF/ppp0/
>	Listening on LPF/eth0/00:40:63:da:3b:e6
>	Sending on   LPF/eth0/00:40:63:da:3b:e6
>	Sending on   Socket/fallback
>	Could be the message "unknown hardware address type 514" the reason


In summary, to function, all the relay agent needs to be able to do 
(for server replies) is to receive them as a unicast packet. As you 
have observed, they will always be sent to the GIAddr the relay agent 
used when forwarding the request.

However, as coded at the moment, the relay agent must explicitly 
listen on any interface it is going to receive packets on, and won't 
work on a ppp interface because it doesn't support the facilities 
expected. My guess is that when the relay agent was first written, 
things like [ASX]DSL were either not common or not even available at 
all and the situation you find was not envisaged. Treating all 
interfaces the same would make sense in that you can use the same bit 
of code for everything.

I suspect there is now an item in the future enhancement list to 
change this IF time & resources permit (or someone presents ISC with 
a patch) - but don't hold your breath.

Until this gets changed, you cannot use a ppp interface with a relay agent.


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