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How to detect rogue DHCP servers, routers and NICs on a network

What are the best techniques for detecting rogue DHCP servers, routers and NICs on a network?

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Usually, performing a simple network scan will uncover many hidden applications and services you never knew were running. Scanning tools range from the venerable open source tool Nmap to high-end and expensive products for enterprise networks. However, the three situations you describe each present a unique issue which standard scanning equipment or Intrusion Detection Systems (IDS) cannot easily identify. Defending against each requires its own approach.

Let's first look at rogue DHCP servers. The crudest -- and most difficult -- approach would be to do a manual check for live DHCP servers using dhcping. This open source tool is a simple utility, like ping, except it tests for running DHCP servers. The results of a dhcping scan can be matched against a list of known DHCP servers on your network. Anything showing up in the scan, and not on your server inventory, should be suspect.

If you use Windows NT 4.0 and later, Windows 2000, XP and Server 2003, there is a command-line tool, Dhcploc, that shows all DHCP servers in your local subnet. While this tool isn't included in the Windows default package, it can be installed from the SupportTools folder of your installation CD.

Unix and Linux users, can use dhcp_probe, a free tool available from the Network Systems Group at Princeton University's Office of Information Technology (http://www.net.princeton.edu/software/dhcp_probe).

If you are looking for a cross-platform tool to monitor network traffic, Traffic Server 4.0 from InMon Corporation can be configured to detect rogue and legitimate DHCP servers.

After you've detected rouge DHCP servers on your campus network, as a preventive measure, I recommend following these two steps. First, increase the physical security of your network. This will block access to anyone who might install an unauthorized DHCP server again. Second, if your network uses Windows 2000 or Windows Server 2003, only include legitimate DHCP servers in Active Directory. This way, any unauthorized DHCP server that attempts to access your network will be denied.

As for rogue routers, if you mean Wireless Access Points (WAP), there are two free tools you can use to scan your network: Netstumbler for Windows and Kismet for both Windows and UNIX environments. Tracking down illegitimate WAPs on campuses can be difficult. WAP sniffing tools need to be close to their target to detect them and therefore can't be managed from a central location. That's how the war driving technique received its name -- from driving around with a laptop loaded with a sniffer for detecting wireless networks.

Rogue NICs are a bit trickier since they can't be detected by traditional scanning technology. However, you'll want to block them because rogue NICs can be indicative of a workstation or server being used by someone scanning your network for open ports to attack. There are two tools that can help detect rogue NICs. Sentinel, for Linux and BSD systems, is a free download from Packetfactory (http://www.packetfactory.net/Projects/sentinel). Windows users can use Microsoft Promqry 1.0, the command-line tool, and its GUI equivalent, PromqryUI 1.0 (http://support.microsoft.com/?kbid=892853).

This was first published in February 2006

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