Share Process Namespace between Containers in a Pod [page]deterministic

This page shows how to configure process namespace sharing for a pod. When process namespace sharing is enabled, processes in a container are visible to all other containers in the same pod.

You can use this feature to configure cooperating containers, such as a log handler sidecar container, or to troubleshoot container images that don't include debugging utilities like a shell.

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## Configure a Pod

Process namespace sharing is enabled using the `shareProcessNamespace` field of `.spec` for a Pod. For example:

1. Create the pod `nginx` on your cluster:

```shell kubectl apply -f https://k8s.io/examples/pods/share-process-namespace.yaml ```

1. Attach to the `shell` container and run `ps`:

```shell kubectl exec -it nginx -c shell -- /bin/sh ```

If you don't see a command prompt, try pressing enter. In the container shell:

```shell # run this inside the "shell" container ps ax ```

The output is similar to this:

```none PID USER TIME COMMAND 1 root 0:00 /pause 8 root 0:00 nginx: master process nginx -g daemon off; 14 101 0:00 nginx: worker process 15 root 0:00 sh 21 root 0:00 ps ax ```

You can signal processes in other containers. For example, send `SIGHUP` to `nginx` to restart the worker process. This requires the `SYS_PTRACE` capability.

```shell # run this inside the "shell" container kill -HUP 8 # change "8" to match the PID of the nginx leader process, if necessary ps ax ```

The output is similar to this:

```none PID USER TIME COMMAND 1 root 0:00 /pause 8 root 0:00 nginx: master process nginx -g daemon off; 15 root 0:00 sh 22 101 0:00 nginx: worker process 23 root 0:00 ps ax ```

It's even possible to access the file system of another container using the `/proc/$pid/root` link.

```shell # run this inside the "shell" container # change "8" to the PID of the Nginx process, if necessary head /proc/8/root/etc/nginx/nginx.conf ```

The output is similar to this:

```none user nginx; worker_processes 1;

error_log /var/log/nginx/error.log warn; pid /var/run/nginx.pid;

events { worker_connections 1024; ```

## Understanding process namespace sharing

Pods share many resources so it makes sense they would also share a process namespace. Some containers may expect to be isolated from others, though, so it's important to understand the differences:

1. The container process no longer has PID 1. Some containers refuse to start without PID 1 (for example, containers using `systemd`) or run commands like `kill -HUP 1` to signal the container process. In pods with a shared process namespace, `kill -HUP 1` will signal the pod sandbox (`/pause` in the above example).

1. Processes are visible to other containers in the pod. This includes all information visible in `/proc`, such as passwords that were passed as arguments or environment variables. These are protected only by regular Unix permissions.

1. **Container filesystems are visible to other containers in the pod through the `/proc/$pid/root` link.** This makes debugging easier, but it also means that filesystem secrets are protected only by filesystem permissions.