simms21 / nsenter

Looking to start a shell inside a Docker container?

Starting from Docker 1.3 you can use Docker exec to enter a Docker container. Example:

docker exec -it CONTAINER_NAME /bin/bash

There are differences between nsenter and docker exec; namely, nsenter doesn't enter the cgroups, and therefore evades resource limitations. The potential benefit of this would be debugging and external audit, but for remote access, docker exec is the current recommended approach.

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This is because my preferred distros (Debian and Ubuntu) ship with an outdated version of util-linux (the package that should contain nsenter).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 && chmod +x /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 toolbox usage for OS X or Windows user

SSH to the Docker Toolbox virtual machine

docker-machine ssh default

Install nsenter, docker-enter, and importenv into the VM

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

You can also install nsenter to another folder. In that case, you will need to specify the full path of nsenter to run it.

docker run --rm -v /tmp:/target jpetazzo/nsenter

Using nsenter

List running containers:

docker ps

Identify the ID of the container that you want to get into; and retrieve its associated PID:

PID=$(docker inspect --format {{.State.Pid}} 08a2a025e05f)

Enter the container:

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

Remember to run those commands in the Docker Toolbox virtual machine; not in your host environment.

Using docker-enter

With docker-enter, you don't need to lookup the container PID.

You can get a shell inside the container:

docker-enter 08a2a025e05f

Or run commands directly:

docker-enter 08a2a025e05f ls /var/log
docker-enter 08a2a025e05f df -h

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 -t 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).