CODEWITHSUNDEEP
dockernetwork-programmingdocker-compose
allan
allan

Reputation: 343

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

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:

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

Upvotes: 4

Views: 5216

Answers (1)

qoomon
qoomon

Reputation: 5306

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
    }
  ]
}

Upvotes: 2

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