How to get the IP address of the docker host from inside a docker container

As of version 18.03, you can use host.docker.internal as the host's IP.

Works in Docker for Mac, Docker for Windows, and perhaps other platforms as well.

This is an update from the Mac-specific docker.for.mac.localhost, available since version 17.06, and docker.for.mac.host.internal, available since version 17.12, which may also still work on that platform.

Note, as in the Mac and Windows documentation, this is for development purposes only.

For example, I have environment variables set on my host:

MONGO_SERVER=host.docker.internal

In my docker-compose.yml file, I have this:

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

For those running Docker in AWS, the instance meta-data for the host is still available from inside the container.

curl http://169.254.169.254/latest/meta-data/local-ipv4

For example:

$ docker run alpine /bin/sh -c "apk update ; apk add curl ; curl -s http://169.254.169.254/latest/meta-data/local-ipv4 ; echo"
fetch http://dl-cdn.alpinelinux.org/alpine/v3.3/main/x86_64/APKINDEX.tar.gz
fetch http://dl-cdn.alpinelinux.org/alpine/v3.3/community/x86_64/APKINDEX.tar.gz
v3.3.1-119-gb247c0a [http://dl-cdn.alpinelinux.org/alpine/v3.3/main]
v3.3.1-59-g48b0368 [http://dl-cdn.alpinelinux.org/alpine/v3.3/community]
OK: 5855 distinct packages available
(1/4) Installing openssl (1.0.2g-r0)
(2/4) Installing ca-certificates (20160104-r2)
(3/4) Installing libssh2 (1.6.0-r1)
(4/4) Installing curl (7.47.0-r0)
Executing busybox-1.24.1-r7.trigger
Executing ca-certificates-20160104-r2.trigger
OK: 7 MiB in 15 packages
172.31.27.238

$ ifconfig eth0 | grep -oP 'inet addr:\K\S+'
172.31.27.238

Update: On Docker for Mac, as of version 18.03, you can use host.docker.internal as the host's IP. See aljabear's answer. For prior versions of Docker for Mac the following answer may still be useful:

On Docker for Mac the docker0 bridge does not exist, so other answers here may not work. All outgoing traffic however, is routed through your parent host, so as long as you try to connect to an IP it recognizes as itself (and the docker container doesn't think is itself) you should be able to connect. For example if you run this from the parent machine run:

ipconfig getifaddr en0

This should show you the IP of your Mac on its current network and your docker container should be able to connect to this address as well. This is of course a pain if this IP address ever changes, but you can add a custom loopback IP to your Mac that the container doesn't think is itself by doing something like this on the parent machine:

sudo ifconfig lo0 alias 192.168.46.49

You can then test the connection from within the docker container with telnet. In my case I wanted to connect to a remote xdebug server:

telnet 192.168.46.49 9000

Now when traffic comes into your Mac addressed for 192.168.46.49 (and all the traffic leaving your container does go through your Mac) your Mac will assume that IP is itself. When you are finish using this IP, you can remove the loopback alias like this:

sudo ifconfig lo0 -alias 192.168.46.49

One thing to be careful about is that the docker container won't send traffic to the parent host if it thinks the traffic's destination is itself. So check the loopback interface inside the container if you have trouble:

sudo ip addr show lo

In my case, this showed inet 127.0.0.1/8 which means I couldn't use any IPs in the 127.* range. That's why I used 192.168.* in the example above. Make sure the IP you use doesn't conflict with something on your own network.

/sbin/ip route|awk '/default/ { print $3 }'

As @MichaelNeale noticed, there is no sense to use this method in Dockerfile (except when we need this IP during build time only), because this IP will be hardcoded during build time.