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Re: IP Telephony pen-test and VLAN's
From: Jason Ostrom <justiceguy () pobox com>
Date: Wed, 17 May 2006 11:59:10 -0500

Frank,

Unless the access switch uses 802.1x port security, there is no negotiation via CDP for allowing your PC access to the voice VLAN. What you need to do is configure your PC to add a 802.1q VLAN tag to your ethernet frame, use the Voice VLAN value, and then your PC will broadcast a DHCP request with the correct VLAN tag. The access switch will allow, and your machine will obtain a DHCP address on the voice VLAN, etc.

I do not know of a tool to do this in Windows, but in Linux, it is trivial:
http://linux-ip.net/html/ether-vlan.html

For injecting ARP MITM and using Cain & Abel, you might have a local configuration issue. In my testing, you didn't need to Hop into the Voice VLAN in order to sniff voice traffic with Cain & Abel. Is your PC terminated ethernet into the back of the IP Phone, or into the wall?

By the way, there was a security notice discovered by Jake Reynolds with Fishnet Security regarding bypassing 802.1x controls and gaining access to the voice VLAN via spoofing CDP. Somewhat similar to your situation, but it should be a non-issue.

http://www.fishnetsecurity.com/csirt/disclosure/cisco/Cisco+802.1x+Advisory.aspx

All the best,
Jason Ostrom

Frank Bussink wrote:
Hello,

I'm trying to do a pen-test on a Cisco IPT system.

Cisco IP telephone are connected on the same network cable as the PC, but
are working on another vlan.
The access switches have a configuration like :
switchport mode access
switchport access vlan 123
switchport voice vlan 124

I think the negotation with the access switch is done by CDP but I'm not sure.

Does anyone have a tool that emulates the PC as a voice device in order to receive a dhcp lease on vlan 124, so I can start to inject ARP MITM with Cain for example ?

Kind regards
Frank

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