Debian: Cannot access opened port in US from EU

Question

On debian 9 I have opened port 0.0.0.0:18000 using python 3, but I cannot access it from outside (target machine actively refused it). I have no firewalls.

I suspected that it might be because I'm using IPv4 and the server is in US while I'm trying to access it from EU via IPv4. But I tried IPv6 too and it's the same error message. I can normally access it via SSH over IPv4. Ports 80/443 also have normal access over IPv4 but it translates it into IPv6.

Proto Recv-Q Send-Q Local Address           Foreign Address         State       PID/Program name
tcp6       0      0 :::18000                :::*                    LISTEN      701/python3
tcp6       0      0 :::22                   :::*                    LISTEN      552/sshd
tcp6       0      0 :::443                  :::*                    LISTEN      572/apache2
tcp6       0      0 MY_SERVER_IPv4:443      MY-PC-IPv4:10991      ESTABLISHED 61154/apache2

Interesting thing is that using putty I successfully connected using SSH over IPv4 (without translation).

tcp        0      0 MY_SERVER_IPv4:22       MY-PC-IPv4:17085      ESTABLISHED 57796/sshd: root@no

Does anybody have an idea what can I do?

Answer 1

As suggested in the comments, I checked with nmap:

nmap -p 18000 MY_SERVER_IP

and got:

STATE:
filtered

So I asked for control panel access and noticed that there is Firewall Policies of which I (because I'm inexperienced) wasn't aware of.

Problem solved. Thank you.