Understanding dhcrelay

[Kay Wanous]

Thanks for the response!

The dhcp server is handing out internal IPs... management wanted it that 
way so that we only had to have one dhcp server.

Here's what ended up fixing it: the dhcp server will always respond to 
the giaddr IP (the internal NIC) of the dhcp server, but the server 
can't reach that 192.168.0.1 IP.  So, adding a static route with the 
external IP of the dhcrelay machine as the gateway to 192.168.0.1 fixed 
the problem.

   -- Kay

Paul wrote:
> Kay Wanous wrote:
>> Hello,
>>
>> I'm trying to set up dhcrelay but based on what I'm reading I think 
>> I'm either missing a critical step or I don't understand how it works.
>>
>> The dhcrelay server receives a DHCP request on its internal interface 
>> (192.168.0.1) and places that IP address in the giaddr field, then 
>> sends it from its outside interface (routable IP) to the dhcp server 
>> (routable IP).  The dhcp server sees the giaddr and instead of 
>> replying to the routable IP of the dhcrelay server, it responds to 
>> 192.168.0.1.  But since the dhcp server can't reach the internal 
>> network, the response is never received on the dhcrelay server.
>>
>> Since the dhcp server can't get to the internal network (isn't that 
>> the whole idea of dhcrelay?), if it's answering to the giaddr field, 
>> how would the response ever get back to the dhcrelay server so that it 
>> could get back to the requesting client?
>>
>> Any help in clearing up this confusion is greatly appreciated.
>>
>> Thanks,
>> Kay
>>
> I don't understand why you would want a DCHP server on a routable 
> network to be providing IP addresses on a non-routable network, but if I 
> had to configure it I would have the DHCP replying to the routable 
> address of the dhcp relay server, otherwise it would never receive 
> anything to relay to the non-routable machine. The problem I have 
> understanding this is, if the machine is non-routable/non-reachable, of 
> what use would a (routable) IP address be to it?
> 
> Cheers,
> 
>