Adds 'useIpFromEnv' parameter to registrator

[ganeshkaila]

Why does it require to use IP from environment variable?

The registrator will register the docker containerized services with the service discovery tools by grabbing the container IP address and exposed ports from the docker inspect information (low-level information of docker object).

• The docker containers which are managed by the kubernetes will not show up the container IP address under docker inspect information. In such environments, the docker containers network information will be offloaded to another container who's reference will be given under HostConfig.NetworkMode value.

In GKE (Google Container Engine), the docker containers will not show up container IP address either in the container inspect information or in the offloaded container inspect information.

If the registrator is provided with the capability to use IP address from the environment variables, then it helps to pass the service IP address (in the consul perspective) whether the pod or node IP address as an environment variable based on the requirement.

How to use the flag useIpFromEnv?

This example information is given based on the consul service discovery perspective.

The POD_IP is the environment variable.

Run the registrator service as shown:

$ registrator -useIpFromEnv=POD_IP consul://<consul-agent-ip>:8500

Add the following k8s spec configuration to the service to be registered:

...
    env:          
    - name: POD_IP
      valueFrom:
        fieldRef:
          fieldPath: status.podIP
...

When the registrator run with the flag -useIpFromEnv=POD_IP, the registrator uses this IP address (the value of POD_IP env) as container IP address and registers with consul agent service.

[progrium]

I never liked that we never switched to purely env based configuration. I would prefer we use environment. As such, I think this could be simplified by just having an environment variable that, if set, would be used for IP. No need for argument to specify what environment variable. Also, thanks for adding to the docs.

[shubhamitc]

do we have an image for this pull request?