How to access the local machine (host machine) in Docker

Question

How to access the local database in Docker? Using 127.0.0.1 is definitely not an option, because this refers to the container itself in the Docker container. Therefore, it is necessary to solve the problem through other channels.

Solution

You can choose one of the following methods according to the type of operating system.

DockerFile:

RUN /sbin/ip route|awk '/default/ { print $3,"\tdockerhost" }' >> /etc/hosts

RunTime:

(may not use) docker run --add-host dockerhost:`/sbin/ip route|awk '/default/ { print $3}'` [my container]
(useful) docker run --add-host=dockerhost:`docker network inspect --format='{{range .IPAM.Config}}{{.Gateway}}{{end}}' bridge` [IMAGE]

Docker for Mac (17.12+):

docker.for.mac.host.internal
MONGO_SERVER=docker.for.mac.host.internal

# docker-compose.yml
version: '3'

services:
 API:
  build: ./api
  volumes:
   - ./api:/usr/src/app:ro
  ports:
   - "8000"
  environment:
   - MONGO_SERVER
  command: /usr/local/bin/gunicorn -c /usr/src/app/gunicorn_config.py -w 1 -b :8000 wsgi

Linux

Solution 1
/sbin/ip route|awk '/default/ { print $3 }'
docker run --add-host dockerhost:`/sbin/ip route|awk '/default/ { print $3}'` [my container]
# Solution 2
-e "DOCKER_HOST=$(ip -4 addr show docker0 | grep -Po 'inet \K[\d.]+')"

Principle

To understand the principle, you need to understand the model of computer networks and the model implemented by Docker. In fact, a virtual bridge docker0 is implemented inside Docker. The virtual address of the external host in the bridge, that is, docker.for.mac.host.internal, is needed to access the external host in the container. If you are interested, you can learn about Docker's network principles, computer network principles, and docker compose.

Reference

[1].(stackoverflow)insert-docker-parent-host-ip-into-containers-hosts-file

[2].(stackoverflow)how-to-get-the-ip-address-of-the-docker-host-from-inside-a-docker-container

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