RE: Can an ethernet bridge be detected ?

[Francois Labreque <flabreq () ca ibm com>]

listbounce () securityfocus com a écrit sur 2011-07-15 19:47:42 :

> De : ijk1137 <ijk1137 () gmail com>
> A : security-basics () securityfocus com
> Date : 2011-07-18 11:55
> Objet : Can an ethernet bridge be detected ?
> Envoyé par : listbounce () securityfocus com
>
> Hi,
>
> I'm wondering when I setup an ethernet bridge like this:
>
> .--------------.            .---------.            .------.
> |workstation pc|------------|bridge 
pc|------------|router|---------internet
> '--------------'        eth0'---------'eth1        '------'
>
> -using these commands on 'bridge pc':
> ||ifconfig eth0 0.0.0.0
> ||ifconfig eth1 0.0.0.0
> brctl addbr bridge0
> brctl addif eth0
> brctl addif eth1
> ifconfig bridge0 up
>
> Is there a way to detect this bridge? Is the 'bridge pc' totally 
> transparent / invisible from both sides?

By default, the bridge will send spanning-tree PDUs ever two seconds on 
each interface.  These packets will have the bridge's MAC as a source, so 
the workstation (or the router) running in promiscuous mode will detect 
that the bridge is there.

Also, the router on your diagram may be set to automatically shutdown 
ports upon reception of BPDUs to prevent the insertion of rogue switches 
(which are just multiport bridges)



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