how to ask a dhcp server to respond a petition only if a condition happens

Simon Hobson dhcp1 at
Tue Mar 16 08:08:37 UTC 2010

Lulu GO wrote:

>My problem is this, I have two dhcp server (dhcp3) in the same 
>network, this network is a link layer network so every host is seen 
>as directly connected. The two servers have debian lenny and there 
>are near 13 AP mikrotik to give connection to the XO laptops (from 
>the project one laptop per child). This is for two public schools 
>(each one with one server) that are connected via a p2p connection 
>and each one provides internet for the school and outdoor too. The 
>servers are also file servers, proxy, etc.

Let me get this clear - you have two schools, each has a network and 
a DHCP server, and the networks are linked so they appear as one big 
network with just one broadcast domain ?

The first thing that comes to mind is ... yuck ! You'll have loads of 
traffic going across the link no matter what you do.

What you will need to do is figure out if there is something unique 
about the access points that will allow you to device a matching 
statement. there has to be something in the received packet that 
differentiates the access point to which the client has connected.

Once you have that, then you can build class statements :

class "school-a" {
   match if <some condition>;
class "school-b" {
   match if <some condition>;

<options> could be things like "next-server" and so on to get the 
client to boot from it's local server.

You can also do things like :

pool {
   range ....;
   deny members of "school-b";
so that the DHCP server in school A won't even respond to the clients 
in school B - but to be honest, it's probably not worth it as the 
DHCP traffic is just so tiny compared to all the other broadcast 
stuff you'll be wasting your bandwidth on.

Another option is to do some filtering at the network level. On one 
or both of the devices that link the two networks, configure a filter 
that will block the client DHCP traffic from going across the link to 
the other school. That will just make the distant DHCP server 
invisible to the client.

Simon Hobson

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This is NOT the same thing as the printer drivers to load on the 
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If anyone knows where I might get hold of one I'd be grateful - 
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Alternatively, if anyone has one of these and would let me image 
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