How can I use the internal ip address of a container as an environment variable in Docker

Question

I'm trying to get the IP address of my docker container as an environment variable within the container. Here is what I've tried:

When starting the container

docker run -dPi -e ip=`hostname -i` myDockerImage

When the container is already booted up

docker exec -it myDockerImage bash -c "export ip=`hostname -i`"

The problem with these two methods is that it uses the ip address of the host running the commands, not the docker container it's being run on.

So then I created a script inside the docker container that looks like this:

#!/bin/bash
export ip=`hostname -i`
echo $ip

And then run this with

docker exec -it myDockerImage bash -c ". ipVariableScript.sh"

When I add my_cmd which in my case is bash to the end of the script, it works in that one session of bash. I can't use it later in the files I need it in. I need to set it as an environment variable, not as a variable for one session.

So I already sourced it with the '.'. But it still won't echo when I'm in the container. If I put an echo $ip in the script, it will give me the correct IP address. But can only be used from within the script it's being set in.

Answer 1

Service names in Docker are more reliable and easier to use. However, here's

How to assign Docker guest IP to environment var inside guest

$ docker run -it ubuntu bash -c 'IP=$(hostname -i); echo ip=$IP'
ip=172.17.0.76

Answer 2

So, this is an old question but I ended up with the same question yesterday and my solution is this: use the docker internal option.

My containers were working fine but at some point the ip changed and I needed to change it on my docker-compose. Of course I can use the "docker network inspect my-container_default" and get my internal IP from that, but this also means changing my docker-compose every time the ip changes (and I'm still not that familiar with docker in order to detect IP changes automatically or make a more sofisticated config). So, I use the "host.docker.internal" flag. Now I no more need to check what's my IP from docker and everything is always connected.

Here an example of a node app which uses elastic search and needs to connect.

version: '3.7'

services:
  api:
    ...configs...
    depends_on:
      - 'elasticsearch'
    volumes:
      - ./:/usr/local/api
    ports:
      - '3000:80'
    links:
      - elasticsearch:els
    environment:
      - PORT=80
      - ELASTIC_NODE=http://host.docker.internal:9200
  elasticsearch:
    container_name: 'els'
    image: docker.elastic.co/elasticsearch/elasticsearch:7.13.4
    ...elastic search container configs...
    ports:
      - '9200:9200'
    expose:
      - 9200
    networks:
      - elastic

networks:
  elastic:
    driver: bridge

Note the "ELASTIC_NODE=http://host.docker.internal:9200" on api environments and the "network" that the elastic search container is using (on bridge mode)

This way you don't need to worry about knowing your IP.

Answer 3

The container name is postgres in this example. It is a bit clumsy, but it delivers.

container_ip=$(docker inspect postgres -f "{{json .NetworkSettings.Networks }}" \
  | awk -v FS=: '{print $9}' \
  | cut -f1 -d\, \
  | echo "${container_ip//\"}")

Make a function out of it:

#!/usr/bin/env bash

set -o errexit
set -o nounset
set -eu -o pipefail
#set -x
#trap read debug
    
#assign container ip address to variable
function get_container_ip () {
    container_ip=$(docker inspect "$1" -f "{{json .NetworkSettings.Networks }}" \
      | awk -v FS=: '{print $9}' \
      | cut -f1 -d\,)

    container_ip=$(echo "${container_ip//\"}")
}

get_container_ip $1
echo "$container_ip"