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assignees:
- sttts

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* TOC
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This document describes how sysctls are used within a Kubernetes cluster.

## What is a Sysctl?

In Linux, the sysctl interface allows an administrator to modify kernel
parameters at runtime. Parameters are available via the `/proc/sys/` virtual
process file system. The parameters cover various subsystems such as:

- kernel (common prefix: `kernel.`)
- networking (common prefix: `net.`)
- virtual memory (common prefix: `vm.`)
- MDADM (common prefix: `dev.`)
- More subsystems are described in [Kernel docs](https://www.kernel.org/doc/Documentation/sysctl/README).

To get a list of all parameters, you can run

```
$ sudo sysctl -a
```

## Namespaced vs. Node-Level Sysctls

A number of sysctls are _namespaced_ in today's Linux kernels. This means that
they can be set independently for each pod on a node. Being namespaced is a
requirement for sysctls to be accessible in a pod context within Kubernetes.

The following sysctls are known to be _namespaced_:

- `kernel.shm*`,
- `kernel.msg*`,
- `kernel.sem`,
- `fs.mqueue.*`,
- `net.*`.

Sysctls which are not namespaced are called _node-level_ and must be set
manually by the cluster admin, either by means of the underlying Linux
distribution of the nodes (e.g. via `/etc/sysctls.conf`) or using a DaemonSet
with privileged containers.

**Note**: it is good practice to consider nodes with special sysctl settings as
_tainted_ within a cluster, and only schedule pods onto them which need those
sysctl settings. It is suggested to use the Kubernetes [_taints and toleration_
feature](/docs/user-guide/kubectl/kubectl_taint.md) to implement this.

## Safe vs. Unsafe Sysctls

Sysctls are grouped into _safe_  and _unsafe_ sysctls. In addition to proper
namespacing a _safe_ sysctl must be properly _isolated_ between pods on the same
node. This means that setting a _safe_ sysctl for one pod

- must not have any influence on any other pod on the node
- must not allow to harm the node's health
- must not allow to gain CPU or memory resources outside of the resource limits
  of a pod.

By far, most of the _namespaced_ sysctls are not necessarily considered _safe_.

For Kubernetes 1.4, the following sysctls are supported in the _safe_ set:

- `kernel.shm_rmid_forced`,
- `net.ipv4.ip_local_port_range`,
- `net.ipv4.tcp_syncookies`.

This list will be extended in future Kubernetes versions when the kubelet
supports better isolation mechanisms.

All _safe_ sysctls are enabled by default.

All _unsafe_ sysctls are disabled by default and must be allowed manually by the
cluster admin on a per-node basis. Pods with disabled unsafe sysctls will be
scheduled, but will fail to launch.

**Warning**: Due to their nature of being _unsafe_, the use of _unsafe_ sysctls
is at-your-own-risk and can lead to severe problems like wrong behavior of
containers, resource shortage or complete breakage of a node.

## Enabling Unsafe Sysctls

With the warning above in mind, the cluster admin can allow certain _unsafe_
sysctls for very special situations like e.g. high-performance or real-time
application tuning. _Unsafe_ sysctls are enabled on a node-by-node basis with a
flag of the kubelet, e.g.:

```shell
$ kubelet --experimental-allowed-unsafe-sysctls 'kernel.msg*,net.ipv4.route.min_pmtu' ...
```

Only _namespaced_ sysctls can be enabled this way.

## Setting Sysctls for a Pod

The sysctl feature is an alpha API in Kubernetes 1.4. Therefore, sysctls are set
using annotations on pods. They apply to all containers in the same pod.

Here is an example, with different annotations for _safe_ and _unsafe_ sysctls:

```yaml
apiVersion: v1
kind: Pod
metadata:
  name: sysctl-example
  annotations:
    security.alpha.kubernetes.io/sysctls: kernel.shm_rmid_forced=1
    security.alpha.kubernetes.io/unsafe-sysctls: net.ipv4.route.min_pmtu=1000,kernel.msgmax=1 2 3
spec:
  ...
```

**Note**: a pod with the _unsafe_ sysctls specified above will fail to launch on
any node which has not enabled those two _unsafe_ sysctls explicitly. As with
_node-level_ sysctls it is recommended to use [_taints and toleration_
feature](/docs/user-guide/kubectl/kubectl_taint.md) or [labels on nodes](/docs
/user-guide/labels.md) to schedule those pods onto the right nodes.
