Old Tired Question: 'Not configured to listen on any interfaces'

[Glenn Satchell]

On 02/26/11 04:28, Alex Bligh wrote:
> Simon,
>
> --On 25 February 2011 16:02:19 +0000 Simon Hobson
> <dhcp1 at thehobsons.co.uk> wrote:
>
>> Alex Bligh wrote:
>>
>>>> When you get a renewal request from a client at (say) 192.168.1.57, how
>>>> do you route the packet back to it ? I'm guessing you don't and rely on
>>>> the client reverting to broadcasts before it's lease runs out, in which
>>>> case your DHCP isn't working as well as you thought.
>>>
>>> Nope. We route the packet back to the relay agent that sent it (using
>>> that relay agent's source IP address, just like is done normally).
>>
>> So you are intercepting the unicast packets at the router then ? For
>> renewals, the client communicates directly with the server, and a relay
>> agent is not required at all. Some net gear has the ability to sniff
>> traffic, and modify DHCP packets - typically by inserting option 82.
>
> Sort of.
>
> We have a virtual device which behaves like a dhcp server, but is
> actually relaying the packets. As far as the client is concerned, it
> is talking to a dhcp server.

Ahh. So more like a dhcp proxy than a relay. The term "dhcp relay" has 
quite specific function as defined in the way dhcp works, and I think 
this has led to some of the confusion in this discussion. Your statement 
above clears up a lot of things.

> Given we are only doing fixed leases, renewals aren't important in
> our config, but I see no reason why such a config could not be
> adapted for dynamic leases.

As I said in another post the client still gets the address dynamically 
from what it believes is the dhcp server and goes through the usual dhcp 
process of requesting a renewal from time to time.

regards,
-glenn