UPDATED: Serving addresses to multiple subnets

[Glenn Satchell]

Hi John

As you have two subnets operating in the one interface you need to
group them using a shared-network statement. This is because the
requests from the dhcp relay have the router's ip address in them as
the gateway. dhcpd uses this to determine the subnet.

A shared-network defines more than one subnet on the same interface.

shared-network "foo" {
   subnet 10.1.200.0 netmask 255.255.254.0 {
      ...
   }
   subnet 10.1.100.0 netmask 255.255.252.0 {
      ...
   }
}

There is no special configuration for remote networks, dhcpd decodes
this automatically for you.

BTW there was no such version 2.0.3, dhcpd --version will give the
version, but that should not matter as the shared-network has been in
for a long time.

regards,
-glenn
--
Glenn Satchell   mailto:glenn.satchell at uniq.com.au | Miss 9: What do you
Uniq Advances Pty Ltd       http://www.uniq.com.au | do at work Dad?
PO Box 70 Paddington NSW Australia 2021            | Miss 6: He just
tel:0409-458-580     fax:02-9380-6416              | types random stuff.

>Date: Thu, 8 Oct 2009 09:08:12 -0700
>From: "John Tabasz (jtabasz)" <jtabasz at cisco.com>
>
> I'm sorry I neglected to include many important points in my query.
> More details here:
> 
> The DHCP server is ISC 2.0.3 running on Solaris 2.8 on a Sun V120. The DHCP 
server has a single live interface on the subnet 10.1.100.0/24. 
> 
> The router interface to which this subnet is connected is part of a 8 port 
switch module installed in a Cisco 3725 and has a secondary subnet defined as 
such:
> 
> interface Vlan1
>  ip address 10.1.200.2 255.255.252.0 secondary
>  ip address 10.1.100.2 255.255.252.0
>  ip helper-address 10.1.100.23 ß------ This is the ip address of the dhcp 
server.
>  standby ip 10.1.100.1
>  standby ip 10.1.200.1 secondary
>  standby priority 240
>  standby preempt
>  standby track FastEthernet0/0
> 
> My dhcpd.conf file (/etc/dhcpd.conf) looks like this:
> 
> subnet 10.1.200.0 netmask 255.255.254.0 {
>   option routers 10.1.200.1;
>   option subnet-mask 255.255.254.0;
>   option domain-name-servers 192.168.199.23,192.168.201.23;
>   option domain-name "mydomain.com";
>   deny unknown-clients;
> }
> 
> subnet 10.1.100.0 netmask 255.255.252.0 {
>   option routers 10.1.100.1;
>   option subnet-mask 255.255.252.0;
>   option domain-name-servers 192.168.199.23,192.168.201.23;
>   option domain-name "mydomain.com";
>   deny unknown-clients;
> }
> 
> host fsr7.mydomain.com                   {
>    hardware ethernet              0:3:ba:1f:be:f1;
>    fixed-address                    10.1.200.38;
>    option host-name                "fsr7";
> }
> 
> host gsr7a.mydomain.com                   {
>    hardware ethernet              0:3:ba:3a:73:c4;
>    fixed-address                    10.1.100.131;
>    option host-name                 "gsr7a";
> }
> 
> The symptom is that hosts on the 10.1.100.0/22 subnet, which is the
> primary subnet on the router interface, are getting IP addresses while
> the hosts on the 10.1.200.0/23 subnet are not.
> 
> Snoop run from the DHCP server tells me that the DHCP request packets
> are hitting the server. Snoop run on the server looking for outbound
> packets destined for the MAC address of the host not getting addresses
> shows no outbound traffic to that address.
> 
> I have tried configuring the router interface with the helper address
> of the DHCP server with no change in results.
> 
> Is there some configuration change that needs to be made to tell the
> DHCP server to reply to requests from a subnet that it is not directly
> connected to? I have subnet declarations in my config file for both
> subnets.
> 
> Thanks,
> John
>