Raspberry pi how to expose a port when a docker running a web server

Question

I have the following configuration in Raspberry Pi for iptables

            pi@raspberrypi:~ $ sudo iptables --list
            Chain INPUT (policy ACCEPT)
            target     prot opt source               destination
            ACCEPT     all  --  anywhere             anywhere
            REJECT     all  --  anywhere             loopback/8           reject-with icmp-port-unreachable
            ACCEPT     all  --  anywhere             anywhere             state RELATED,ESTABLISHED
            ACCEPT     tcp  --  anywhere             anywhere             tcp dpt:http
            ACCEPT     tcp  --  anywhere             anywhere             tcp dpt:https
            ACCEPT     tcp  --  anywhere             anywhere             tcp dpt:http-alt
            ACCEPT     tcp  --  anywhere             anywhere             tcp dpt:8181
            ACCEPT     tcp  --  anywhere             anywhere             tcp dpt:8182
            ACCEPT     tcp  --  anywhere             anywhere             state NEW tcp dpt:ssh
            ACCEPT     icmp --  anywhere             anywhere
            LOG        all  --  anywhere             anywhere             limit: avg 5/min burst 5 LOG level debug prefix "iptables denied: "
            DROP       all  --  anywhere             anywhere

            Chain FORWARD (policy DROP)
            target     prot opt source               destination
            DOCKER-USER  all  --  anywhere             anywhere
            DOCKER-ISOLATION  all  --  anywhere             anywhere
            ACCEPT     all  --  anywhere             anywhere             ctstate RELATED,ESTABLISHED
            DOCKER     all  --  anywhere             anywhere
            ACCEPT     all  --  anywhere             anywhere
            ACCEPT     all  --  anywhere             anywhere
            DROP       all  --  anywhere             anywhere

            Chain OUTPUT (policy ACCEPT)
            target     prot opt source               destination
            ACCEPT     all  --  anywhere             anywhere

            Chain DOCKER (1 references)
            target     prot opt source               destination
            ACCEPT     tcp  --  anywhere             172.17.0.2           tcp dpt:8181

            Chain DOCKER-ISOLATION (1 references)
            target     prot opt source               destination
            RETURN     all  --  anywhere             anywhere

            Chain DOCKER-USER (1 references)
            target     prot opt source               destination
            RETURN     all  --  anywhere             anywhere

and I have exposed port 8181 in Docker file. This device has a static IP of 192.168.100.20 and I can ping in to it from my host machine.

This is the docker file configuration

            FROM myapp

            COPY . /app
            RUN pip install -e /app
            WORKDIR /app/node

            EXPOSE 8181

            ENTRYPOINT [ "myapp", "run" ]

I am running as docker run -p 8181:8181 <image_id>

But when I tried to access it by its IP, it says address not found and when I do a port scan, I'm getting the following result

            Discovered closed port 8181/tcp on 192.168.100.20
            8181/tcp  closed   intermapper          conn-refused

What prevents accessing it from outside the Raspberry Pi device ?

Answer 1

What does 'myapp' do ?

Are you sure it does listen on port 8181 of the container's external interface (or on all interfaces) ?

Are you sure 'myapp' starts correctly ?

Does

$ docker logs <container id>

give you any error or debug message ?

If your images contains netstat, the output of

$ docker exec <container id> netstat -lnp

would show if anything in the container listens on the public port 8181.

To debug such situations I'd recommend to start your image in interactive mode:

$ docker run -it --rm -p 8181:8181 <image_id> /bin/bash

and then start your app manually inside the container with

$ myapp run

ideally adding some flags to add more verbosity to your app if your app supports it.

It doesn't seem to be a raspberry issue but a configuration of your image.