Jenkins - Other Images | Using Images

Overview
Images
Configuration and Customization
Usage
Jenkins Plug-ins

Overview

OKD provides a container image for running Jenkins. This image provides a Jenkins server instance, which can be used to set up a basic flow for continuous testing, integration, and delivery.

This image also includes a sample Jenkins job, which triggers a new build of a BuildConfig defined in OKD, tests the output of that build, and then on successful build, retags the output to indicate the build is ready for production. For more details, see the README.

OKD follows the LTS release of Jenkins. OKD provides an image containing Jenkins 2.x. A separate image with Jenkins 1.x was previously made available but is now no longer maintained.

Images

The OKD Jenkins image comes in two flavors:

RHEL 7 Based Image

The RHEL 7 image is available through the Red Hat Registry:

$ docker pull registry.access.redhat.com/openshift3/jenkins-2-rhel7

CentOS 7 Based Image

This image is available on Docker Hub:

$ docker pull openshift/jenkins-2-centos7

To use these images, you can either access them directly from these registries or push them into your OKD Docker registry. Additionally, you can create an ImageStream that points to the image, either in your Docker registry or at the external location. Your OKD resources can then reference the ImageStream. You can find example ImageStream definitions for all the provided OKD images.

Configuration and Customization

Authentication

You can manage Jenkins authentication in two ways:

OKD OAuth authentication provided by the OpenShift Login plug-in.
Standard authentication provided by Jenkins

OKD OAuth authentication

OAuth authentication is activated by configuring the Configure Global Security panel in the Jenkins UI, or by setting the OPENSHIFT_ENABLE_OAUTH environment variable on the Jenkins Deployment Config to anything other than false. This activates the OpenShift Login plug-in, which retrieves the configuration information from pod data or by interacting with the OKD API server.

Valid credentials are controlled by the OKD identity provider. For example, if Allow All is the default identity provider, you can provide any non-empty string for both the user name and password.

Jenkins supports both browser and non-browser access.

Valid users are automatically added to the Jenkins authorization matrix at log in, where OKD Roles dictate the specific Jenkins permissions the user will have.

Users with the admin role will have the traditional Jenkins administrative user permissions. Users with the edit or view role will have progressively less permissions. See the Jenkins image source repository README for the specifics on the OpenShift roles to Jenkins permissions mappings.

The admin user that is pre-populated in the OKD Jenkins image with administrative privileges will not be given those privileges when OKD OAuth is used, unless the OKD cluster administrator explicitly defines that user in the OKD identity provider and assigns the admin role to the user.

Jenkins' users permissions can be changed after the users are initially established. The OpenShift Login plug-in polls the OKD API server for permissions and updates the permissions stored in Jenkins for each user with the permissions retrieved from OKD. If the Jenkins UI is used to update permissions for a Jenkins user, the permission changes are overwritten the next time the plug-in polls OKD.

You can control how often the polling occurs with the OPENSHIFT_PERMISSIONS_POLL_INTERVAL environment variable. The default polling interval is five minutes.

The easiest way to create a new Jenkins service using OAuth authentication is to use a template as described below.

Jenkins Standard Authentication

Jenkins authentication is used by default if the image is run directly, without using a template.

The first time Jenkins starts, the configuration is created along with the administrator user and password. The default user credentials are admin and password. Configure the default password by setting the JENKINS_PASSWORD environment variable when using (and only when using) standard Jenkins authentication.

To create a new Jenkins application using standard Jenkins authentication:

$ oc new-app -e \
    JENKINS_PASSWORD=<password> \
    openshift/jenkins-2-centos7

Environment Variables

The Jenkins server can be configured with the following environment variables:

OPENSHIFT_ENABLE_OAUTH (default: false)

Determines whether the OpenShift Login plug-in manages authentication when logging into Jenkins. To enable, set to true.
JENKINS_PASSWORD (default: password)

The password for the admin user when using standard Jenkins authentication. Not applicable when OPENSHIFT_ENABLE_OAUTH is set to true.
OPENSHIFT_JENKINS_JVM_ARCH

Set to x86_64 or i386 to override the JVM used to host Jenkins. For memory efficiency, by default the Jenkins image dynamically uses a 32-bit JVM if running in a container with a memory limit under 2GiB.
JAVA_MAX_HEAP_PARAM
CONTAINER_HEAP_PERCENT (default: 0.5, or 50%)
JENKINS_MAX_HEAP_UPPER_BOUND_MB

These values control the maximum heap size of the Jenkins JVM. If JAVA_MAX_HEAP_PARAM is set (example setting: -Xmx512m), its value takes precedence. Otherwise, the maximum heap size is dynamically calculated as CONTAINER_HEAP_PERCENT% (example setting: 0.5, or 50%) of the container memory limit, optionally capped at JENKINS_MAX_HEAP_UPPER_BOUND_MB MiB (example setting: 512).

By default, the maximum heap size of the Jenkins JVM is set to 50% of the container memory limit with no cap.
JAVA_INITIAL_HEAP_PARAM
CONTAINER_INITIAL_PERCENT

These values control the initial heap size of the Jenkins JVM. If JAVA_INITIAL_HEAP_PARAM is set (example setting: -Xms32m), its value takes precedence. Otherwise, the initial heap size may be dynamically calculated as CONTAINER_INITIAL_PERCENT% (example setting: 0.1, or 10%) of the dynamically calculated maximum heap size.

By default, the initial heap sizing is left to the JVM.
CONTAINER_CORE_LIMIT

If set, specifies an integer number of cores used for sizing numbers of internal JVM threads. Example setting: 2.
JAVA_TOOL_OPTIONS (default: -XX:+UnlockExperimentalVMOptions -XX:+UseCGroupMemoryLimitForHeap -Dsun.zip.disableMemoryMapping=true)

Specifies options to be heeded by all JVMs running in this container. It is not recommended to override this.
JAVA_GC_OPTS (default: -XX:+UseParallelGC -XX:MinHeapFreeRatio=5 -XX:MaxHeapFreeRatio=10 -XX:GCTimeRatio=4 -XX:AdaptiveSizePolicyWeight=90)

Specifies Jenkins JVM garbage collection parameters. It is not recommended to override this.
JENKINS_JAVA_OVERRIDES

Specifies additional options for the Jenkins JVM. These options are appended to all other options, including the Java options above, and may be used to override any of them if necessary. Separate each additional option with a space; if any option contains space characters, escape them with a backslash. Example settings: -Dfoo -Dbar; -Dfoo=first\ value -Dbar=second\ value.
JENKINS_OPTS

Specifies arguments to Jenkins.
INSTALL_PLUGINS

Specifies additional Jenkins plug-ins to install when the container is first run or when OVERRIDE_PV_PLUGINS_WITH_IMAGE_PLUGINS is set to true (see below). Plug-ins are specified as a comma-delimited list of name:version pairs. Example setting: git:3.7.0,subversion:2.10.2.
OPENSHIFT_PERMISSIONS_POLL_INTERVAL (default: 300000 - 5 minutes)

Specifies in milliseconds how often the OpenShift Login plug-in polls OKD for the permissions associated with each user defined in Jenkins.
OVERRIDE_PV_CONFIG_WITH_IMAGE_CONFIG (default: false)

When running this image with an OKD persistent volume for the Jenkins config directory, the transfer of configuration from the image to the persistent volume is only done the first startup of the image as the persistent volume is assigned by the persistent volume claim creation. If you create a custom image that extends this image and updates configuration in the custom image after the initial startup, by default it will not be copied over, unless you set this environment variable to true.
OVERRIDE_PV_PLUGINS_WITH_IMAGE_PLUGINS (default: false)

When running this image with an OKD persistent volume for the Jenkins config directory, the transfer of plugins from the image to the persistent volume is only done the first startup of the image as the persistent volume is assigned by the persistent volume claim creation. If you create a custom image that extends this image and updates plugins in the custom image after the initial startup, by default they will not be copied over, unless you set this environment variable to true.

Cross Project Access

If you are going to run Jenkins somewhere other than as a deployment within your same project, you will need to provide an access token to Jenkins to access your project.

Identify the secret for the service account that has appropriate permissions to access the project Jenkins needs to access:

$ oc describe serviceaccount jenkins
Name:       default
Labels:     <none>
secrets:    {  jenkins-token-uyswp    }
            {  jenkins-dockercfg-xcr3d    }
Tokens:     jenkins-token-izv1u
            jenkins-token-uyswp

In this case the secret is named jenkins-token-uyswp

Retrieve the token from the secret:

$ oc describe secret <secret name from above> # for example, jenkins-token-uyswp
Name:       jenkins-token-uyswp
Labels:     <none>
Annotations:    kubernetes.io/service-account.name=jenkins,kubernetes.io/service-account.uid=32f5b661-2a8f-11e5-9528-3c970e3bf0b7
Type:   kubernetes.io/service-account-token
Data
====
ca.crt: 1066 bytes
token:  eyJhbGc..<content cut>....wRA

The token field contains the token value Jenkins needs to access the project.

Volume Mount Points

The Jenkins image can be run with mounted volumes to enable persistent storage for the configuration:

/var/lib/jenkins - This is the data directory where Jenkins stores configuration files including job definitions.

Customizing the Jenkins Image through Source-To-Image

To customize the official OKD Jenkins image, you have two options:

Use Docker layering.
Use the image as a Source-To-Image builder, described here.

You can use S2I to copy your custom Jenkins Jobs definitions, additional plug-ins or replace the provided config.xml file with your own, custom, configuration.

In order to include your modifications in the Jenkins image, you need to have a Git repository with the following directory structure:

plugins: This directory contains those binary Jenkins plug-ins you want to copy into Jenkins.
plugins.txt: This file lists the plug-ins you want to install:

pluginId:pluginVersion

configuration/jobs: This directory contains the Jenkins job definitions.
configuration/config.xml: This file contains your custom Jenkins configuration.

The contents of the configuration/ directory will be copied into the /var/lib/jenkins/ directory, so you can also include additional files, such as credentials.xml, there.

The following is an example build configuration that customizes the Jenkins image in OKD:

apiVersion: v1
kind: BuildConfig
metadata:
  name: custom-jenkins-build
spec:
  source:                       (1)
    git:
      uri: https://github.com/custom/repository
    type: Git
  strategy:                     (2)
    sourceStrategy:
      from:
        kind: ImageStreamTag
        name: jenkins:latest
        namespace: openshift
    type: Source
  output:                       (3)
    to:
      kind: ImageStreamTag
      name: custom-jenkins:latest

1	The `source` field defines the source Git repository with the layout described above.
2	The `strategy` field defines the original Jenkins image to use as a source image for the build.
3	The `output` field defines the resulting, customized Jenkins image you can use in deployment configuration instead of the official Jenkins image.

Configuring the Jenkins Kubernetes Plug-in

The OKD Jenkins image includes the pre-installed Kubernetes plug-in that allows Jenkins agents to be dynamically provisioned on multiple container hosts using Kubernetes and OKD.

To use the Kubernetes plug-in, OKD provides five images suitable for use as Jenkins agents: the Base, Maven, and Node.js images. See Jenkins Agents for more information.

the jenkins-slave-maven-* and jenkins-slave-nodejs-* images are being marked as deprecated during the v3.10 release cycle. The images will still exist in the interim so users can migrate their applications to the newer jenkins-agent-maven-* and jenkins-agent-nodejs-* images.

Both the Maven and Node.js agent images are automatically configured as Kubernetes Pod Template images within the OKD Jenkins image’s configuration for the Kubernetes plug-in. That configuration includes labels for each of the images that can be applied to any of your Jenkins jobs under their "Restrict where this project can be run" setting. If the label is applied, execution of the given job will be done under an OKD pod running the respective agent image.

The Jenkins image also provides auto-discovery and auto-configuration of additional agent images for the Kubernetes plug-in. With the OpenShift Sync plug-in, the Jenkins image on Jenkins start-up searches within the project that it is running, or the projects specifically listed in the plug-in’s configuration for the following:

Image streams that have the label role set to jenkins-slave.
Image stream tags that have the annotation role set to jenkins-slave.
ConfigMaps that have the label role set to jenkins-slave.

When it finds an image stream with the appropriate label, or image stream tag with the appropriate annotation, it generates the corresponding Kubernetes plug-in configuration so you can assign your Jenkins jobs to run in a pod running the container image provided by the image stream.

The name and image references of the image stream or image stream tag are mapped to the name and image fields in the Kubernetes plug-in pod template. You can control the label field of the Kubernetes plug-in pod template by setting an annotation on the image stream or image stream tag object with the key slave-label. Otherwise, the name is used as the label.

When it finds a ConfigMap with the appropriate label, it assumes that any values in the key-value data payload of the ConfigMap contains XML consistent with the config format for Jenkins and the Kubernetes plug-in pod templates. A key differentiator to note when using ConfigMaps, instead of image streams or image stream tags, is that you can control all the various fields of the Kubernetes plug-in pod template.

The following is an example ConfigMap:

kind: ConfigMap
apiVersion: v1
metadata:
  name: jenkins-agent
  labels:
    role: jenkins-slave
data:
  template1: |-
    <org.csanchez.jenkins.plugins.kubernetes.PodTemplate>
      <inheritFrom></inheritFrom>
      <name>template1</name>
      <instanceCap>2147483647</instanceCap>
      <idleMinutes>0</idleMinutes>
      <label>template1</label>
      <serviceAccount>jenkins</serviceAccount>
      <nodeSelector></nodeSelector>
      <volumes/>
      <containers>
        <org.csanchez.jenkins.plugins.kubernetes.ContainerTemplate>
          <name>jnlp</name>
          <image>openshift/jenkins-agent-maven-35-centos7:v3.10</image>
          <privileged>false</privileged>
          <alwaysPullImage>true</alwaysPullImage>
          <workingDir>/tmp</workingDir>
          <command></command>
          <args>${computer.jnlpmac} ${computer.name}</args>
          <ttyEnabled>false</ttyEnabled>
          <resourceRequestCpu></resourceRequestCpu>
          <resourceRequestMemory></resourceRequestMemory>
          <resourceLimitCpu></resourceLimitCpu>
          <resourceLimitMemory></resourceLimitMemory>
          <envVars/>
        </org.csanchez.jenkins.plugins.kubernetes.ContainerTemplate>
      </containers>
      <envVars/>
      <annotations/>
      <imagePullsecrets/>
      <nodeProperties/>
    </org.csanchez.jenkins.plugins.kubernetes.PodTemplate>

After startup, the OpenShift Sync plug-in monitors the API server of OKD for updates to ImageStreams, ImageStreamTags, and ConfigMaps and adjusts the configuration of the Kubernetes plug-in.

In particular, the following rules will apply:

Removal of the label or annotation from the ConfigMap, ImageStream, or ImageStreamTag will result in the deletion of any existing PodTemplate from the configuration of the Kubernetes plug-in.
Similarly, if those objects are removed, the corresponding configuration is removed from the Kubernetes plug-in.
Conversely, either the creation of appropriately labeled or annotated ConfigMap, ImageStream, or ImageStreamTag objects, or the adding of labels after their initial creation, leads to the creation of a PodTemplate in the Kubernetes-plugin configuration.
In the case of the PodTemplate via ConfigMap form, changes to the ConfigMap data for the PodTemplate will be applied to the PodTemplate settings in the Kubernetes plug-in configuration, and will override any changes made to the PodTemplate through the Jenkins UI in the interim between changes to the ConfigMap.

To use a container image as a Jenkins agent, the image must run the slave agent as an entrypoint. For more details about this, refer to the official Jenkins documentation.

Permission Considerations

In the previous ConfigMap example, the <serviceAccount> element of the Pod Template XML is the OKD Service Account used for the resulting Pod. The service account credentials mounted into the Pod, with permissions associated with the service account, control which operations against the OKD master are allowed from the Pod.

Consider the following with service accounts used for the Pod, launched by the Kubernetes Plug-in running in the OKD Jenkins image:

If you use the example template for Jenkins provided by OKD, the jenkins service account is defined with the edit role for the project Jenkins is running in, and the master Jenkins Pod has that service account mounted.
The two default Maven and NodeJS Pod Templates injected into the Jenkins configuration are also set to use the same service account as the master.
Any Pod Templates auto-discovered by the OpenShift Sync plug-in as a result of Image streams or Image stream tags having the required label or annotations have their service account set to the master’s service account.
For the other ways you can provide a Pod Template definition into Jenkins and the Kubernetes plug-in, you have to explicitly specify the service account to use.
Those other ways include the Jenkins console, the podTemplate pipeline DSL provided by the Kubernetes plug-in, or labeling a ConfigMap whose data is the XML configuration for a Pod Template.
If you do not specify a value for the service account, the default service account is used.
You need to ensure that whatever service account is used has the necessary permissions, roles, and so on defined within OKD to manipulate whatever projects you choose to manipulate from the within the Pod

Usage

Creating a Jenkins Service from a Template

Templates provide parameter fields to define all the environment variables (password) with predefined defaults. OKD provides templates to make creating a new Jenkins service easy. The Jenkins templates should have been registered in the default openshift project by your cluster administrator during the initial cluster setup. See Loading the Default Image Streams and Templates for more details, if required.

a deployment configuration and a service.

A pod may be restarted when it is moved to another node, or when an update of the deployment configuration triggers a redeployment.

jenkins-persistent uses a persistent volume store. Data survives a pod restart.

must instantiate the template to be able to use Jenkins:

Using the Jenkins Kubernetes Plug-in

Creating a New Jenkins Service

In the below sample, the openshift-jee-sample BuildConfig causes a Jenkins maven agent Pod to be dynamically provisioned. The Pod clones some Java source, builds a WAR file, then causes a second BuildConfig (openshift-jee-sample-docker) to run to layer the newly created WAR file into a Docker image.

A fuller sample which achieves a similar goal is available here.

Example 1. Example BuildConfig using the Jenkins Kubernetes Plug-in

kind: List
apiVersion: v1
items:
- kind: ImageStream
  apiVersion: v1
  metadata:
    name: openshift-jee-sample
- kind: BuildConfig
  apiVersion: v1
  metadata:
    name: openshift-jee-sample-docker
  spec:
    strategy:
      type: Docker
    source:
      type: Docker
      dockerfile: |-
        FROM openshift/wildfly-101-centos7:latest
        COPY ROOT.war /wildfly/standalone/deployments/ROOT.war
        CMD $STI_SCRIPTS_PATH/run
      binary:
        asFile: ROOT.war
    output:
      to:
        kind: ImageStreamTag
        name: openshift-jee-sample:latest
- kind: BuildConfig
  apiVersion: v1
  metadata:
    name: openshift-jee-sample
  spec:
    strategy:
      type: JenkinsPipeline
      jenkinsPipelineStrategy:
        jenkinsfile: |-
          node("maven") {
            sh "git clone https://github.com/openshift/openshift-jee-sample.git ."
            sh "mvn -B -Popenshift package"
            sh "oc start-build -F openshift-jee-sample-docker --from-file=target/ROOT.war"
          }
    triggers:
    - type: ConfigChange

It is also possible to override the specification of the dynamically created Jenkins agent Pod. The following is a modification to the above example which overrides the container memory and specifies an environment variable:

Example 2. Example BuildConfig using the Jenkins Kubernetes Plug-in, specifying memory limit and environment variable

kind: BuildConfig
apiVersion: v1
metadata:
  name: openshift-jee-sample
spec:
  strategy:
    type: JenkinsPipeline
    jenkinsPipelineStrategy:
      jenkinsfile: |-
        podTemplate(label: "mypod", (1)
                    cloud: "openshift", (2)
                    inheritFrom: "maven", (3)
                    containers: [
            containerTemplate(name: "jnlp", (4)
                              image: "openshift/jenkins-agent-maven-35-centos7:v3.10", (5)
                              resourceRequestMemory: "512Mi", (6)
                              resourceLimitMemory: "512Mi", (7)
                              envVars: [
              envVar(key: "CONTAINER_HEAP_PERCENT", value: "0.25") (8)
            ])
          ]) {
          node("mypod") { (9)
            sh "git clone https://github.com/openshift/openshift-jee-sample.git ."
            sh "mvn -B -Popenshift package"
            sh "oc start-build -F openshift-jee-sample-docker --from-file=target/ROOT.war"
          }
        }
  triggers:
  - type: ConfigChange

1	A new Pod template called "mypod" is defined on-the-fly. The new Pod template name is referenced in the node stanza below.
2	The "cloud" value must be set to "openshift".
3	The new Pod template can inherit its configuration from an existing Pod template. In this case, we inherit from the "maven" Pod template which is pre-defined by OKD.
4	We are overriding values in the pre-existing Container, therefore we must specify it by name. All Jenkins agent images shipped with OKD use the Container name "jnlp".
5	The Container image must be re-specified. This is a known issue.
6	A memory request of 512Mi is specified.
7	A memory limit of 512Mi is specified.
8	An environment variable CONTAINER_HEAP_PERCENT, with value "0.25", is specified.
9	The node stanza references the name of the Pod template newly defined above.

For more information on Kubernetes plug-in configuration, see the Kubernetes plug-in documentation.

Memory Requirements

When deployed by the provided Jenkins Ephemeral or Jenkins Persistent templates, the default memory limit is 512MiB.

See Sizing OpenJDK on OKD for background information on tuning the JVM used by Jenkins.

For memory efficiency, by default the Jenkins image dynamically uses a 32-bit JVM if running in a container with a memory limit under 2GiB. This behavior can be overridden by the OPENSHIFT_JENKINS_JVM_ARCH environment variable.

By default the Jenkins JVM uses 50% of the container memory limit for its heap. This value can be modified by the CONTAINER_HEAP_PERCENT environment variable. It can also be capped at an upper limit or overridden entirely. See Environment Variables for more details.

Consider that by default all other processes executed in the Jenkins container, such as shell scripts or oc commands run locally from pipelines, are not likely to be able to use more than the remaining 256MiB memory combined without provoking an OOM kill. It is therefore highly recommended that pipelines run external commands in a agent container wherever possible.

It is recommended to specify memory request and limit values on agent containers created by the Jenkins Kubernetes Plug-in. As admin, defaults can be set on a per-agent image basis through the Jenkins configuration. The memory request and limit can also be overridden on a per-container basis as documented above.

You can increase the amount of memory available to Jenkins by overriding the MEMORY_LIMIT paramenter when instantiating the Jenkins Ephemeral or Jenkins Persistent template.

Jenkins Plug-ins

The following plug-ins are provided to integrate Jenkins with OKD. They are available by default in the Jenkins image.

OKD Client Plug-in

The OKD Client Plug-in aims to provide a readable, concise, comprehensive, and fluent Jenkins Pipeline syntax for rich interactions with OKD. The plug-in leverages the oc binary, which must be available on the nodes executing the script.

This plug-in is fully supported and is included in the Jenkins image. It provides:

A Fluent-style syntax for use in Jenkins Pipelines.
Use of and exposure to any option available with oc.
Integration with Jenkins credentials and clusters.
Continued support for classic Jenkins Freestyle jobs.

See the OpenShift Pipeline Builds tutorial and the plug-in’s README for more information.

OKD Pipeline Plug-in

The OKD Pipeline Plug-in is a prior integration between Jenkins and OKD which provides less functionality than the OKD Client Plug-in. It remains available and supported.

See the plug-in’s README for more information.

OKD Sync Plug-in

To facilitate OKD Pipeline build strategy for integration between Jenkins and OKD, the OpenShift Sync Plug-in monitors the API server of OKD for updates to BuildConfigs and Builds that employ the Pipeline strategy and either creates Jenkins Pipeline projects (when a BuildConfig is created) or starts jobs in the resulting projects (when a Build is started).

As noted in Configuring the Jenkins Kubernetes Plug-in, this plug-in can create PodTemplate configurations for the Kubernetes plug-in based on specifically cited ImageStream, ImageStreamTag, or ConfigMap objects defined in OKD.

This plug-in can now take secret objects with a label key of credential.sync.jenkins.openshift.io and label value of true and construct Jenkins credentials which are placed in the default global domain within the Jenkins credentials hierarchy. The ID of the credential will be composed of the namespace the secret is defined in, a hyphen (-), followed by the name of the secret.

Similar to the handling of ConfigMaps for PodTemplates, the secret object defined in OKD is considered the master configuration. Any subsequent updates to the object in OKD will be applied to the Jenkins credential (overwriting any changes to the credential made in the interim).

Removal of the credential.sync.jenkins.openshift.io property, setting of that property to something other than true, or deletion of the secret in OKD will result in deletion of the associated credential in Jenkins.

The type of secret will be mapped to the jenkins credential type as follows:

With Opaque type secret objects the plug-in looks for username and password in the data section and constructs a Jenkins UsernamePasswordCredentials credential. Remember, in OKD the password field can be either an actual password or the user’s unique token. If those are not present, it will look for the ssh-privatekey field and create a Jenkins BasicSSHUserPrivateKey credential.
With kubernetes.io/basic-auth type `secret`objects the plug-in creates a Jenkins UsernamePasswordCredentials credential.
With kubernetes.io/ssh-auth type secret objects the plug-in creates a Jenkins BasicSSHUserPrivateKey credential.

Kubernetes Plug-in

The Kubernetes plug-in is used to run Jenkins agents as pods on your cluster. The auto-configuration of the Kubernetes plug-in is described in Using the Jenkins Kubernetes Plug-in.