DHCP Relay + PPP (your mail)

Simon Hobson dhcp at thehobsons.co.uk
Tue May 23 15:53:34 UTC 2006

David W. Hankins wrote:
>On Tue, May 23, 2006 at 12:15:34PM +0530, JOBY THAMPAN wrote:
>>	When I made DHCP Relay listening on a machine containing PPP
>>  Interface, the DHCP Relay is not recieving the offer packet from dhcp server
>>	But when I made DHCP Relay listening on ethernet interface on a
>>  different machine other than the machine containing PPP Interface DHCP Offer
>>  Successfully recieved.
>>	If somebody knows, Please explain why this is so???
>The short answer is that is not presently a supported configuration.
>If the relay and server are on the same box, you're better off just
>extending the server to cover the additional interfaces directly than
>to introduce a middleman.

I don't think that is what he is meaning, I assume he is referring to 
the diagram he posted yesterday.

He runs a relay agent on a machine connected to the ethernet segment 
of the client, and with a PPPoE link back to a router and onwards to 
the server. The replies from the server don't reach the client.

If he puts an extra machine between the client network and and the 
PPPoE link (so the relay agent talks "upstream" via an ethernet 
interface) then the relay works.

So the problem seems to be that if the upstream interface of the 
machine running the relay agent is a PPP link, it ignores packets 
returning from the server.

Thoughts that come to mind ...

The relay agent needs to listen on the PPP interface to handle 
packets from the server - does it do so by default, or does it need 
to be told to ?

How is the relay agent being started, and is it being started with 
only one interface listed ?

Does the relay agent have to start after the PPP I/F comes up ? If 
so, is it, or is it starting before ?


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