Using NSEnter with Boot2Docker

September 1, 2014

NSEnter is a nice way to connect to a running Docker container. This post presents a script to simplify the usage of nsenter together with Boot2Docker.

There is still quite some dust around Docker and after gaining more and more experience, new patterns and anti-patterns are emerging.

One of those anti-patterns is the usage of an SSH daemon inside an image for debugging, backup and troubleshooting purposes. Jérôme Petazzoni’s Blog Post explains this nicely. In addition it provides proper solutions for common use cases for which SSH is currently used.

Nevertheless I still have this irresistible urge to login into a container. And if it is only for looking around and checking out the environment (call me old-fashioned, that’s ok ;-)

Luckily Jérôme provides a perfect solution to satisfy this thirst: nsenter. This allows you to enter into container namespaces. On the GitHub page you find the corresponding recipe for installing and using nsenter on a Linux host.

If you want to use it from OS X with e.g. Boot2Docker you need to login into the VM hosting the Docker daemon and then connect to a running container.

As described in the NSenter README you can use a simple alias for doing this transparently

	docker-enter() {
	  boot2docker ssh '[ -f /var/lib/boot2docker/nsenter ] || docker run --rm -v /var/lib/boot2docker/:/target jpetazzo/nsenter'
	  boot2docker ssh -t sudo /var/lib/boot2docker/docker-enter "$@"
	}

For a bit more comfort with usage information and error checking you can convert this to a small shell script like docker-enter which needs to be installed within the path (on OS X). As arguments it expects a container id or name and optionally a command (with args) to execute in the container. This script also will automatically install nsenter on the boot2docker VM if not already present (like the shell function above does this as well):

	10:20 [~] $ docker ps -q
	5bf8a161cceb
	
	10:20 [~] $ docker-enter 5bf8a161cceb bash
	
	Unable to find image 'jpetazzo/nsenter' locally
	Pulling repository jpetazzo/nsenter
	Installing nsenter to /target
	Installing docker-enter to /target
	
	[email protected]:/#

If you want even more comfort with bash completion you can add the small Bash completion script docker-enter_commands (inspired by and copied from Docker’s bash completion) to your ~/.bash_completion_scripts/ directory (or wherever your completion scripts are located, e.g. /usr/local/etc/bash_completion.d if you installed bash-completion via brew). This setup completes on container names and ids on the arguments for docker-enter. Alternatively you can put the commands together with the shell function code above directly into your ~/.bashrc, too.

P.S. After writing this post, I’ve found out, that this topic has been already covered in another blog post previously by Lajos Papp. That’s also where the shell function definition in the nsenter README originates from. Give credit to whom it’s due.