In Mike’s post this morning on network security he made the outlandish suggestion that rather than trying to fix your firewall rules, you could just block everything and wait for the calls to figure out what really needs to be open.

I made the exact same recommendation at the SANS data security event I was at earlier this week, albeit about blocking access to files with sensitive content.

I call this “management by complaint”, and it’s a pretty darn effective tactic. Many times in security we’re called in to fix something after the fact, or in the position of trying to clean up something that’s gotten messy over time. Nothing wrong with that – my outbound firewall rules set on my Mac (Little Snitch) are loaded with stuff that’s built up since I set up this system – including many out of date permissions for stale applications.

It can take a lot less time to turn everything off, then turn things back on as they are needed. For example, I once talked with a healthcare organization in the midst of a content discovery project. The slowest step was identifying the various owners of the data, then determining if it was needed. If it isn’t known to be part of a critical business process, they could just quarantine the data and leave a note (file) with a phone number.

There are four steps:

  1. Identify known rules you absolutely need to keep, e.g., outbound port 80, or an application’s access to its supporting database.
  2. Turn off everything else.
  3. Sit by the phone. Wait for the calls.
  4. As requests come in, evaluate them and turn things back on.

This only works if you have the right management support (otherwise, I hope you have a hell of a resume, ‘cause you won’t be there long). You also need the right granularity so this makes a difference. For example, one organization would create web filtering exemptions by completely disabling filtering for the users – rather than allowing what they needed.

Think about it – this is exactly how we go about debugging (especially when hardware hacking). Turn everything off to reduce the noise, then turn things on one by one until you figure out what’s going on. Works way better than trying to follow all the wires while leaving all the functionality in place.

Just make sure you have a lot of phone lines. And don’t duck up anything critical, even if you do have management approval. And for a big project, make sure someone is around off-hours for the first week or so… just in case.