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nsenter in a can

This is a small Docker recipe to build nsenter easily and install it in your
system.

What is nsenter?

It is a small tool allowing to enter into namespaces. Technically,
it can enter existing namespaces, or spawn a process into a new set of
namespaces. "What are those namespaces you're blabbering about?"
We are talking about container namespaces.

nsenter can do many useful things, but the main reason why I'm so
excited about it is because it lets you enter into a Docker container.

Why build nsenter in a container?

This is because my preferred distros (Debian and Ubuntu) ship with an
outdated version of util-linux (the package that should contain nsenter).
Therefore, if you need nsenter on those distros, you have to juggle with
APT repository, or compile from source, or… Ain't nobody got time for that.

I'm going to make a very bold assumption: if you landed here, it's because
you want to enter a Docker container. Therefore, you won't mind if my
method to build nsenter uses Docker itself.

How do I install nsenter with this?

If you want to install nsenter into /usr/local/bin, just do this:

docker run --rm -v /usr/local/bin:/target jpetazzo/nsenter

The jpetazzo/nsenter container will detect that /target is a
mountpoint, and it will copy the nsenter binary into it.

If you don't trust me, and prefer to extract the nsenter binary,
rather than allowing my container to potentially wreak havoc into
your system's $PATH, you can also do this:

docker run --rm jpetazzo/nsenter cat /nsenter > /tmp/nsenter

Then do whatever you want with the binary in /tmp/nsenter.

How do I use nsenter?

First, figure out the PID of the container you want to enter:

PID=$(docker inspect --format {{.State.Pid}} <container_name_or_ID>)

Then enter the container:

nsenter --target $PID --mount --uts --ipc --net --pid

What's that docker-enter thing?

It's just a small shell script that wraps up the steps described above into
a tiny helper. It takes the name or ID of a container and optionally the name
of a program to execute inside the namespace. If no command is specified a
shell will be invoked instead.

# list the root filesystem
docker-enter my_awesome_container ls -la

docker-enter with boot2docker

If you are using boot2docker, you can use the function below, to:

  • install nsenter and docker-enter into boot2docker's /var/lib/boot2docker/ directory,
    so they survive restarts.
  • execute docker-enter inside of boot2docker combined with ssh
docker-enter() {
  boot2docker ssh '[ -f /var/lib/boot2docker/nsenter ] || docker run --rm -v /var/lib/boot2docker/:/target jpetazzo/nsenter'
  boot2docker ssh sudo /var/lib/boot2docker/docker-enter "$@"
}

You can use it directly from your host (OS X/Windows), no need to ssh into boot2docker.

Caveats

  • This only works on Intel 64 bits platforms. Arguably, this is the
    only officially supported platform for Docker; so it's not a big deal.
    As soon as the Docker registry supports other architectures, I will
    see how to build nsenter for those!
  • nsenter still needs to run from the host; it cannot run inside a
    container (yet).
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