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Share Process Namespace between Containers in a Pod

FEATURE STATE: Kubernetes v1.12 beta
This feature is currently in a beta state, meaning:

  • The version names contain beta (e.g. v2beta3).
  • Code is well tested. Enabling the feature is considered safe. Enabled by default.
  • Support for the overall feature will not be dropped, though details may change.
  • The schema and/or semantics of objects may change in incompatible ways in a subsequent beta or stable release. When this happens, we will provide instructions for migrating to the next version. This may require deleting, editing, and re-creating API objects. The editing process may require some thought. This may require downtime for applications that rely on the feature.
  • Recommended for only non-business-critical uses because of potential for incompatible changes in subsequent releases. If you have multiple clusters that can be upgraded independently, you may be able to relax this restriction.
  • Please do try our beta features and give feedback on them! After they exit beta, it may not be practical for us to make more changes.

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 that 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.

Before you begin

You need to have a Kubernetes cluster, and the kubectl command-line tool must be configured to communicate with your cluster. If you do not already have a cluster, you can create one by using Minikube, or you can use one of these Kubernetes playgrounds:

Your Kubernetes server must be version v1.10 or later. To check the version, enter kubectl version.

Process Namespace Sharing is a beta feature that is enabled by default. It may be disabled by setting --feature-gates=PodShareProcessNamespace=false.

Configure a Pod

Process Namespace Sharing is enabled using the ShareProcessNamespace field of v1.PodSpec. For example:

pods/share-process-namespace.yaml 
apiVersion: v1
kind: Pod
metadata:
  name: nginx
spec:
  shareProcessNamespace: true
  containers:
  - name: nginx
    image: nginx
  - name: shell
    image: busybox
    securityContext:
      capabilities:
        add:
        - SYS_PTRACE
    stdin: true
    tty: true

  1. Create the pod nginx on your cluster:

    kubectl create -f https://k8s.io/examples/pods/share-process-namespace.yaml

  2. Attach to the shell container and run ps:

    ```
kubectl attach -it nginx -c shell
```

If you don't see a command prompt, try pressing enter.

```
/ # ps ax
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.

    / # kill -HUP 8
    / # ps ax
    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 another container image using the /proc/$pid/root link.

    / # head /proc/8/root/etc/nginx/nginx.conf

    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 container images may expect to be isolated from other containers, though, so it’s important to understand these differences:

  1. The container process no longer has PID 1. Some container images 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.)

  2. 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.

  3. 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.

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