Docker command can't connect to Docker daemon

Question

I want to make a move to Docker, so I've just started to mess around with it. I've installed Docker on a VirtualBox Ubuntu 15.10 (Wily Werewolf) installation and as suggested here I then tried running a basic nginx Docker image:

$ docker run --name mynginx1 -P -d nginx
Cannot connect to the Docker daemon. Is the docker daemon running on this host?

So I checked out whether Docker was running:

$ sudo service docker status
● docker.service - Docker Application Container Engine
   Loaded: loaded (/lib/systemd/system/docker.service; enabled; vendor preset: enabled)
   Active: active (running) since vr 2015-11-06 08:41:48 CET; 15min ago
     Docs: https://docs.docker.com
 Main PID: 7542 (docker)
   CGroup: /system.slice/docker.service
           └─7542 /usr/bin/docker daemon -H fd://

nov 06 08:41:47 kramer65-VirtualBox systemd[1]: Starting Docker Application Container Engine...
nov 06 08:41:47 kramer65-VirtualBox docker[7542]: time="2015-11-06T08:41:47.900410966+01:00" level=info msg="API ...ock"
nov 06 08:41:48 kramer65-VirtualBox docker[7542]: time="2015-11-06T08:41:48.033514149+01:00" level=info msg="Fire...lse"
nov 06 08:41:48 kramer65-VirtualBox docker[7542]: time="2015-11-06T08:41:48.141594321+01:00" level=info msg="Defa...ess"
nov 06 08:41:48 kramer65-VirtualBox docker[7542]: time="2015-11-06T08:41:48.416294436+01:00" level=warning msg="Y...it."
nov 06 08:41:48 kramer65-VirtualBox docker[7542]: time="2015-11-06T08:41:48.565507576+01:00" level=info msg="Load...rt."
nov 06 08:41:48 kramer65-VirtualBox docker[7542]: time="2015-11-06T08:41:48.567907022+01:00" level=info msg="Load...ne."
nov 06 08:41:48 kramer65-VirtualBox docker[7542]: time="2015-11-06T08:41:48.567945214+01:00" level=info msg="Daem...ion"
nov 06 08:41:48 kramer65-VirtualBox docker[7542]: time="2015-11-06T08:41:48.567969891+01:00" level=info msg="Dock....9.0
nov 06 08:41:48 kramer65-VirtualBox systemd[1]: Started Docker Application Container Engine.
Hint: Some lines were ellipsized, use -l to show in full.

This suggests that the Docker daemon is actually already running, but to be sure I just started the Docker daemon manually:

$ sudo docker daemon
INFO[0000] API listen on /var/run/docker.sock           
INFO[0000] [graphdriver] using prior storage driver "aufs" 
INFO[0000] Firewalld running: false                     
INFO[0000] Default bridge (docker0) is assigned with an IP address XXX.XX.X.X/XX. Daemon option --bip can be used to set a preferred IP address 
WARN[0000] Your kernel does not support swap memory limit. 
INFO[0000] Loading containers: start.                   

INFO[0000] Loading containers: done.                    
INFO[0000] Daemon has completed initialization          
INFO[0000] Docker daemon                                 commit=76d6bc9 execdriver=native-0.2 graphdriver=aufs version=1.9.0

I then tried running the image again, but with the same result:

$ docker run --name mynginx1 -P -d nginx
Cannot connect to the Docker daemon. Is the docker daemon running on this host?

I tried sudo'ing the command, but to no avail. What am I doing wrong here?

Answer 1

Usually, the following command does the trick:

sudo service docker restart

This, instead of docker start for the cases where Docker seems to already be running.

If that works then, as suggested and in another answer and on this GitHub issue, if you haven't added yourself in the docker group do it by running:

sudo usermod -aG docker <your-username> 

And you're most likely good to go.

---

As for anybody else bumping into this, in some OS's docker doesn't start right after you install it and, as a result, the same can't connect to daemon message appears. In this case you can first verify that Docker is indeed not running by checking the status of your docker service by executing:

sudo service docker status

If the output looks something like: docker stop/waiting instead of docker start/running, process 15378 then it obviously means Docker is not active. In this case make sure you start it with:

sudo service docker start

And, as before, you'll most likely be good to go.

Answer 2

1. I also had the same issue. The problem was in sockets allocated to docker-daemon and docker-client.
2. First, permission was not set for the docker-client on docker.sock You can set it using "sudo usermod -aG docker $USER"
3. Then check your bash file where the docker-client is running, For me it was on 0.0.0.0:2375, while docker-daemon was running on unix socket.(It was set in the configuration file of dockerd).
4. Just comment the bash-line and it'll work fine.
5. But if you want to make it work on TCP port instead of unix socket, change the configuration file of dockerd and set it on 0.0.0.0.2375 and keep the line in bash as it is if present or set it to 0.0.0.0:2375.

Answer 3

Had the same issue and what worked for me was:
Checking the ownership of /var/run/docker.sock

ls -l /var/run/docker.sock

If you're not the owner then change ownership with the command

sudo chown *your-username* /var/run/docker.sock

Then you can go ahead and try executing the docker commands hassle-free :D

Answer 4

I had the same problem. Been struggling for two days to solve it.

It only worked when I did:

1. According to Docker's Tutorial, you need to add the Docker key if not already added using:

   $ sudo wget -qO- https://get.docker.com/gpg | sudo apt-key add -

2. Then make sure you grant docker privileges to yourself using:

   $ sudo usermod -aG docker $USER

Hope this helps you too.

Answer 5

For the ones who already tried restarting your machine, unsetting the environment variable DOCKER_HOST as told in the docker env documentation and all the rest just try to go with the

sudo service docker restart

Only this did the trick for me even after restarting the machine.