This is the multi-page printable view of this section. Click here to print.

Return to the regular view of this page.

Node Reference Information

This section contains the following reference topics about nodes:

You can also read node reference details from elsewhere in the Kubernetes documentation, including:

1 - Kubelet Checkpoint API

FEATURE STATE: Kubernetes v1.30 [beta] (enabled by default: true)

Checkpointing a container is the functionality to create a stateful copy of a running container. Once you have a stateful copy of a container, you could move it to a different computer for debugging or similar purposes.

If you move the checkpointed container data to a computer that's able to restore it, that restored container continues to run at exactly the same point it was checkpointed. You can also inspect the saved data, provided that you have suitable tools for doing so.

Creating a checkpoint of a container might have security implications. Typically a checkpoint contains all memory pages of all processes in the checkpointed container. This means that everything that used to be in memory is now available on the local disk. This includes all private data and possibly keys used for encryption. The underlying CRI implementations (the container runtime on that node) should create the checkpoint archive to be only accessible by the root user. It is still important to remember if the checkpoint archive is transferred to another system all memory pages will be readable by the owner of the checkpoint archive.

Operations

post checkpoint the specified container

Tell the kubelet to checkpoint a specific container from the specified Pod.

Consult the Kubelet authentication/authorization reference for more information about how access to the kubelet checkpoint interface is controlled.

The kubelet will request a checkpoint from the underlying CRI implementation. In the checkpoint request the kubelet will specify the name of the checkpoint archive as checkpoint-<podFullName>-<containerName>-<timestamp>.tar and also request to store the checkpoint archive in the checkpoints directory below its root directory (as defined by --root-dir). This defaults to /var/lib/kubelet/checkpoints.

The checkpoint archive is in tar format, and could be listed using an implementation of tar. The contents of the archive depend on the underlying CRI implementation (the container runtime on that node).

HTTP Request

POST /checkpoint/{namespace}/{pod}/{container}

Parameters

  • namespace (in path): string, required

    Namespace

  • pod (in path): string, required

    Pod

  • container (in path): string, required

    Container

  • timeout (in query): integer

    Timeout in seconds to wait until the checkpoint creation is finished. If zero or no timeout is specified the default CRI timeout value will be used. Checkpoint creation time depends directly on the used memory of the container. The more memory a container uses the more time is required to create the corresponding checkpoint.

Response

200: OK

401: Unauthorized

404: Not Found (if the ContainerCheckpoint feature gate is disabled)

404: Not Found (if the specified namespace, pod or container cannot be found)

500: Internal Server Error (if the CRI implementation encounter an error during checkpointing (see error message for further details))

500: Internal Server Error (if the CRI implementation does not implement the checkpoint CRI API (see error message for further details))

2 - Linux Kernel Version Requirements

Many features rely on specific kernel functionalities and have minimum kernel version requirements. However, relying solely on kernel version numbers may not be sufficient for certain operating system distributions, as maintainers for distributions such as RHEL, Ubuntu and SUSE often backport selected features to older kernel releases (retaining the older kernel version).

Pod sysctls

On Linux, the sysctl() system call configures kernel parameters at run time. There is a command line tool named sysctl that you can use to configure these parameters, and many are exposed via the proc filesystem.

Some sysctls are only available if you have a modern enough kernel.

The following sysctls have a minimal kernel version requirement, and are supported in the safe set:

  • net.ipv4.ip_local_reserved_ports (since Kubernetes 1.27, needs kernel 3.16+);
  • net.ipv4.tcp_keepalive_time (since Kubernetes 1.29, needs kernel 4.5+);
  • net.ipv4.tcp_fin_timeout (since Kubernetes 1.29, needs kernel 4.6+);
  • net.ipv4.tcp_keepalive_intvl (since Kubernetes 1.29, needs kernel 4.5+);
  • net.ipv4.tcp_keepalive_probes (since Kubernetes 1.29, needs kernel 4.5+);
  • net.ipv4.tcp_syncookies (namespaced since kernel 4.6+).
  • net.ipv4.tcp_rmem (since Kubernetes 1.32, needs kernel 4.15+).
  • net.ipv4.tcp_wmem (since Kubernetes 1.32, needs kernel 4.15+).
  • net.ipv4.vs.conn_reuse_mode (used in ipvs proxy mode, needs kernel 4.1+);

kube proxy nftables proxy mode

For Kubernetes 1.32, the nftables mode of kube-proxy requires version 1.0.1 or later of the nft command-line tool, as well as kernel 5.13 or later.

For testing/development purposes, you can use older kernels, as far back as 5.4 if you set the nftables.skipKernelVersionCheck option in the kube-proxy config. But this is not recommended in production since it may cause problems with other nftables users on the system.

Version 2 control groups

Kubernetes cgroup v1 support is in maintained mode starting from Kubernetes v1.31; using cgroup v2 is recommended. In Linux 5.8, the system-level cpu.stat file was added to the root cgroup for convenience.

In runc document, Kernel older than 5.2 is not recommended due to lack of freezer.

Other kernel requirements

Some features may depend on new kernel functionalities and have specific kernel requirements:

  1. Recursive read only mount: This is implemented by applying the MOUNT_ATTR_RDONLY attribute with the AT_RECURSIVE flag using mount_setattr(2) added in Linux kernel v5.12.
  2. Pod user namespace support requires minimal kernel version 6.5+, according to KEP-127.
  3. For node system swap, tmpfs set to noswap is not supported until kernel 6.3.

Linux kernel long term maintenance

Active kernel releases can be found in kernel.org.

There are usually several long term maintenance kernel releases provided for the purposes of backporting bug fixes for older kernel trees. Only important bug fixes are applied to such kernels and they don't usually see very frequent releases, especially for older trees. See the Linux kernel website for the list of releases in the Longterm category.

What's next

3 - Articles on dockershim Removal and on Using CRI-compatible Runtimes

This is a list of articles and other pages that are either about the Kubernetes' deprecation and removal of dockershim, or about using CRI-compatible container runtimes, in connection with that removal.

Kubernetes project

You can provide feedback via the GitHub issue Dockershim removal feedback & issues. (k/kubernetes/#106917)

External sources

4 - Node Labels Populated By The Kubelet

Kubernetes nodes come pre-populated with a standard set of labels.

You can also set your own labels on nodes, either through the kubelet configuration or using the Kubernetes API.

Preset labels

The preset labels that Kubernetes sets on nodes are:

What's next

5 - Local Files And Paths Used By The Kubelet

The kubelet is mostly a stateless process running on a Kubernetes node. This document outlines files that kubelet reads and writes.

The kubelet typically uses the control plane as the source of truth on what needs to run on the Node, and the container runtime to retrieve the current state of containers. So long as you provide a kubeconfig (API client configuration) to the kubelet, the kubelet does connect to your control plane; otherwise the node operates in standalone mode.

On Linux nodes, the kubelet also relies on reading cgroups and various system files to collect metrics.

On Windows nodes, the kubelet collects metrics via a different mechanism that does not rely on paths.

There are also a few other files that are used by the kubelet as well as kubelet communicates using local Unix-domain sockets. Some are sockets that the kubelet listens on, and for other sockets the kubelet discovers them and then connects as a client.

Configuration

Kubelet configuration files

The path to the kubelet configuration file can be configured using the command line argument --config. The kubelet also supports drop-in configuration files to enhance configuration.

Certificates

Certificates and private keys are typically located at /var/lib/kubelet/pki, but can be configured using the --cert-dir kubelet command line argument. Names of certificate files are also configurable.

Manifests

Manifests for static pods are typically located in /etc/kubernetes/manifests. Location can be configured using the staticPodPath kubelet configuration option.

Systemd unit settings

When kubelet is running as a systemd unit, some kubelet configuration may be declared in systemd unit settings file. Typically it includes:

State

Checkpoint files for resource managers

All resource managers keep the mapping of Pods to allocated resources in state files. State files are located in the kubelet's base directory, also termed the root directory (but not the same as /, the node root directory). You can configure the base directory for the kubelet using the kubelet command line argument --root-dir.

Names of files:

Checkpoint file for device manager

Device manager creates checkpoints in the same directory with socket files: /var/lib/kubelet/device-plugins/. The name of a checkpoint file is kubelet_internal_checkpoint for Device Manager

Pod status checkpoint storage

FEATURE STATE: Kubernetes v1.27 [alpha] (enabled by default: false)

If your cluster has
in-place Pod vertical scaling
enabled (feature gate
name InPlacePodVerticalScaling), then the kubelet stores a local record of allocated Pod resources.

The file name is pod_status_manager_state within the kubelet base directory (/var/lib/kubelet by default on Linux; configurable using --root-dir).

Container runtime

Kubelet communicates with the container runtime using socket configured via the configuration parameters:

  • containerRuntimeEndpoint for runtime operations
  • imageServiceEndpoint for image management operations

The actual values of those endpoints depend on the container runtime being used.

Device plugins

The kubelet exposes a socket at the path /var/lib/kubelet/device-plugins/kubelet.sock for various Device Plugins to register.

When a device plugin registers itself, it provides its socket path for the kubelet to connect.

The device plugin socket should be in the directory device-plugins within the kubelet base directory. On a typical Linux node, this means /var/lib/kubelet/device-plugins.

Pod resources API

Pod Resources API will be exposed at the path /var/lib/kubelet/pod-resources.

DRA, CSI, and Device plugins

The kubelet looks for socket files created by device plugins managed via DRA, device manager, or storage plugins, and then attempts to connect to these sockets. The directory that the kubelet looks in is plugins_registry within the kubelet base directory, so on a typical Linux node this means /var/lib/kubelet/plugins_registry.

Note, for the device plugins there are two alternative registration mechanisms. Only one should be used for a given plugin.

The types of plugins that can place socket files into that directory are:

  • CSI plugins
  • DRA plugins
  • Device Manager plugins

(typically /var/lib/kubelet/plugins_registry).

Security profiles & configuration

Seccomp

Seccomp profile files referenced from Pods should be placed in /var/lib/kubelet/seccomp. See the seccomp reference for details.

AppArmor

The kubelet does not load or refer to AppArmor profiles by a Kubernetes-specific path. AppArmor profiles are loaded via the node operating system rather then referenced by their path.

Locking

FEATURE STATE: Kubernetes v1.2 [alpha]

A lock file for the kubelet; typically /var/run/kubelet.lock. The kubelet uses this to ensure that two different kubelets don't try to run in conflict with each other. You can configure the path to the lock file using the the --lock-file kubelet command line argument.

If two kubelets on the same node use a different value for the lock file path, they will not be able to detect a conflict when both are running.

What's next

6 - Kubelet Configuration Directory Merging

When using the kubelet's --config-dir flag to specify a drop-in directory for configuration, there is some specific behavior on how different types are merged.

Here are some examples of how different data types behave during configuration merging:

Structure Fields

There are two types of structure fields in a YAML structure: singular (or a scalar type) and embedded (structures that contain scalar types). The configuration merging process handles the overriding of singular and embedded struct fields to create a resulting kubelet configuration.

For instance, you may want a baseline kubelet configuration for all nodes, but you may want to customize the address and authorization fields. This can be done as follows:

Main kubelet configuration file contents:

apiVersion: kubelet.config.k8s.io/v1beta1
kind: KubeletConfiguration
port: 20250
authorization:
  mode: Webhook
  webhook:
    cacheAuthorizedTTL: "5m"
    cacheUnauthorizedTTL: "30s"
serializeImagePulls: false
address: "192.168.0.1"

Contents of a file in --config-dir directory:

apiVersion: kubelet.config.k8s.io/v1beta1
kind: KubeletConfiguration
authorization:
  mode: AlwaysAllow
  webhook:
    cacheAuthorizedTTL: "8m"
    cacheUnauthorizedTTL: "45s"
address: "192.168.0.8"

The resulting configuration will be as follows:

apiVersion: kubelet.config.k8s.io/v1beta1
kind: KubeletConfiguration
port: 20250
serializeImagePulls: false
authorization:
  mode: AlwaysAllow
  webhook:
    cacheAuthorizedTTL: "8m"
    cacheUnauthorizedTTL: "45s"
address: "192.168.0.8"

Lists

You can overide the slices/lists values of the kubelet configuration. However, the entire list gets overridden during the merging process. For example, you can override the clusterDNS list as follows:

Main kubelet configuration file contents:

apiVersion: kubelet.config.k8s.io/v1beta1
kind: KubeletConfiguration
port: 20250
serializeImagePulls: false
clusterDNS:
  - "192.168.0.9"
  - "192.168.0.8"

Contents of a file in --config-dir directory:

apiVersion: kubelet.config.k8s.io/v1beta1
kind: KubeletConfiguration
clusterDNS:
  - "192.168.0.2"
  - "192.168.0.3"
  - "192.168.0.5"

The resulting configuration will be as follows:

apiVersion: kubelet.config.k8s.io/v1beta1
kind: KubeletConfiguration
port: 20250
serializeImagePulls: false
clusterDNS:
  - "192.168.0.2"
  - "192.168.0.3"
  - "192.168.0.5"

Maps, including Nested Structures

Individual fields in maps, regardless of their value types (boolean, string, etc.), can be selectively overridden. However, for map[string][]string, the entire list associated with a specific field gets overridden. Let's understand this better with an example, particularly on fields like featureGates and staticPodURLHeader:

Main kubelet configuration file contents:

apiVersion: kubelet.config.k8s.io/v1beta1
kind: KubeletConfiguration
port: 20250
serializeImagePulls: false
featureGates:
  AllAlpha: false
  MemoryQoS: true
staticPodURLHeader:
  kubelet-api-support:
  - "Authorization: 234APSDFA"
  - "X-Custom-Header: 123"
  custom-static-pod:
  - "Authorization: 223EWRWER"
  - "X-Custom-Header: 456"

Contents of a file in --config-dir directory:

apiVersion: kubelet.config.k8s.io/v1beta1
kind: KubeletConfiguration
featureGates:
  MemoryQoS: false
  KubeletTracing: true
  DynamicResourceAllocation: true
staticPodURLHeader:
  custom-static-pod:
  - "Authorization: 223EWRWER"
  - "X-Custom-Header: 345"

The resulting configuration will be as follows:

apiVersion: kubelet.config.k8s.io/v1beta1
kind: KubeletConfiguration
port: 20250
serializeImagePulls: false
featureGates:
  AllAlpha: false
  MemoryQoS: false
  KubeletTracing: true
  DynamicResourceAllocation: true
staticPodURLHeader:
  kubelet-api-support:
  - "Authorization: 234APSDFA"
  - "X-Custom-Header: 123"
  custom-static-pod:
  - "Authorization: 223EWRWER"
  - "X-Custom-Header: 345"

7 - Kubelet Device Manager API Versions

This page provides details of version compatibility between the Kubernetes device plugin API, and different versions of Kubernetes itself.

Compatibility matrix

v1alpha1v1beta1
Kubernetes 1.21-✓
Kubernetes 1.22-✓
Kubernetes 1.23-✓
Kubernetes 1.24-✓
Kubernetes 1.25-✓
Kubernetes 1.26-✓

Key:

  • ✓ Exactly the same features / API objects in both device plugin API and the Kubernetes version.
  • + The device plugin API has features or API objects that may not be present in the Kubernetes cluster, either because the device plugin API has added additional new API calls, or that the server has removed an old API call. However, everything they have in common (most other APIs) will work. Note that alpha APIs may vanish or change significantly between one minor release and the next.
  • - The Kubernetes cluster has features the device plugin API can't use, either because server has added additional API calls, or that device plugin API has removed an old API call. However, everything they share in common (most APIs) will work.

8 - Kubelet Systemd Watchdog

FEATURE STATE: Kubernetes v1.32 [beta] (enabled by default: true)

On Linux nodes, Kubernetes 1.32 supports integrating with systemd to allow the operating system supervisor to recover a failed kubelet. This integration is not enabled by default. It can be used as an alternative to periodically requesting the kubelet's /healthz endpoint for health checks. If the kubelet does not respond to the watchdog within the timeout period, the watchdog will kill the kubelet.

The systemd watchdog works by requiring the service to periodically send a keep-alive signal to the systemd process. If the signal is not received within a specified timeout period, the service is considered unresponsive and is terminated. The service can then be restarted according to the configuration.

Configuration

Using the systemd watchdog requires configuring the WatchdogSec parameter in the [Service] section of the kubelet service unit file:

[Service]
WatchdogSec=30s

Setting WatchdogSec=30s indicates a service watchdog timeout of 30 seconds. Within the kubelet, the sd_notify() function is invoked, at intervals of WatchdogSec ÷ 2, to send WATCHDOG=1 (a keep-alive message). If the watchdog is not fed within the timeout period, the kubelet will be killed. Setting Restart to "always", "on-failure", "on-watchdog", or "on-abnormal" will ensure that the service is automatically restarted.

Some details about the systemd configuration:

  1. If you set the systemd value for WatchdogSec to 0, or omit setting it, the systemd watchdog is not enabled for this unit.
  2. The kubelet supports a minimum watchdog period of 1.0 seconds; this is to prevent the kubelet from being killed unexpectedly. You can set the value of WatchdogSec in a systemd unit definition to a period shorter than 1 second, but Kubernetes does not support any shorter interval. The timeout does not have to be a whole integer number of seconds.
  3. The Kubernetes project suggests setting WatchdogSec to approximately a 15s period. Periods longer than 10 minutes are supported but explicitly not recommended.

Example Configuration

[Unit]
Description=kubelet: The Kubernetes Node Agent
Documentation=https://kubernetes.io/docs/home/
Wants=network-online.target
After=network-online.target

[Service]
ExecStart=/usr/bin/kubelet
# Configures the watchdog timeout
WatchdogSec=30s
Restart=on-failure
StartLimitInterval=0
RestartSec=10

[Install]
WantedBy=multi-user.target

What's next

For more details about systemd configuration, refer to the systemd documentation

9 - Node Status

The status of a node in Kubernetes is a critical aspect of managing a Kubernetes cluster. In this article, we'll cover the basics of monitoring and maintaining node status to ensure a healthy and stable cluster.

Node status fields

A Node's status contains the following information:

You can use kubectl to view a Node's status and other details:

kubectl describe node <insert-node-name-here>

Each section of the output is described below.

Addresses

The usage of these fields varies depending on your cloud provider or bare metal configuration.

  • HostName: The hostname as reported by the node's kernel. Can be overridden via the kubelet --hostname-override parameter.
  • ExternalIP: Typically the IP address of the node that is externally routable (available from outside the cluster).
  • InternalIP: Typically the IP address of the node that is routable only within the cluster.

Conditions

The conditions field describes the status of all Running nodes. Examples of conditions include:

Node conditions, and a description of when each condition applies.
Node ConditionDescription
ReadyTrue if the node is healthy and ready to accept pods, False if the node is not healthy and is not accepting pods, and Unknown if the node controller has not heard from the node in the last node-monitor-grace-period (default is 50 seconds)
DiskPressureTrue if pressure exists on the disk size—that is, if the disk capacity is low; otherwise False
MemoryPressureTrue if pressure exists on the node memory—that is, if the node memory is low; otherwise False
PIDPressureTrue if pressure exists on the processes—that is, if there are too many processes on the node; otherwise False
NetworkUnavailableTrue if the network for the node is not correctly configured, otherwise False

In the Kubernetes API, a node's condition is represented as part of the .status of the Node resource. For example, the following JSON structure describes a healthy node:

"conditions": [
  {
    "type": "Ready",
    "status": "True",
    "reason": "KubeletReady",
    "message": "kubelet is posting ready status",
    "lastHeartbeatTime": "2019-06-05T18:38:35Z",
    "lastTransitionTime": "2019-06-05T11:41:27Z"
  }
]

When problems occur on nodes, the Kubernetes control plane automatically creates taints that match the conditions affecting the node. An example of this is when the status of the Ready condition remains Unknown or False for longer than the kube-controller-manager's NodeMonitorGracePeriod, which defaults to 50 seconds. This will cause either an node.kubernetes.io/unreachable taint, for an Unknown status, or a node.kubernetes.io/not-ready taint, for a False status, to be added to the Node.

These taints affect pending pods as the scheduler takes the Node's taints into consideration when assigning a pod to a Node. Existing pods scheduled to the node may be evicted due to the application of NoExecute taints. Pods may also have tolerations that let them schedule to and continue running on a Node even though it has a specific taint.

See Taint Based Evictions and Taint Nodes by Condition for more details.

Capacity and Allocatable

Describes the resources available on the node: CPU, memory, and the maximum number of pods that can be scheduled onto the node.

The fields in the capacity block indicate the total amount of resources that a Node has. The allocatable block indicates the amount of resources on a Node that is available to be consumed by normal Pods.

You may read more about capacity and allocatable resources while learning how to reserve compute resources on a Node.

Info

Describes general information about the node, such as kernel version, Kubernetes version (kubelet and kube-proxy version), container runtime details, and which operating system the node uses. The kubelet gathers this information from the node and publishes it into the Kubernetes API.

Heartbeats

Heartbeats, sent by Kubernetes nodes, help your cluster determine the availability of each node, and to take action when failures are detected.

For nodes there are two forms of heartbeats:

  • updates to the .status of a Node
  • Lease objects within the kube-node-lease namespace. Each Node has an associated Lease object.

Compared to updates to .status of a Node, a Lease is a lightweight resource. Using Leases for heartbeats reduces the performance impact of these updates for large clusters.

The kubelet is responsible for creating and updating the .status of Nodes, and for updating their related Leases.

  • The kubelet updates the node's .status either when there is change in status or if there has been no update for a configured interval. The default interval for .status updates to Nodes is 5 minutes, which is much longer than the 40 second default timeout for unreachable nodes.
  • The kubelet creates and then updates its Lease object every 10 seconds (the default update interval). Lease updates occur independently from updates to the Node's .status. If the Lease update fails, the kubelet retries, using exponential backoff that starts at 200 milliseconds and capped at 7 seconds.

10 - Seccomp and Kubernetes

Seccomp stands for secure computing mode and has been a feature of the Linux kernel since version 2.6.12. It can be used to sandbox the privileges of a process, restricting the calls it is able to make from userspace into the kernel. Kubernetes lets you automatically apply seccomp profiles loaded onto a node to your Pods and containers.

Seccomp fields

FEATURE STATE: Kubernetes v1.19 [stable]

There are four ways to specify a seccomp profile for a pod:

pods/security/seccomp/fields.yaml 
apiVersion: v1
kind: Pod
metadata:
  name: pod
spec:
  securityContext:
    seccompProfile:
      type: Unconfined
  ephemeralContainers:
  - name: ephemeral-container
    image: debian
    securityContext:
      seccompProfile:
        type: RuntimeDefault
  initContainers:
  - name: init-container
    image: debian
    securityContext:
      seccompProfile:
        type: RuntimeDefault
  containers:
  - name: container
    image: docker.io/library/debian:stable
    securityContext:
      seccompProfile:
        type: Localhost
        localhostProfile: my-profile.json

The Pod in the example above runs as Unconfined, while the ephemeral-container and init-container specifically defines RuntimeDefault. If the ephemeral or init container would not have set the securityContext.seccompProfile field explicitly, then the value would be inherited from the Pod. The same applies to the container, which runs a Localhost profile my-profile.json.

Generally speaking, fields from (ephemeral) containers have a higher priority than the Pod level value, while containers which do not set the seccomp field inherit the profile from the Pod.

The following values are possible for the seccompProfile.type:

Unconfined
The workload runs without any seccomp restrictions.
RuntimeDefault
A default seccomp profile defined by the container runtime is applied. The default profiles aim to provide a strong set of security defaults while preserving the functionality of the workload. It is possible that the default profiles differ between container runtimes and their release versions, for example when comparing those from CRI-O and containerd.
Localhost
The localhostProfile will be applied, which has to be available on the node disk (on Linux it's /var/lib/kubelet/seccomp). The availability of the seccomp profile is verified by the container runtime on container creation. If the profile does not exist, then the container creation will fail with a CreateContainerError.

Localhost profiles

Seccomp profiles are JSON files following the scheme defined by the OCI runtime specification. A profile basically defines actions based on matched syscalls, but also allows to pass specific values as arguments to syscalls. For example:

{
  "defaultAction": "SCMP_ACT_ERRNO",
  "defaultErrnoRet": 38,
  "syscalls": [
    {
      "names": [
        "adjtimex",
        "alarm",
        "bind",
        "waitid",
        "waitpid",
        "write",
        "writev"
      ],
      "action": "SCMP_ACT_ALLOW"
    }
  ]
}

The defaultAction in the profile above is defined as SCMP_ACT_ERRNO and will return as fallback to the actions defined in syscalls. The error is defined as code 38 via the defaultErrnoRet field.

The following actions are generally possible:

SCMP_ACT_ERRNO
Return the specified error code.
SCMP_ACT_ALLOW
Allow the syscall to be executed.
SCMP_ACT_KILL_PROCESS
Kill the process.
SCMP_ACT_KILL_THREAD and SCMP_ACT_KILL
Kill only the thread.
SCMP_ACT_TRAP
Throw a SIGSYS signal.
SCMP_ACT_NOTIFY and SECCOMP_RET_USER_NOTIF.
Notify the user space.
SCMP_ACT_TRACE
Notify a tracing process with the specified value.
SCMP_ACT_LOG
Allow the syscall to be executed after the action has been logged to syslog or auditd.

Some actions like SCMP_ACT_NOTIFY or SECCOMP_RET_USER_NOTIF may be not supported depending on the container runtime, OCI runtime or Linux kernel version being used. There may be also further limitations, for example that SCMP_ACT_NOTIFY cannot be used as defaultAction or for certain syscalls like write. All those limitations are defined by either the OCI runtime (runc, crun) or libseccomp.

The syscalls JSON array contains a list of objects referencing syscalls by their respective names. For example, the action SCMP_ACT_ALLOW can be used to create a whitelist of allowed syscalls as outlined in the example above. It would also be possible to define another list using the action SCMP_ACT_ERRNO but a different return (errnoRet) value.

It is also possible to specify the arguments (args) passed to certain syscalls. More information about those advanced use cases can be found in the OCI runtime spec and the Seccomp Linux kernel documentation.

Further reading