[DHCP] Re: Multiple subnets, wired/wireless

Glenn Satchell Glenn.Satchell at uniq.com.au
Thu Oct 1 16:26:46 UTC 2009

>Date: Thu, 01 Oct 2009 09:46:02 -0600
>From: "Ashley M. Kirchner" <ashley at pcraft.com>
>To: Users of ISC DHCP <dhcp-users at lists.isc.org>
>Subject: Re: [DHCP] Re: Multiple subnets, wired/wireless
>X-BeenThere: dhcp-users at lists.isc.org
>    First of all, my apologies for in a way "dropping the ball" on this 
>one.  I posted my original question, received replies and simply got too 
>busy with other projects and didn't reply back to this thread.  Thanks 
>to both Niall and Alan for their replies as well.
>Glenn Satchell wrote:
>> First up you need to connect your dhcp server to the second network.
>> You can do this directly through a second NIC, or via the first network
>> through a router.
>    It will be through a third NIC, though whether it's second, third, 
>or nth shouldn't make any difference.

That's correct.

>> Secondly you configure the subnet declaration in dhcpd.conf. This looks
>> exactly the same whether it is a local subnet (like your first one) or
>> a remote subnet using a DHCP relay.
>> There is an example in the dhcp.conf man page, but essentially it will
>> look something like this:
>> # global options (eg dns servers, domain name and so on)
>> option domain-name "example.com.au";
>> option domain-name-servers server1.example.com.au, server2.example.com.au;
>> default-lease-time 14400;
>> # wired subnet
>> subnet netmask {
>> 	option routers;
>> 	range;
>> }
>> # wireless subnet
>> subnet netmask {
>> 	option routers;
>> 	range;
>> 	default-lease-time 3600;
>> }
>    This is what I wasn't entirely sure about, mainly because we have 
>several options set on our current subnet and I didn't quite understand 
>how to separate them from this new subnet.  Somewhere I saw mention of a 
>"shared-network" option and things started getting fuzzy.  So I 
>appreciate your example above as that made things a lot clearer.
>    There are still bits and pieces I have to figure out here.  For 
>example, we allow our internal firewall to update the external DNS and 
>thus have lines like:

I'd put these in the global configuration space:

>option ip-forwarding off;
>ddns-update-style interim;
>ddns-domainname "int.pcraft.com";

And these in the subnet space for the wired subnet

>ddns-updates on;
>do-forward-updates on;
>allow client-updates;

For the wireless subnet, if you don't want dns entries then disable
them in this subnet using

ddns-updates off;

>    There's a DHCP_UPDATER key being used for the updates.  How will 
>this affect a second subnet?  Do I have to move these lines inside of 
>the subnet{} statement?  Will it even work?  I don't know till I try it 
>I guess.

Leave all the DHCP_UPDATER, zone definitions, etc, in the global conf space.


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