SSH, Interactive Sessions, Tunneling, Docker Daemon and Batch Shipyard

The focus of this article is to explain how Azure Batch compute nodes exist with an Azure deployment, interactive SSH, and the concept of SSH tunneling to a Docker Host on an Azure Batch compute node from your local machine.

Azure Batch Deployments and Port Exposure

Azure Batch compute nodes which comprise a pool are behind a NAT/load balancer which have certain endpoints exposed on the public IP of the deployment to specific instances (i.e., compute nodes).

For instance, port 12345 may map to port 22 of the first instance of a compute node in the pool for the public IP address The next compute node in the pool may have port 22 mapped to port 12346 on the load balancer.

This allows many compute nodes to sit behind one public IP address.

SSH Keypair Generation

In order to use SSH, you will need to generate a public/private RSA keypair that SSH requires for asymmetric key authentication. If you are running Batch Shipyard on Linux/Mac (or Windows with ssh-keygen accessible in your %PATH% or current working directory), you can opt to leave ssh_public_key and ssh_private_key unspecified or empty in ssh configuration blocks and Batch Shipyard will automatically generate the keypair for you. Alternatively, you can specify the location of pre-generated keypairs that you may have on your system.

On Windows, if you don't have ssh-keygen available as per above, you can use PuTTYgen to pre-generate public/private keys and then specify the file path in ssh_public_key and ssh_private_key in ssh configuration blocks. To create compatible keys for use with Batch Shipyard, perform the following actions:

  1. Launch PuTTYgen
  2. Click on the Generate button on the bottom right
  3. Move the mouse around as directed to generate randomness for the keys
  4. Save the RSA private key portion as a file
    • Click on Conversions file menu at the top
    • Click Export OpenSSH key
    • A prompt will ask if it is ok to save the key without a passphrase. Click Yes.
    • Save the file to a path accessible by Batch Shipyard
  5. Save the RSA public key portion either as text data or as a file. This is done by selecting all of the text under the box labeled Public key for pasting into OpenSSH authorized_keys file: and pressing CTRL+C or right-click and Copy.
    • If using the raw data, populate your ssh config property named ssh_public_key_data with the key data from the box
    • If you are saving the key data to a file, then populate your ssh config property named ssh_public_key and point it to the file

Interactive SSH

By adding an SSH user to the pool (which can be automatically done for you via the ssh block in the pool config upon pool creation or through the pool user add command), you can interactively log in to compute nodes in the pool and execute any command on the remote machine, including Docker commands via sudo.

You can utilize the pool ssh command to automatically connect to any compute node in the pool without having to manually resort to pool nodes grls and issuing the ssh command with the appropriate parameters. If you have the SSH private key in the default location or as specified in the generated_file_export_path, then an interactive SSH session will be created to the compute node specified.

pool ssh can accept either option --cardinal or the option --nodeid. If using --cardinal it requires the natural counting number from zero associated with the list of nodes as enumerated by pool nodes grls. If using --nodeid, then the exact compute node id within the pool specified in the pool config must be used. If neither option is specified, the default is --cardinal 0. For example:

SHIPYARD_CONFIGDIR=. shipyard pool ssh

would create an interactive SSH session with the first compute node in the pool as listed by pool nodes grls.

Securely Connecting to the Docker Socket Remotely via SSH Tunneling

To take advantage of this feature, you must install Docker locally on your machine and have ssh available. You can find guides to install Docker on various operating systems here.

The typical recommendation is to secure the Docker daemon if being accessed remotely via certificates and TLS. Because SSH is already configured on all of the nodes with authorized users to use the Docker daemon with Batch Shipyard, we can simply use SSH tunneling instead which simplifies the process and is less likely to be blocked in outbound firewall rules. This method is secure as the tunnel is opened and encrypted via ssh with a public/private RSA key pair. Please note that the Docker daemon port is not mapped on the NAT/load balancer, so it is impossible to connect to the port remotely without an SSH tunnel.

By specifying generate_docker_tunnel_script as true in the ssh configuration block in the pool config, a file named will be generated on pool add if an SSH user is specified, on pool user add when a pool user is added, on pool resize when a pool is resized, or on pool nodes grls when a pool's remote login settings are listed.

This script simplifies creating an SSH tunnel to the Docker socket from your local machine. It accepts a cardinal number of the node to connect to, similar to the --cardinal option for pool ssh. So if you were connecting to the first node in the pool, you would execute the docker tunnel script as:

./ 0

This will background the SSH tunnel to the remote Docker daemon and output something similar to the following:

tunneling to docker daemon on tvm-2522076272_3-20161214t213502z at
ssh tunnel pid is 22204
execute docker commands with DOCKER_HOST=: or with option: -H :

Now you can run the docker command locally but have these actions work remotely through the tunnel on the compute node with the appropriate -H option as noted above. For instance:

docker -H : run --rm -it busybox

would place the current shell context inside the busybox container running remotely on the Batch compute node.

Alternatively you can export an environment variable named DOCKER_HOST which will work for all docker invocations until the environment variable is unset. For example:

export DOCKER_HOST=:
docker run --rm -it busybox
# other docker commands after this will automatically run on the compute node

would create a busybox container on the remote compute node similar to the prior command.

To run a CUDA/GPU enabled docker image remotely with nvidia-docker, first you must install nvidia-docker locally in addition to docker as per the initial requirement. You can install nvidia-docker locally even without an Nvidia GPU or CUDA installed. It is simply required for the local command execution. If you do not have an Nvidia GPU available and install nvidia-docker you will most likely encounter an error with the nvidia docker service failing to start, but this is ok. You can then launch your CUDA-enabled Docker image on the remote compute node on Azure N-series VMs the same as any other Docker image except invoking with the nvidia-docker command instead:

DOCKER_HOST=: nvidia-docker run --rm -it nvidia/cuda nvidia-smi

# or, export the DOCKER_HOST env var first

export DOCKER_HOST=:
nvidia-docker run --rm -it nvidia/cuda nvidia-smi
# other docker or nvidia-docker commands after this will automatically
# run on the compute node

Once you are finished with running your docker and/or nvidia-docker commands remotely, you can terminate the SSH tunnel by sending a SIGTERM to the SSH tunnel process. In this example, the pid is 22204 as displayed by the script, thus we would terminate the SSH tunnel with the following:

kill 22204
# unset DOCKER_HOST if exported so docker commands are routed back to localhost

Finally, please remember that the script is generated and is specific for the pool as specified in the pool configuration file at the time of pool creation, resize, when an SSH user is added or when the remote login settings are listed.