How can work be done as a firewall in the Nextcloud? Is there an app for this?
Can you explain your issue? Do you want to use a firewall? I think you must implement a firewall on your operating system. But mostly it is not really useful. Please do not install a firewall.
For more security of your Nextcloud read Hardening and security guidance. The risk of Nextcloud is the webservice you must secure. A firewall must always let the webservice through (port 80/443). The firewall can not really help you.
Please describe in more detail what your actual goal is. And why do you add random tags to your post that seem to have nothing to do with your actual question?
My answer to the question would be: “No it can’t be done, Nextcloud cannot act as a firewall.” But I’m not sure if this is what you actually meant, and I am not in the mood to play guessing games. 
Nextcloud is not a firewall.
If you need a firewall, I have two Nextcloud instances running behind OPNsense. It works well. You might have a look at that. It also has a plugin to use Nextcloud to back up its configuration.
- Firewall monitors incoming and outgoing network traffic and decides whether to allow or block specific traffic based on a defined set of security rules.
- Nextcloud is a software appliance for file sharing, which would normally reside within the firewall. Anyone using it is already past the firewall.
Think of it like your front door. Anyone eating dip in your kitchen has already made it through the front door.
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