Re: Block LAN DHCP broadcast

[Wakko Warner <wakko@animx.eu.org>]

Please keep list mail on the list.

Gnarlodious wrote:
> On 7/16/07, Wakko Warner wrote:
> >Gnarlodious wrote:
> >> I would need to have another
> >> device on the UPS battery. I want to have only two devices taking
> >> power from the UPS battery, the modem and the server.
> >
> >A diagram might be more helpful.
> http://etc.Gnarlodious.com/Images/Lan1.png

So you have a dsl connection with 3 computers and a wireless router
connected directly to it.  I would assume that you want to keep those 3
computers from getting an IP via DHCP from the dsl modem?  From the
networking perspective, unless the ups is actually networked, has nothing to
do with the network.  (Personally, I would assign a static IP to the UPS).

Does the DSL modem have a built in hub?

From the AP1 you have 2 PCs and another wireless router.  Are these getting
their IP from AP1?

From AP2 you have 2 PCs and nothing else.  Since I don't know the interfaces
on the AP devices, I don't know if they are routing traffic or switching
traffic.

I guess the real question is, does the 4 devices connected (according to
your diagram) directly to your DSL modem have non-private IPs?
(private IP ranges: http://tools.ietf.org/html/rfc1918 section 3).

> >DHCP is broadcast when one requests an IP.  That's why it's D(ynamic)HCP.
> OK, I'm starting to understand that what I want can't be done.
> 
> >The best way I can think of is to have a system (computer, router, whatever
> >running linux) with 2 bridged interfaces and block DHCP traffic going 
> >across
> >the bridge.
> Any page that explains how to set that up? I'm not a network pro...

The man page for ebtables and brctl.  You'll need a linux kernel with
bridging (802.1d support), ebtables enabled (Personaly, I just enable all
the netfilter modules and let the system decide at runtime which onces to
load), the drivers for 2 nics (I used 3c905b cards on a celeron 600 pc,
throughput is around 8-9mb/sec).

> And thanks for the hint about ebtables.

You could probably do it with iptables on a bridging interface, but ebtables
might be easier.

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