DHCP Relay & Option 82
Simon Hobson
dhcp1 at thehobsons.co.uk
Wed Aug 25 15:29:17 UTC 2010
Bruce Hudson wrote:
> Assuming I understand what you are trying to do, you need to enclose
>the two IP sunets in one "shared-network" stanza. That tells the server
>that it is okay to use the 192.168.1.0/24 addresses to satisfy requests
>from a 10.0.0.0/8 relay.
>
> shared-network "Tower1" {
> subnet 192.168.1.0 netmask 255.255.255.0 {
> ...
> }
>
> subnet 10.0.0.0 255.0.0.0 {
> }
> }
>
>This will work for the example cited. If your network is still flat, you
>really want a single "shared-subnet" for the entire campus that surrounds
>all of your subnet stanzas.
But I think what he is actually trying to do is convert FROM a flat
network TO a routed network.
So to answer the original question :
Each separate network MUST have a distinct and non-overlapping IP subnet.
dhcpd MUST be configured to know about all these subnets, and if it
is to be able to issue a lease to clients located there, then the
config must also allocate a range of addresses and/or some host
statements with fixed-address declarations.
Note that shared-network is generally NOT required, and if misused
will cause dhcp to "not work properly".
Note also that you do NOT have to use option 82 for this to work, you
only have to configure a relay agent in each network that doesn't
connect directly to the server AND correctly configure the server.
Eg, consider this minimal routed network :
dhcp -- 192.168.1.0/24 -- router/agent -- 192.168.2.0/24 -- client
A DHCP broadcast by the client will be detected by the agent. The
agent will insert it's local address in the 192.168.2.0 subnet into
the Gateway-address field of the packet and forward the packet to the
server. The server will spot the presence of the gateway-address and
determine from it that the client is in the 192.168.2.0 network - and
allocate an address that is appropriate.
The server will then reply to the relay agent, which in turn will
broadcast the reply to the client.
This all happens automagically.
The (somewhat abbreviated) dhcp config would be along the lines of :
<global options>
subnet 192.168.1.0 ... {
<subnet options>
range 192.168.1.10 192.168.1.100 ;
}
subnet 192.168.2.0 ... {
<subnet options>
range 192.168.2.10 192.168.2.100 ;
}
Note that teh two subnets are **NOT** a shared network.
--
Simon Hobson
Visit http://www.magpiesnestpublishing.co.uk/ for books by acclaimed
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