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OpenSnitch is a GNU/Linux port of the Little Snitch application firewall.

The daemon is implemented in Go and needs to run as root in order to interact with the Netfilter packet queue, edit iptables rules and so on, in order to compile it you will need to install the protobuf-compiler, libpcap-dev and libnetfilter-queue-dev packages on your system, then just:

cd daemon
go build .

The user interface is a python script running as a gRPC server on a unix socket, to order to install its dependencies:

cd ui
pip install -r requirements.txt

You will also need to install the package python-pyqt5 for your system (if anyone finds a way to make this work from the requirements.txt file feel free to send a PR).

First, you need to decide in which folder opensnitch rules will be saved, it is suggested that you just:

mkdir -p ~/.opensnitch/rules

Now run the daemon:

sudo /path/to/daemon -ui-socket unix:///tmp/osui.sock -rules-path ~/.opensnitch/rules

And the UI service as your user:

python /path/to/ui/main.py --socket unix:///tmp/osui.sock

You can also use --socket "[::]:50051" to have the UI use TCP instead of a unix socket and run the daemon on another computer with -ui-socket "x.x.x.x:50051" (where x.x.x.x is the IP of the computer running the UI service).

Rules are stored as JSON files inside the -rule-path folder, in the simplest cast a rule looks like this:

{
   "created": "2018-04-07T14:13:27.903996051+02:00",
   "updated": "2018-04-07T14:13:27.904060088+02:00",
   "name": "deny-simple-www-google-analytics-l-google-com",
   "enabled": true,
   "action": "deny",
   "duration": "always",
   "operator": {
     "type": "simple",
     "operand": "dest.host",
     "data": "www-google-analytics.l.google.com"
   }
}

An example with a regular expression:

{
   "created": "2018-04-07T14:13:27.903996051+02:00",
   "updated": "2018-04-07T14:13:27.904060088+02:00",
   "name": "deny-any-google-analytics",
   "enabled": true,
   "action": "deny",
   "duration": "always",
   "operator": {
     "type": "regexp",
     "operand": "dest.host",
     "data": "(?i).*analytics.*\\.google\\.com"
   }
}

An example whitelisting a whole process:

{
   "created": "2018-04-07T15:00:48.156737519+02:00",
   "updated": "2018-04-07T15:00:48.156772601+02:00",
   "name": "allow-simple-opt-google-chrome-chrome",
   "enabled": true,
   "action": "allow",
   "duration": "always",
   "operator": {
     "type": "simple",
     "operand": "process.path",
     "data": "/opt/google/chrome/chrome"
   }
 }

I tried, but for very fast updates it failed bad on my configuration (failed bad = SIGSEGV), moreover I find Qt5 layout system superior and easier to use.

The UI service is able to use a TCP listener instead of a UNIX socket, that means the UI service itself can be executed on any operating system, while receiving messages from a single local daemon instance or multiple instances from remote computers in the network, therefore DBUS would have made the protocol and logic uselessly GNU/Linux specific.