DOCKER_OPTS do not work in config file /etc/default/docker

I have changed /etc/default/docker with DOCKER_OPTS="-H tcp://127.0.0.1:2375 -H unix:///var/run/docker.sock" (docker version 1.4.1 in ubuntu 14.04), but it do not take any effect for me (not listening at port 2375). It seems that docker do not read this initial config file because I found export http_proxy enviroment do not work too.

Only sudo docker -H tcp://127.0.0.1:2375 -H unix:///var/run/docker.sock -d works.

It really confused me!

According to docker documentation, The recommended way to configure the daemon flags and environment variables for your Docker daemon is to use a systemd drop-in file.

So, for this specific case, do the following:

1. Use the command sudo systemctl edit docker.service to open an override file for docker.service in a text editor.

2. Add or modify the following lines, substituting your own values.

   [Service]
   ExecStart=
   ExecStart=/usr/bin/dockerd -H tcp://127.0.0.1:2375 -H unix:///var/run/docker.sock
   

3. Save the file.

4. Reload the systemctl configuration.

    $ sudo systemctl daemon-reload
   

5. Restart Docker:

    $ sudo systemctl restart docker.service
   

6. Check to see whether the change was honored by reviewing the output of netstat to confirm dockerd is listening on the configured port.

   $ sudo netstat -lntp | grep dockerd
   tcp        0      0 127.0.0.1:2375          0.0.0.0:*               LISTEN      3758/dockerd