dumb-init

dumb-init is a simple process designed to run as PID 1 inside Docker containers and proxy signals to a single child process.

In Docker containers, a process typically runs as PID 1, which means that signals like TERM will just bounce off your process unless it goes out of its way to handle them (see the "Why" section below). This is a big problem with scripts in languages like Python, Bash, or Ruby, and can lead to leaking Docker containers if you're not careful.

Why you need a signal proxy

Normally, when processes are sent a signal like TERM, the Linux kernel will try to trigger any custom handlers the process has registered for that signal.

If the process hasn't registered custom handlers, the kernel will fall back to default behavior for that signal (such as killing the process, in the case of TERM).

However, processes which run as PID 1 get special treatment by the kernel, and default signal handlers won't be applied. If your process doesn't explicitly handle these signals, a TERM will have no effect at all.

For example, if you have Jenkins jobs that do docker run my-container script, sending TERM to the docker run process will typically kill the docker run command, but leave the container running in the background.

What dumb-init does

dumb-init runs as PID 1, acting like a simple init system. It launches a single process, and then proxies all received signals to that child process.

Since your actual process is no longer PID 1, when it receives signals from dumb-init, the default signal handlers will be applied, and your process will behave as you would expect.

If your process dies, dumb-init will also die.

Installing inside Docker containers

You have a few options for using dumb-init:

Option 1: Installing via an internal apt server

If you have an internal apt server, uploading the .deb to your server is the recommended way to use dumb-init. In your Dockerfiles, you can simply apt-get install dumb-init and it will be available.

Option 2: Installing the .deb package manually

If you don't have an internal apt server, you can use dpkg -i to install the .deb package. You can choose how you get the .deb onto your container (mounting a directory or wget-ing it are some options).

Option 3: Installing from PyPI

dumb-init can be installed from PyPI using pip. Since dumb-init is written in C, you'll want to first install a C compiler (on Debian/Ubuntu, apt-get install gcc is sufficient), then just pip install dumb-init.

Usage

Once installed inside your Docker container, simply prefix your commands with dumb-init. For example:

$ docker run my_container dumb-init python -c 'while True: pass'

Running this same command without dumb-init would result in being unable to stop the container without SIGKILL, but with dumb-init, you can send it more humane signals like TERM.

See also

• Docker and the PID 1 zombie reaping problem (Phusion Blog)
• Trapping signals in Docker containers (@gchudnov)
• pgctl