Well, not a single answer for such a long period, but I'm still trying to solve this problem. Here's what I've come up with so far. Firstly, on the server side, using ufw, I blocked all incoming connections, except for ssh, http, htps, and API_PORT for manager and ACCESS_KEY_PORT client. Secondly, I completely blocked all outgoing connections so that the outline client connects to a server isolated from the outside world. However, at this stage, I found strange errors - the client could not connect to the server, saying that it was necessary to contact the server administrator (which I am). After looking at the network logs, I saw that the connection is working, however, several requests to external addresses do not go through and therefore the client cannot work as it should. Where is the server trying to reach after the client connects and why? What specific ports does the server need for normal operation? Why can't the client connect to an accessible local machine that is isolated from the external Internet?