Home » DevOps » Adding an existing Docker host to Docker Machine : a few tips

About Anthony Dahanne

Anthony Dahanne
Anthony Dahanne is a Java software developer for 8 years, his favorite topics are Android, building tools, Continuous Integration and, of course, core Java development. Working for Terracotta, he currently implements the REST management interface for EhCache.

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 :

$ docker-machine ls
NAME               ACTIVE   DRIVER       STATE     URL                           SWARM
my.host.com                 generic      Running   tcp://my.host.com:2376
default                     virtualbox   Running   tcp://192.168.99.104:2376
docker-vm          *        virtualbox   Running   tcp://192.168.99.102:2376

To talk to the my.host.com daemon, just issue this command :

$ eval "$(docker-machine env my.host.com)"

and you’re now talking to your my.host.com daemon :

$ docker ps
CONTAINER ID        IMAGE                  COMMAND                  CREATED             STATUS              PORTS                          NAMES
77aa96cf6db9        flexget                "/usr/local/bin/flexg"   46 hours ago        Up 21 hours     

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 my.host.com, 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 :
# visudo
%sudo   ALL=(ALL) NOPASSWD:ALL

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 my.host.com) to Docker Machine :

$ docker-machine create --driver generic \
 --generic-ip-address my.host.com \
 --generic-ssh-user anthony \
 my.host.com

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

SSH cmd error!
command: sudo hostname my.host.com && echo "my.host.com" | sudo tee /etc/hostname
err    : exit status 1
output : Saving password to keychain failed
Identity added: /Users/anthony/.docker/machine/machines/my.host.com/id_rsa (/Users/anthony/.docker/machine/machines/my.host.com/id_rsa)
sudo: no tty present and no askpass program specified

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

 	
%sudo   ALL=(ALL) NOPASSWD:ALL

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,

sudo tail -f /var/log/syslog

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

Oct  7 01:26:54 my.host kernel: [1859460.411738] [UFW BLOCK] IN=eth0 OUT= MAC=XX:XX SRC=199.99.99.99 DST=92.12.12.12 LEN=64 TOS=0x00 PREC=0x00 TTL=52 ID=37574 DF PROTO=TCP SPT=49855 DPT=2376 WINDOW=65535 RES=0x00 SYN URGP=0

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

$ sudo ufw allow 2376

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

sudo tail -f /var/log/auth.log

and maybe you saw those 2 lines :

Oct  7 01:04:41 host sshd[29920]: error: Could not load host key: /etc/ssh/ssh_host_ecdsa_key
Oct  7 01:04:41 host sshd[29920]: error: Could not load host key: /etc/ssh/ssh_host_ed25519_key

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

$ sudo ssh-keygen -A

and they will be generated

ssh-keygen: generating new host keys: ECDSA ED25519

Failed to execute operation: File exists

SSH cmd error!
command: sudo systemctl enable docker
err    : exit status 1
output : Synchronizing state for docker.service with sysvinit using update-rc.d...
Executing /usr/sbin/update-rc.d docker defaults
Executing /usr/sbin/update-rc.d docker enable
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

Importing SSH key...
Warning: got an invalid line error parsing /etc/os-release: %sExpected Saving password to keychain failed to split by '=' char into two strings, instead got 1 strings
Warning: got an invalid line error parsing /etc/os-release: %sExpected Identity added: /Users/anthony/.docker/machine/machines/my.host.com/id_rsa (/Users/anthony/.docker/machine/machines/my.host.com/id_rsa) to split by '=' char into two strings, instead got 1 strings

You can safely ignore those warnings

Victory !

$ docker-machine ls
NAME               ACTIVE   DRIVER       STATE     URL                           SWARM
my.host.com      *      generic      Running   tcp://my.host.com:2376

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

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