Dedicated Documentation Page

Oct 13, 2025 by pinkandtrash.com 51 views

Problem Statement

In the current implementation of K0s, there is no clear way to configure arbitrary sysctl values and load arbitrary kernel modules at runtime. This limitation arises from the need for additional OS settings that are based on image-builder image configurations, which are not supported by vanilla K8s.

Solution Overview

To address this issue, we propose adding a new Kernel field to the ClusterSpec struct, which would list extra sysctl values and kernel modules that need to be configured or loaded during runtime. This feature would build upon the existing kernel configuration capabilities in K0s, as demonstrated in the kernelsetup_linux.go file.

KernelSpec Configuration

The KernelSpec struct would contain two primary fields:

Modules: A list of kernel modules that should be loaded during runtime.
SysctlParams: A map of sysctl parameters that should be configured during runtime.

Here is an example of the KernelSpec struct:

// KernelSpec defines kernel-related configurations for the cluster
type KernelSpec struct {
    // Modules is a list of kernel modules that should be loaded
    Modules []string `json:"modules,omitempty"`
    
    // SysctlParams is a map of sysctl parameters that should be configured
    SysctlParams map[string]string `json:"sysctlParams,omitempty"`
}

Example `sysctl` Configurations

The following sysctl configurations are commonly used in production environments and would be included in the SysctlParams map:

fs.inotify.max_user_instances = 8192
fs.inotify.max_user_watches = 524288
kernel.panic = 10
kernel.panic_on_oops = 1
net.bridge.bridge-nf-call-ip6tables = 1
net.bridge.bridge-nf-call-iptables = 1
net.ipv4.conf.all.rp_filter = 1
net.ipv4.ip_forward = 1
net.ipv4.tcp_congestion_control = bbr
net.ipv6.conf.all.disable_ipv6 = 0
net.ipv6.conf.all.forwarding = 1
vm.overcommit_memory = 1

Controller Behavior

If any errors occur during the application of sysctl configurations or loading kernel modules, the controller should hard-fail to ensure that the cluster is not compromised.

Alternatives Considered

Two alternative solutions were considered:

Dedicated Documentation Page: Create a dedicated documentation page for kernel tuning parameters that should be used in production environments. This page would serve as a reference guide for users who want to configure their kernel settings manually.
Quick-Start Guide: Add a link to the kernel tuning documentation in the Quick-Start guide, providing users with a clear path to configuring their kernel settings.

Additional Context

In both cases, documentation would be created to support the solution. This would ensure that users have access to the necessary information to configure their kernel settings effectively.

Benefits

The proposed solution would provide several benefits, including:

Improved flexibility: Users would have more control over their kernel settings, allowing them to customize their cluster to meet specific use-case requirements.
Simplified configuration: The KernelSpec struct would provide a clear and concise way to configure kernel settings, reducing the complexity of the configuration process.
Enhanced security: By providing a way to configure kernel settings, users would be able to harden their cluster's security posture, reducing the risk of vulnerabilities and attacks.

Conclusion

Q: What is the purpose of the `KernelSpec` struct?

A: The KernelSpec struct is designed to provide a clear and concise way to configure kernel settings, including sysctl values and kernel modules, at runtime.

Q: What are the benefits of using the `KernelSpec` struct?

A: The KernelSpec struct provides several benefits, including:

Improved flexibility: Users have more control over their kernel settings, allowing them to customize their cluster to meet specific use-case requirements.
Simplified configuration: The KernelSpec struct provides a clear and concise way to configure kernel settings, reducing the complexity of the configuration process.
Enhanced security: By providing a way to configure kernel settings, users can harden their cluster's security posture, reducing the risk of vulnerabilities and attacks.

Q: What types of `sysctl` values can be configured using the `KernelSpec` struct?

A: The KernelSpec struct can be used to configure a wide range of sysctl values, including but not limited to:

fs.inotify.max_user_instances
fs.inotify.max_user_watches
kernel.panic
kernel.panic_on_oops
net.bridge.bridge-nf-call-ip6tables
net.bridge.bridge-nf-call-iptables
net.ipv4.conf.all.rp_filter
net.ipv4.ip_forward
net.ipv4.tcp_congestion_control
net.ipv6.conf.all.disable_ipv6
net.ipv6.conf.all.forwarding
vm.overcommit_memory

Q: How do I load kernel modules using the `KernelSpec` struct?

A: To load kernel modules using the KernelSpec struct, you can simply add the module names to the Modules field of the struct. For example:

KernelSpec {
    Modules: []string{"module1", "module2", "module3"},
    SysctlParams: map[string]string{
        "fs.inotify.max_user_instances": "8192",
        "fs.inotify.max_user_watches": "524288",
    },
}

Q: What happens if errors occur during the application of `sysctl` configurations or loading kernel modules?

A: If any errors occur during the application of sysctl configurations or loading kernel modules, the controller will hard-fail to ensure that the cluster is not compromised.

Q: Can I use the `KernelSpec` struct in conjunction with other K0s features?

A: Yes, the KernelSpec struct can be used in conjunction with other K0s features, such as the image-builder feature, to provide a more comprehensive and flexible way to configure kernel settings.

Q: How do I get started with using the `KernelSpec` struct?

A: To get started with using the KernelSpec struct, simply create a new KernelSpec struct and add the desired sysctl values and kernel modules to the SysctlParams and Modules fields, respectively. Then, pass the KernelSpec struct to the K0s controller to apply the configurations.

Q: What is the future roadmap for the `KernelSpec` struct?

A: The future roadmap for the KernelSpec struct includes continued development and refinement of the feature, as well as integration with other K0s features and tools. Additionally, the KernelSpec struct will be used as a foundation for future kernel configuration features and tools.