Hi @auggi3 and welcome to the Manjaro community
Uncomplicated Firewall (UFW) will help achieve your goal.
See the Manjaro Wiki Firewalls article for information on how to install and enable UFW. Setting UFW to the Home profile will deny all incoming traffic, while allowing all outgoing traffic. From that point, you will need to actively allow only traffic you specifically want to let in.
More information about UFW may also be found in the Arch Wiki Uncomplicated Firewall article.
UFW is one of the simpler solutions, however, requires effort on your part to learn the rules syntax and understand how firewalls work in Linux.
Here are some additional resources:
In addition to UFW, there is OpenSnitch which aims to replicate the functionality of the macOS LittleSnitch app. OpenSnitch will allow more fine-grained control over which applications may (or may not) send outgoing connections. You might find this an easier solution to limit outgoing destinations.
As you are new to the community, I also suggest the following links/tutorials which will help use the forum effectively.
- [How to] Request Support
- [How to] Find System Information
- [How to] Find Error Logs
- [How to] Post Command Output and File Content as Formatted Text
- [How to] Post Screenshots and Links
I hope this is helpful. Cheers.