Suppose an individual were able to introduce a malicious DHCP server into your environment. That server could then wreak all sorts of havoc on your network. In the best case, it could simply offer every client an identical IP address, resulting in a network outage when all hosts believe they were leased the same address. In the worst case, the rogue server could set the default gateway to be the IP address of an attacker's proxy server. In such a scenario, an attacker could intercept all traffic leaving the host.
For this reason, it's important to ensure that you don't have rogue DHCP servers on your network. Microsoft's DHCP Server Location Utility is a great tool that allows you to search your network for active DHCP servers. You can also provide it with a list of authorized DHCP servers and configure it to alert you immediately if a rogue server is detected. Due to the risks described above, you should take action to immediately disconnect any rogue servers on your network.
This was first published in April 2007