Here's how you can download the DroidWall app free for Android devices.
1. Go to the Google Play page for the free app DroidWall.
2. Tap the Install button.
Here's a description of DroidWall:
"Front-end application for the powerful iptables Linux firewall.
Allows you to restrict which apps can access the network.
This is the perfect solution if you don't have an unlimited data plan, or just wants to see your battery lasting longer.
For advanced users, it also allows you to manually define custom iptables rules."