[docker] How to detect a docker daemon port

I have installed Ubuntu and Docker. I am trying to launch Raik container:

$ DOCKER_RIAK_AUTOMATIC_CLUSTERING=1 DOCKER_RAIK_CLUSTER_SIZE=5 DOCKER_RIAK_BACKEND=leveldb make start-cluster ./bin/start

and get the error message:

It looks like the environment variable DOCKER_HOST has not been set. The Riak cluster cannot be started unless this has been set appropriately. For example:

export DOCKER_HOST="tcp://127.0.0.1:2375"

If I set

export DOCKER_HOST="tcp://127.0.0.1:2375"

all my other containers stop working and said, that can not find the Docker daemon.

It looks like my Docker damon use other than 2375 port. How can I check it ?

1. Prepare extra configuration file. Create a file named /etc/systemd/system/docker.service.d/docker.conf. Inside the file docker.conf, paste below content:

[Service]
ExecStart=
ExecStart=/usr/bin/dockerd -H tcp://0.0.0.0:2375 -H unix:///var/run/docker.sock

Note that if there is no directory like docker.service.d or a file named docker.conf then you should create it.

2. Restart Docker. After saving this file, reload the configuration by systemctl daemon-reload and restart Docker by systemctl restart docker.service.

3. Check your Docker daemon. After restarting docker service, you can see the port in the output of systemctl status docker.service like /usr/bin/dockerd -H tcp://0.0.0.0:2375 -H unix:///var/run/docker.sock.

Hope this may help

Thank you!

Try add -H tcp://0.0.0.0:2375(at end of Execstart line) instead of -H 0.0.0.0:2375.

Reference docs of docker: https://docs.docker.com/install/linux/linux-postinstall/#configure-where-the-docker-daemon-listens-for-connections

There are 2 ways in configuring the docker daemon port

1) Configuring at /etc/default/docker file:

DOCKER_OPTS="-H tcp://127.0.0.1:5000 -H unix:///var/run/docker.sock"

2) Configuring at /etc/docker/daemon.json:

{
"debug": true,
"hosts": ["tcp://127.0.0.1:5000", "unix:///var/run/docker.sock"]
}

If the docker default socket is not configured Docker will wait for infinite period.i.e

Waiting for /var/run/docker.sock
Waiting for /var/run/docker.sock
Waiting for /var/run/docker.sock
Waiting for /var/run/docker.sock
Waiting for /var/run/docker.sock

NOTE : BUT DON'T CONFIGURE IN BOTH THE CONFIGURATION FILES, the following error may occur :

Waiting for /var/run/docker.sock
unable to configure the Docker daemon with file /etc/docker/daemon.json: the following directives are specified both as a flag and in the configuration file: hosts: (from flag: [tcp://127.0.0.1:5000 unix:///var/run/docker.sock], from file: tcp://127.0.0.1:5000)

The reason for adding both the user port[ tcp://127.0.0.1:5000] and default docker socket[unix:///var/run/docker.sock] is that the user port enables the access to the docker APIs whereas the default socket enables the CLI. In case the default port[unix:///var/run/docker.sock] is not mentioned in /etc/default/docker file the following error may occur:

# docker ps
Cannot connect to the Docker daemon at unix:///var/run/docker.sock. Is the docker daemon running?

This error is not because that the docker is not running, but because of default docker socket is not enabled.

Once the configuration is enabled restart the docker service and verify the docker port is enabled or not:

# netstat -tunlp | grep -i 5000
tcp        0      0 127.0.0.1:5000          0.0.0.0:*               LISTEN      31661/dockerd 

Applicable for Docker Version 17.04, may vary with different versions of docker.

Since I also had the same problem of "How to detect a docker daemon port" however I had on OSX and after little digging in I found the answer. I thought to share the answer here for people coming from osx.

If you visit known-issues from docker for mac and github issue, you will find that by default the docker daemon only listens on unix socket /var/run/docker.sock and not on tcp. The default port for docker is 2375 (unencrypted) and 2376(encrypted) communication over tcp(although you can choose any other port).

On OSX its not straight forward to run the daemon on tcp port. To do this one way is to use socat container to redirect the Docker API exposed on the unix domain socket to the host port on OSX.

docker run -d -v /var/run/docker.sock:/var/run/docker.sock -p 127.0.0.1:2375:2375 bobrik/socat TCP-LISTEN:2375,fork UNIX-CONNECT:/var/run/docker.sock

and then

export DOCKER_HOST=tcp://localhost:2375

However for local client on mac os you don't need to export DOCKER_HOST variable to test the api.

If you run ps -aux | dockerd you should see the tcp endpoint it is running on.