Oracle Linux - enable Docker daemon socket option

Installing Docker on a Oracle Linux instance is relative easy and you can get things to work extremely fast and easy. Within a very short timeframe you will have your Docker engine running and you first containers up and running. However, at one point in time you do want to start interacting with docker in a more interactive manner and not only use the docker command from the CLI. In a more integrated situation you do want to communicate over an API with Docker.

In our case the need was to have Jenkins build a Maven project with would build a Docker container with the help from the Docker Maven Plugin build by the people at Spotify. The first run we did hit an issue stating that the build failed with the below message:

[INFO] I/O exception (java.io.IOException) caught when processing request to {}->unix://localhost:80: Permission denied
[INFO] Retrying request to {}->unix://localhost:80

The message need to be solved by taking two steps, (1) ensuring you have your docker Daemon listening on an external socket and (2) ensuring you set an environment variable.

Setting the Docker daemon socket option:
To ensure the docker daemon will listen, on port 2375 you have to make some changes to /etc/sysconfig/docker , location of this configuration file differs per Linux distribution however on Oracle Linux you will need this file.

You will have to ensure that other_args is stating that you want to run the daemon sockets. In the below example we have made the explicit configuration that it needs to run on the localhost IP and the external IP of the docker host.

other_args="-H tcp://127.0.0.1:2375 -H tcp://192.168.56.4:2375 -H unix:///var/run/docker.sock"

Setting DOCKER_HOST environment variable:
To make sure that Jenkins knows where to find the Docker API you will have to set the DOCKER_HOST environment variable. You can do so from the command line with the below command:

export DOCKER_HOST="tcp://192.168.56.4:2375"

Even though the above export works, if you would only need this for Jenkins you can also set a global environment var within Jenkins. Setting it in Jenkins when you only need it in Jenkins might be a better idea. You can set global environment variables within Jenkins under "Manage Jenkins" -"Configure System" - "Global Properties"

Now, when you run a build the build should connect to docker on port 2375 (not 80) and the build should finish without any issue.