I'm not sure why you'd want to build the two systems in isolation. As you mentioned in your question, the difference between IPS and IDS systems is slight: They are extremely closely related and tightly integrated. In fact, there are few IDS systems on the market today that do not have some IPS capability. An IPS is simply an IDS deployed on the network in a fashion that allows it to block traffic.
The general IPS and IDS deployment strategy I recommend is to first deploy this type of device in pure IDS mode, without blocking any traffic. Then, after evaluating the device's performance on the network, slowly change the rules producing the best results (e.g. low false positive rates) from IDS rules that simply produce alerts, to IPS rules that block the offending traffic. Not long ago I wrote a tip on IPS best practices covering some other deployment strategies, including:
- Running the IPS in "monitor" mode until it's clear the system is properly tuned.
- Keeping the number of "block" mode rules to a small, finely tuned set.
- Considering a fail-open configuration so that if the device fails, it doesn't prevent the flow of network traffic.
Good luck with your deployment!
For more information:
- Network intrusion prevention systems: Should enterprises deploy now? Read more.
- Find out more about the difference between intrusion detection and intrusion prevention.
This was first published in May 2009