Q:

Unable to open port

I try to convince apache to listen to two ports (80 and 8091). That works perfectly locally. 
I configured iptables so that port 8091 is shown as follows using iptables -L
Chain INPUT (policy ACCEPT)
target     prot opt source               destination         
ACCEPT     tcp  --  anywhere             anywhere             state NEW tcp dpt:8091

and expected that I can reach the port/web page from outside, which does not work.
When I use nmap -p 80,8091 -v -sV with the external ip-address from the server itself (locally) it looks as expected
PORT     STATE SERVICE VERSION
80/tcp   open  http    Apache httpd 2.4.29 ((Ubuntu))
8091/tcp open  http    Apache httpd 2.4.29 ((Ubuntu))

When I do the same call from a remote machine nmap returns
PORT     STATE    SERVICE VERSION
80/tcp   open     http    Apache httpd 2.4.29 ((Ubuntu))
8091/tcp filtered jamlink

and I cannot reach the web page on 8091, which as mentioned above is perfectly reachable locally using the external address. Has anyone any idea what might be wrong here.

A:

In some VPS providers, there are extra firewalls in play to protect the systems.
This is the case with Google Cloud, Azure Cloud, VMware Cloud, DigitalOcean, AWS, and others.
Make sure that there's no additional firewall in play, and if there is add the port into the allowed list on the firewall.

