A simple way to block access to the internet per app
AFWall+ (Android Firewall +) - iptables based firewall for Android
Android application for online privacy and security
TrackerControl Android: monitor and control trackers and ads
Manage port forwardings via UPnP
A java PCAP and DPI library
Bypass your ISP's firewalls and connect to the internet!
Jateway is a general purpose TCP/IP connection proxy
Personal firewall for linux desktop and android phone
Lomalarga is a VPN-like service based on SSH and Linux.