Is there a way to detect programmatically whether a Docker container is running natively or using Docker Machine?

Question

Is there a way to find out from inside a Docker container whether that container is running natively on a Linux host, or in Docker Machine?

The background of the question is that I want to enable JMX in my Tomcat-based container, but for that, I need to know the external IP address of the host:

When running in Docker-Machine, this is something like 192.168.99.100.
Running natively on Linux, this is the IP address of the host.
Running in Docker for Mac, this is also the IP address of the host.

I have a Docker Tomcat image that I want to roll out with JMX enabled, but I don't know whether people are going to use it natively or running in Docker-Machine, hence I don't know which IP address to provide for the java.rmi.server.hostname parameter when starting Tomcat.

Is there a way to either

find that IP address from inside of the container (or find out where it's running), or
find it out from the outside (e.g. in docker-compose) and then pass it into the container upon start? Can the DOCKER_HOST environment variable help in this case? When running Docker Machine, it's set to tcp://192.168.99.100:2376- is this variable also used when not running Docker Machine?

I would like to avoid the situation where people have to provide the IP address manually when starting the container (or Docker Compose).

regarding to your second option, you can use [docker api](https://docs.docker.com/engine/reference/api/docker_remote_api/) and get information regarding to your container! — Mohsen ZareZardeyni, Dec 07 '16 at 09:23
@Mohsen89z what would you query in docker to find out what type of host you are running on? — Matt, Dec 07 '16 at 09:34
I think that's going to be hard to do without injecting an environment variable at run time from the host running it. You could possibly set up a script for people to run your containers that collects the info for them. or maybe try [jolokia](https://jolokia.org) which suits being run in docker. — Matt, Dec 07 '16 at 09:36

score 0 · Accepted Answer · answered Dec 12 '16 at 13:16

Here's how I have solved this by using the DOCKER_HOST variable. My assumption is that if this variable is set, it will contain the IP address of the host, in the form of tcp://192.168.99.100:2367.

In my docker-compose.yaml file, I have mapped the host's DOCKER_HOST variable into the container:

version: "2"

services:
  foo:
    environment:
      DOCKER_HOST: $DOCKER_HOST

Then in the container, I evaluate this variable as part of the script that is run when the container is started, and then use it to set Tomcat's JMX properties:

# Allow to specify the JMX port from the outside. Use 10992 as a default if not provided
DOCKER_JMX_PORT=${DOCKER_JMX_PORT:-10992}

# Evaluate the $DOCKER_HOST variable
if [ -n "DOCKER_HOST" ]; then
  # DOCKER_HOST variable is set
  DOCKER_JMX_HOST=$(echo "$DOCKER_HOST" | sed -e "s|tcp://||g" -e "s/:.*//g")
else
  # DOCKER_HOST variable is not set
  DOCKER_JMX_HOST=$(hostname)
fi

# Set the JMX properties for Tomcat
export CATALINA_OPTS="-Dcom.sun.management.jmxremote=true \
  -Dcom.sun.management.jmxremote.port=$DOCKER_JMX_PORT \
  -Dcom.sun.management.jmxremote.rmi.port=$DOCKER_JMX_PORT \
  -Dcom.sun.management.jmxremote.authenticate=false \
  -Dcom.sun.management.jmxremote.ssl=false \
  -Djava.rmi.server.hostname=$DOCKER_JMX_HOST"

When the DOCKER_HOST environment variable is not set, I assume that the container is running natively, e.g. on Linux, and I use the hostname for the JMX properties.

Is there a way to detect programmatically whether a Docker container is running natively or using Docker Machine?

1 Answers1