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My Goal

I have a web frontend application which communicates via a specific port (REST API) with a backend application (lets call it "My_App"). My_App needs to send Multicast frames to communicate with something in the outside world. And I need to put everything into Docker Containers.

What I tried (and failed)

First I tried to have nginx and the app inside a single Container using host networking for the whole Container. Like depicted in the following picture:

enter image description here

This works on my developement PC but does not on the target system due to some firewall rules preventing me from accessing Port 8081 and 12345 from outside the target system. If I log into the target system e.g. via ssh I can access those Ports just fine. So the situation on the target system is like that: enter image description here

What I am trying to achieve

So now I am trying to split the app and nginx into separate Containers (Port Mapping for bridged networks is allowed on the host system and I can access maped Ports from the outside).

What I want to to do is connect a Container A using Bridge Networking with another Container B using host networking, so that an Application (e.g. Nginx as reverse Proxy) inside Container A can send Requests to Container B on a specific Port. Something like shown in the following picture:

Container A with bridge Networking talking to another Container with Host networking

But now I do not know how to achieve this. I know that I can communicate between Containers if both are using bridge Networking by using Dockers DNS and the name of each Container. But how to do this if one Container uses host networking, bypassing Dockers DNS?

Here is my current docker-compose file:

version: "2"
services:
  Container_B:
    image: my_app
    network_mode: host
  
  Container_A:
    image: nginx:alpine
    ports: 
      - 80:8081
      - 12342:12342  // apparently does not work because in use by Container_B
    depends_on:
      - "Container_B"

The actual question

Is it possible to access a Port, inside a Container_B using host networking, from within a Container_A using Bridge networking? Without some external application doing the routing?

A.K.
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1 Answers1

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If you are on Mac or Windows, you can use host.docker.internal to connect to containers in network_mode: 'host'.
For linux, this feature is not available yet. It will be available in the new release of docker. Till then, you can use a docker image dockerhost (docker pull qoomon/docker-host) and connect to containers in host mode using dockerhost:{port} method.

add this in your docker-compose.yml file:

dockerhost:
      image: qoomon/docker-host
      cap_add:
        - NET_ADMIN
        - NET_RAW
      restart: on-failure
      networks:
        - back_end

Container_A should also be in the same network as dockerhost.
After this, you can connect to containers from within bridge network to containers in host mode using dockerhost:{port} where port is where container in host is listening to.

thecodeboxed
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