connect from inside a docker container to IP address on host network

Question

Question: (the short version)

When using Docker for application development, how do I configure a Docker-compose file to let me access the IP address of a server on the host network from inside the docker container? I don't want to access the host itself -- but instead want to access another server on the same network as the host. I also need to have various docker containers be able to talk to each other.

What is the cleanest way to allow access to host-network IPv4 resources from inside of a docker container and still allow inter-container communication?

All the background and details are below (aka the long version).

---

Setup:

I'm writing an application that polls a Modbus server to collect measured data at a given interval. The Modbus server is at 192.168.1.50 port 502 on my local network. I can successfully ping that server from my computer.

Here's my development approach:

• application is inside a docker container
• developing in a Mac environment
• using Django-3.1/postgres to build the UI and store the data (for now)
• using the pymodbus library to manage communication between the application and the remote Modbus server

Here's the Docker-compose file that creates the application environment:

version: '3.8'

services:

  db:
    container_name: db
    image: postgres:10.7
    environment:
      POSTGRES_PASSWORD: pwd
      POSTGRES_DB: app_db
    volumes:
      - ./data/postgres:/var/lib/postgresql/data
    ports:
      - 5432:5432

  web:
    container_name: web
    build:
      context: ../.
      dockerfile: ./_app/dev/Dockerfile
    volumes:
    - ../.:/app
    ports:
    - 8000:8000
    depends_on:
      - db
    env_file:
      - app.env
    command: ./manage.py runserver 0.0.0.0:8000

And here's the Dockerfile:

FROM python:3.8.5-buster

RUN mkdir /app
WORKDIR /app

RUN apt-get update -y
RUN apt-get install inetutils-traceroute -y

COPY ./_app/dev/python_reqs.txt /app/python_reqs.txt
RUN pip install --upgrade pip
RUN pip install -r python_reqs.txt

# Install wait-for-it
COPY ./_app/wait-for-it.sh /usr/local/bin/
RUN chmod u+x /usr/local/bin/wait-for-it.sh

# Configure custom entrypoint to wait for postgres and run webserver
COPY ./_app/dev/custom-entrypoint /usr/local/bin/
RUN chmod u+x /usr/local/bin/custom-entrypoint
ENTRYPOINT ["custom-entrypoint"]

EXPOSE 8000

Problem:

From inside the 'web' docker container I cannot access the Modbus server on the host LAN. I cannot ping its IP address (192.168.1.50).

traceroute 192.168.1.50 provides no useful info. It just fails.

When I run docker-compose up a docker network called '_app_default' is created. Here's what I get when I run docker network inspect _app_default:

[
    {
        "Name": "_app_default",
        "Id": "59f0836c2f3017c537f973a38ab3a55ad96e2ed737c4ac111fc0e911fa0c8a67",
        "Created": "2020-09-13T18:51:14.6395366Z",
        "Scope": "local",
        "Driver": "bridge",
        "EnableIPv6": false,
        "IPAM": {
            "Driver": "default",
            "Options": null,
            "Config": [
                {
                    "Subnet": "192.168.112.0/20",
                    "Gateway": "192.168.112.1"
                }
            ]
        },
        "Internal": false,
        "Attachable": true,
        "Ingress": false,
        "ConfigFrom": {
            "Network": ""
        },
        "ConfigOnly": false,
        "Containers": {
            "28af77ef2d6f99812cb392c8f9eca045b150256c464befc3bb249c9de9e653c1": {
                "Name": "db",
                "EndpointID": "d657f2f29699ada859447c919ad2c94195a6e9a7ae7eeb35989560d2717183c5",
                "MacAddress": "02:42:c0:a8:70:02",
                "IPv4Address": "192.168.112.2/20",
                "IPv6Address": ""
            },
            "a8a619d664a27286e276614e549514dff2c15c5c3c44c39e127fec54ec2b5c6b": {
                "Name": "web",
                "EndpointID": "8f1661c6fa24104a70171d1f830e1fe34ed8d89bacd55c9b9d05dc62f64112c0",
                "MacAddress": "02:42:c0:a8:70:03",
                "IPv4Address": "192.168.112.3/20",
                "IPv6Address": ""
            }
        },
        "Options": {},
        "Labels": {
            "com.docker.compose.network": "default",
            "com.docker.compose.project": "_app",
            "com.docker.compose.version": "1.27.0"
        }
    }
]

What I Already Tried:

I tried using network_mode: "host" in the 'web' service definition. That did not help.

I did my best to read through the Docker networking documentation but I can't find a straightforward solution to this situation and I get all turned around -- I'm unfortunately not good with networking topics -- yet :) . Do I need an 'overlay' network? Or a 'macvlan'? Help!

Any assistance that you StackOverflow superstars can send would be greatly appreciated. Thank you!!

-Allan

Answer 1

It looks like you have overlapping ip ranges of docker network and local network. To solve this you need to override docker network address pool e.g.

edit docker daemon config

$ cat ~/.docker/daemon.json
{
  "debug" : true,
  "default-address-pools" : [
    {
      "base" : "172.31.0.0/16",
      "size" : 24
    }
  ]
}