Adding an existing Docker host to Docker Machine : a few tips

Quick intro to Docker Machine

After switching from boot2docker (deprecated) to Docker Machine (or should I rather say the Docker Toolbox on mac) on my laptop, and after playing around with Google Cloud Engine through Docker Machine, I was curious to see if I could manage all my Docker hosts, running on VPS or dedicated servers, from Docker Machine.

Docker Machine is a convenient tool to choose which Docker daemon you want to use : that could be a Docker daemon running in a Virtual box VM, or a Docker daemon running on EC2, GCE, Digital Ocean, etc. or a Docker daemon running on a Linux box (it is just a matter of which driver is being used)

The virtualbox driver allows you to create a new VM in Virtual Box, starts it and configures the client / daemon tcp link with tls (over port 2376 by default); cloud drivers such as google allow you to do the same thing but within the « cloud », whereas the generic driver does not create a new host, instead uses ssh to configure the client / daemon tcp link with tls on an existing host.

Once set up,  « docker-machine ls » gives you the list of Docker daemons to interact with :

To talk to the daemon, just issue this command :

and you’re now talking to your daemon :

Convenient isn’t it ?

Integrating all my docker hosts (daemons) with docker-machine on my laptop would give the following benefits :

  • No need to ssh anymore to my (remote) Docker boxes
  • All my docker image work (Dockerfiles) could stay on my laptop, I could start new containers from local Dockerfiles on my remote hosts from the comfort of my laptop

Docker Machine : the generic driver

To integrate hosts you access through ssh, such as, into Docker Machine list of daemons, you need to use the generic driver. (this driver will take care of configuring your remote daemon to listen on port 2376 and generate and distribute the necessary keys for tls)

As of Docker Machine 0.4.2 with Docker 1.8.2, you need to respect some constraints to integrate your ssh enabled Docker host; here’s a list (obtained the hard way….) :

  • make sure you can connect, from your laptop to your remote Docker host using public key authentication (it means you generated a public key on your laptop and added to the list of authorized keys on the remote host : ssh-keygen, ssh-copy-id would help for that)
  • If you are using « sudo » on the host, make sure you configure passwordless sudo :

if you don’t, well unless this issue is closed, that’s not going to work

  • make sure the Docker daemon version on the server is the same one as the Docker client on the laptop
  • open the port 2376 (« ufw allow 2376 » on Debian / Ubuntu) on your remote host : your laptop Docker client will connect via TCP on port 2376 to talk to remote daemons

Those verifications made, I suggest you now add your remote Docker daemon (running on to Docker Machine :

It did not work ? well I got you covered with next chapter !

Docker machine generic driver common issues

On Mac OS X, it keeps asking me for my secret key passphrase

Well, that’s not _that_ bad, but still there’s a workaround described here

sudo: no tty present and no askpass program specified

Make sure your remote user has passwordless sudo configured, as described in the previous chapter :

Follow this issue for more info

docker-machine will wait forever

Probably it’s trying to connect to port 2376 and it’s blocked; have a look at the syslog on your remote host,

and check if there are no lines similar to this one :

Hum, UFW BLOCK and 2376 on the same line ? Sounds like you need to update your firewall configuration :

Triple check the docker versions on your laptop and the remote host are aligned
Could not load host key: /etc/ssh/ssh_host_ecdsa_key

If you are running into trouble, you might be looking into the ssh log

and maybe you saw those 2 lines :

You just need to generate those keys, on the remote host :

and they will be generated

Failed to execute operation: File exists

Apparently this is a Debian 8 specific issue, the good news is : it did not prevent the addition of your remote host to Docker Machine

Unimportant warnings

You can safely ignore those warnings

Victory !

You can now enjoy docker running on your remote host from your laptop !

One last thing ! docker-machine, remote hosts (not VirtualBox), and volumes

Do not expect volumes mounting (-v local_folder:container_folder) to work… Because the « local_folder » in question can only be a path on the remote host for now… Not a path on your laptop.

So you can either

9 réflexions sur « Adding an existing Docker host to Docker Machine : a few tips »

  1. Very detailed and helpful.

    %sudo ALL=(ALL) NOPASSWD:ALL did not work for me in redhat

    I had to do

  2. It’is very useful!
    Now I easy use my home server from desktop.
    « kind of Feel Like God » (c) )

    Many, many Thanks!

  3. On Ubuntu 15.10 after doing
    I also had to remove ‘user’ from the group sudo to enable sudo without password.

  4. Hi there,

    is this working for you when using a DNS instead of an IP? Not for me (for existing AWS instances)


  5. just use ssh key login
    « `
    docker-machine -D create –driver generic –generic-ip-address –generic-ssh-key ~/.ssh/docker/qcloud_rsa qcloud
    « `

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