Serving multiple subnets

Todd Snyder tsnyder at
Wed Sep 12 14:33:33 UTC 2007

I'm sorry - I'm relatively new to advanced DHCP stuff, and yes, I could
very well be mixing terminology :)  I've spent too much time reading man
pages the last 2 days trying to figure this out and the words are a
jumble now :)

As for the function, it sounds like DHCPd does exactly what I want it to
do out of the box.

The only instance where the builtin automagic pool selection by relay
address function would break down, as I see it, is if I am given the
same address range in two different cores.  If that comes up (and
hopefully it won't), I'll have to dig deeper.

Thank you to you, and to Glenn, for your quick answers.  I've already
got it configured and running now, and me boss is right pleased.



-----Original Message-----
From: dhcp-users-bounce at [mailto:dhcp-users-bounce at] On
Behalf Of Bruce Hudson
Sent: Wednesday, September 12, 2007 10:27 AM
To: dhcp-users at
Subject: Re: Serving multiple subnets

> I am running a DHCP server in a particular segment.  On that segment, 
> I need to serve a small DHCP pool.
> Connecting to this server are also relays (Cisco) in various other 
> places.
> Each of these other places has a different pool of addresses assigned 
> to them.
> So far, I have been unable to find a way to say "if you come from this

> relay, you are this group, if you come from that relay, you are that 
> group" etc.

    Everything was easy up to this point but you have suddently switched
from talking about different pools to different groups. Perhaps you mean
the same thing but it is not entirely clear.

> When I look at the packets, I see there is a relay server ip address 
> passed - is there a way to match based on that, or only on proper 
> options?

    The DHCP server uses the relay address to determine what part of the
network, as described by the configuration file, a request came from;
and hence what addresses can properly be given out to satisfy that
This is extremely basic to how the server works. It happens without any
extra configuration required. If you have two subnets defined, each with
a pool of addresses, the server will assign an address from the correct
pool. Any options defined for the pool or the subnet will be assigned to
the client lease.

    Perhaps your requirements are more subtle than I read from your
message. It is possible to assign the client to a group based on the
relay address in the packet but very rarely would you need to.
Bruce A. Hudson				| Bruce.Hudson at Dal.CA
UCIS, Networks and Systems		|
Dalhousie University			|
Halifax, Nova Scotia, Canada		| (902) 494-3405

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