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I am toying around with my Raspberry Pi. Today I set up a container running Etherpad Lite and I would like to make it accessible through the internet. Accessing it via local/internal IP address on its port on our home network works.

Also on this Raspberry Pi I have a running Apache which serves at port 80 and can be access via domain name using dynamic DNS (Two-DNS).

Using the internal IP address, I can access both by specifying the correct port. But using the domain name, I can only reach Apache at port 80. Shouldn't I be able to connect to the other port as well? Is this a problem with docker? Or is there something I overlooked with DynDNS?

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It sounds more like your router/firewall is blocking the second port.

Because you're trying to access your server via the domain name, you're technically attempting to visit your site by going through your router out to the Internet, and then being re routed back to your internal network. As a result, your router/firewall is probably blocking the secondary port from entering your network.

Client -> DNS resolved to external IP -> request sent to router/default gateway -> router forwards to external side of firewall -> router realizes that the request is hosted on internal side of router -> your router/firewall blocks access

You'll most likely need to forward the secondary port in your router config.

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    Yes, you are right! Thank you very much for the explanation! I forgot that I did this for port 80 as well. – anjuta Jan 3 at 19:13

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