Docker Environment Variables and Container Linking Example

example `WEBDB_PORT_5432_TCP_PORT=5432`. * The `prefix_PROTO` variable contains just the protocol from the URL for example `WEBDB_PORT_5432_TCP_PROTO=tcp`. If the container exposes multiple ports, an environment variable set is defined for each one. This means, for example, if a container exposes 4 ports that Docker creates 12 environment variables, 3 for each port. Additionally, Docker creates an environment variable called `<alias>_PORT`. This variable contains the URL of the source container's first exposed port. The 'first' port is defined as the exposed port with the lowest number. For example, consider the `WEBDB_PORT=tcp://172.17.0.82:5432` variable. If that port is used for both tcp and udp, then the tcp one is specified. Finally, Docker also exposes each Docker originated environment variable from the source container as an environment variable in the target. For each variable Docker creates an `<alias>_ENV_<name>` variable in the target container. The variable's value is set to the value Docker used when it started the source container. Returning back to our database example, you can run the `env` command to list the specified container's environment variables. ```console $ docker run --rm --name web2 --link db:db training/webapp env <...> DB_NAME=/web2/db DB_PORT=tcp://172.17.0.5:5432 DB_PORT_5432_TCP=tcp://172.17.0.5:5432 DB_PORT_5432_TCP_PROTO=tcp DB_PORT_5432_TCP_PORT=5432 DB_PORT_5432_TCP_ADDR=172.17.0.5 <...> ``` You can see that Docker has created a series of environment variables with useful information about the source `db` container. Each variable is prefixed with `DB_`, which is populated from the `alias` you specified above. If the `alias` were `db1`, the variables would be prefixed with `DB1_`. You can use these environment variables to configure your applications to connect to the database on the `db` container. The connection is secure and private; only the linked `web` container can communicate with the `db` container. ### Important notes on Docker environment variables Unlike host entries in the [`/etc/hosts` file](#updating-the-etchosts-file), IP addresses stored in the environment variables are not automatically updated if the source container is restarted. We recommend using the host entries in `/etc/hosts` to resolve the IP address of linked containers.

This passage details how Docker exposes environment variables from a linked container to the target container, including variables for the container's name, exposed ports (address, port, protocol), and Docker-originated environment variables from the source. It provides an example using a database container (`db`) linked to a web application container (`web2`), demonstrating the resulting environment variables. It concludes by advising the use of host entries in `/etc/hosts` for IP address resolution of linked containers over environment variables due to the latter's lack of automatic updating upon source container restart.