[Dnsmasq-discuss] Request for brain-storm: Rogue dhcp-servers on the lan

Simon Kelley simon at thekelleys.org.uk
Thu Aug 21 15:03:46 BST 2008


Rune Kock wrote:
> I know this is a bit off topic, but maybe someone on the list has some
> thoughts on this:
> 
> I'm running a router for a group of people connected by lan.  And I
> use a dhcp-server (dnsmasq) on the router to configure the clients.
> 
> But increasingly often, someone has connected another router to the
> lan, usually to use it as a wifi access point.  And since they don't
> know what they are doing, they connect their own router's lan-port to
> the big lan instead of using the wan-port.  And so we get a wrong
> dhcp-server competing with dnsmasq.
> 
> Every time this happens, I have to track down the rogue router by
> testing each cable of the lan.  Quite time consuming, and until I get
> it done, the network is very unstable for the users.
> 
> Does anyone have some ideas as how to mitigate this problem?

Talking to the network guys of my aquaintance, it's not an easy problem
to fix unless you have enterprise-grade networking kit.

You could try something which broadcasts a DHCPDISCOVER packet, that
should give you replies from every DHCP server on the net, with their IP
addresses.

Cheers,

Simon.

> 
> 
> Rune
> 
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