MAC to IP assignment not working - dhcprelay

Edwin Whitelaw Edwin.Whitelaw at
Wed Apr 25 12:06:14 UTC 2007


The shared-subnet comment was the key.  I'm a wireless ISP and each 
remote access point (Voyage Linux on a single board computer) has two 
nets per system, one NATed, for customers' routers and the other a  
non-NATed, IP for internal network devices.  This being my first attempt 
to use the relay approach, I'd failed to realize I still needed the 
shared-network statement back on the central DHCP server.

The small range declared allows both client routers and network bridges 
to acquire a NATed IP automagically with a short lease pending their 
addition to the include file for a fixed MAC-IP relationship.  It's 
worked well in practice but needed to be consolidated via relay.

Assuming my other configuration is correct, I've not had a problem with 
clients getting range addresses if they have an entry in the included 
host assignment file.  I assume the deny known-clients declaration 
simply reinforces the default behavior of use the fixed assignment 
before referring to the range?  I'd rather have the client get some 
address, even from the range, rather than none at all.  Periodic review 
of the leases file lets me see where I've overlooked a fixed assignment 
since my client radios have known MAC ranges.

Thanks for the insightful comments.


> Hi Edwin
> You said the include file was copied directly from the standalone dhcp
> server. Were the subnet definitions also copied exactly?
> The dhcp server will ignore a fixed-address statement if that address
> is not valid for the subnet where the request came from. So your hosts
> with 172.17.20.x address will get an address from if the
> relay forwards a request stating that they are coming from
> network.
> Perhaps you need a shared-network statement around the two subnet
> definitions if both are valid in the remote location?
> You may also want to change the dynamic subnet defintion to explicitly
> exclude the fixed hosts, eg:
> subnet netmask {
> 	pool {
> 		deny known-clients;
>         	range;
>         }
>         default-lease-time 3600;
>         max-lease-time 3600;
>         option routers;
>         option broadcast-address;
>         option subnet-mask;
> } # End of subnet
> regards,
> -glenn

Edwin Whitelaw, P.E.
New River Valley Unwired, LLC
2200 Lonesome Dove Dr
Christiansburg, VA 24073

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