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

Lulu GO lulumailgo at
Wed Mar 17 19:00:56 UTC 2010

Hello Simon, thanks for answering. Your are right about the network, that is
how it works, but I can't change any of that, I have to solve the problem of
deciding wich server is better to associate with but I can't add equipment
such as routers or changing too much the way the network was planned to
work. This is a project from my university that was given to me but the
decisions made of how the schools were going to operate was made many time
I agree with you about the broadcast traffic but I think they thought of
that and bought mikrotik to configure them like routers when they think is
begining to seem necesary.

I was hoping to figure out wich AP the client connect to measuring (with
something like arping or rarping) latency to the server from every AP and
compare it with latency to the laptop's MAC (the MAC I will know it from de
dhcp discover packet).

Then comparing many data from snmp consults I will decide wich server is
optimum, that condition is the one both servers must check before
I couldn't understand from your reply if the classes you define are in the
dhcpd.conf, and if that the case I think I can use it for the purpose I
need, thanks! I will try it and let you know.

The thing about blocking the clients from the other school or the outdoor
clients is, that is not the solution we were asked to do, in fact they want
the clients to associate to the optimum server despite where they are.


2010/3/16 Simon Hobson <dhcp1 at>

> 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>;
>  <options>;
> }
> class "school-b" {
>  match if <some condition>;
>  <options>;
> }
> <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
> WANTED: "Software CD ROM Kit" for Canon CLBP 360-PS printer (Canon part no
> RH6-3612, or possibly RH6-3810, or RH6-3610 might do). I've a dead HD and
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> This is NOT the same thing as the printer drivers to load on the computer -
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> Canon drew a blank, it's been out of support for years.
> Alternatively, if anyone has one of these and would let me image their hard
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