Flying Buttress has been discontinued. Similar apps include Little Snitch. It's paid, but Karma Firewall is free. Other options are Netmute, Little Snitch Mini, EasyFirewall and Radio Silence. In total there are seven Firewalls like Flying Buttress for Mac, Windows, Linux and Android.
Firewall application for monitoring and controlling outbound network connections, notifying users of attempted access, supporting rule-based blocking, real-time alerts on suspicious activity, and preventing unauthorized data transmission on macOS systems.
Austria
EU
As soon as you’re connected to the Internet, your apps can send whatever they want to wherever they want, unnoticed by you, because all of this happens invisibly under the hood.
AustriaEU
Perfects the Windows Firewall offering additional security. Get the most out of your Windows firewall, create new security rules for your online applications, alerts you to anomalies with real-time protection.
GermanyEU
Karma Firewall app lets you take control back of your phone and block internet traffic to and from specific apps. This is a simple yet powerful firewall with an easy to use intuitive GUI which allows you to protect your sensitive data.
Radio Silence lets you block individual apps from accessing the internet. It never throws pop-ups on the screen, and is completely invisible.
FinlandEU
Italy
