dhcp config - 2 subnets on one phys net

Igor Antokhin igor at sai.msu.ru
Fri Sep 29 09:50:21 UTC 2006

On Fri, 29 Sep 2006, Simon Hobson wrote:

> Igor Antokhin wrote:
>> Sorry for a question from a newbie, but I could not find a clear answer to
>> mu problem - only pieces of the puzzle...
>> I have a computer on a physical network It runs Linux
>> Fedora Core 5. It is just a client machine not providing any general
>> use services like dns printing etc. Now for some reasons I want to create
>> a private network for my department using the same physical ethernet.
>> I understand that what I have to do is this:
> The problem is that you CANNOT provide DHCP to only one of the
> subnets - at least not without co-ordination with the admin of the
> existing DHCP.
> The problem is that even though you can apparently run two interfaces
> (one 'real', one 'virtual'), DHCP relies on broadcasts which do not
> respect the distinction between the subnets/interfaces.

I know that and I am willing to coordinate my efforts with the sysadmins.

> To make it work you will have to be able to identify every client
> that will be part of your private network (MAC address is usually
> easiest). On your new dhcp server you will have to service ONLY those
> clients and ignore all others.

Right, this is what I do.

> The admin of the existing dhcp server
> will have to explicitly ignore your clients.

Hmm, here I am not quite sure. All my clients are on the private network I thought their broadcast requests would not be transferred 
to the external network (if I do not explicitely relay them). As for the 
other clients on the main network, there is not need to worry - my server 
will never serve their requests so they will always be servised by another 
dhcp server. The problem you mention would potentially affect my clients 
only - has to be solved, of course...

> If you don't do this
> then you will have two dhcp servers fighting each other - yours will
> offer a private address, the other will offer a public address, and
> whichever the client decides to accept, one or other dhcp server will
> reply with a NAK.

So if I am correct above there should be no fight...

> Unless you specifically want to play with the dhcp, you might be
> better talking with the network admin and see if he can just set up
> the dhcp for your private network on the existing server - be a whole
> lot less effort overall.

That's the whole problem. Unfortunately our local administration is not 
very reliable and professional :(. Right now the main dhcp server is not 
working (for two days already) and nobody seem to care. Actually nobody 
of those two people who are responsible are at work... My clients 
complain... So I just forsed to provide an independent service. As I said 
in my first message my current problem is that I am not sure how I setup 
my computer as a router and how to setup NAT.

Thanks for your help.


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