Jenkins ssh-agent plug in

https://support.cloudbees.com/hc/en-us/articles/222838288-SSH-Credentials-Management-with-Jenkins

https://support.cloudbees.com/hc/en-us/articles/222978868-How-to-Connect-to-Remote-SSH-Agents-#:~:text=In%20Jenkins%3A&text=Choose%20SSH%20Username%20with%20private,useful%20Description%20for%20this%20credential.

Jenkins ssh-agent only takes user and private key as credentials for build jobs, not user and password, otherwise you will get error below:

FATAL: [ssh-agent] Could not find specified credentials

[ssh-agent] Looking for ssh-agent implementation... 

[ssh-agent] Exec ssh-agent (binary ssh-agent on a remote machine)

.....

Permission denied, please try again. 

Permission denied (publickey,password).

Target remote server must have completed setting for ssh login via private key before. (public key in ~/.ssh/authorized_keys)

Jenkins on K8S

Jenkins can deploy via Rancher catalog or Helm chart.

Jenkins K8S deployment will need persistent storage. Jenkins service can be exposed or access via ingress rules.

Jenkins master -> Credentials -> add Credentials -> kind: Kubernetes Service Account

Manage Jenkins -> Manager Nodes and Cloud -> Configure Cloud (for new version of Jenkins)

• Test Connection 
• Pod Retention: Never (Jenkins slave pod will be terminated after build job completes)
• Slave pod will be placed into jenkins namespace by default

Jenkins slave on K8S

Use K8S as Jenkins slave/agent via Jenkins kubernetes plugin. (continuous integration)

• From Jenkins master, install Kubernetes plugin (This plugin integrates Jenkins with Kubernetes)
• Credentials -> Add Credentials -> Kind: Secret file (kubeconfig file from K8S)
• Manage Jenkins -> Manage Nodes and Cloud -> Configure Clouds (new version of Jenkins) 

1. Credentials: select Secret file name just created from drop-down menu, and Test Connection
2. Jenkins URL and tunnel match Jenkins master. No need https:// for tunnel
3. Pod label will be used for slave pod label in K8S
4. Pod Template: name will be prefix for slave pod name prefix in K8S; blank namespace will create slave pod in default namespace in K8S;
5. Pod Template -> Labels is critical, which help Jenkins master decides which builder will be used for the build job. 
6. Pod Template: Usage: only builds job with label expression matching this label
7. Define container template details: name: jnlp; docker image: jenkins/jnlp-slave:latest; working directory: /home/jenkins/; also environment variable value pair: {JENKINS_URL: http://jenkins-master:8080} (or adding other agent as Pod Template)

When creating a new build job, check Restrict where this project can be run, enter Label expression: Pod template label in previous step 5, so Jenkins master will use K8S pod to execute build task.

Jenkins slave started in K8S, then terminated in K8S after build job completes with proper pod retention setting in Pod Template.

Use K8S as Jenkins slave via Jenkin Kubernetes Continuous Deploy Plugin (continuous deployment on kubernetes)

• From Jenkins master, install Kubernetes Continuous Deploy Plugin
• Credentials -> Add Credentials -> Kind: Kubernetes Configuration (KubeConfig); directly copy content of  kubeconfig file into textext. The ID of this Jenkins credential will match value of  kubeconfigId in file: Jenkinsfile, which is in source code Repositories, such as github, or private repository
• If Kubernetes plugin is already configured, no more configuration is required.

When creating a new pipeline in Jenkins, in Pipeline section: select Pipeline script from SCM, then select proper SCM, repository and branch. Default script path is: Jenkinsfile

Jenkins pipeline checkout source from SCM, and start slave pod on K8S to build images and upload container images into registry such Docker hub or any private/public registry defined in Jenkinsfile from SCM. Then slave pod on K8S is terminated with the completion of Continuous Integration/Delivery process. Finally, pipeline script will deploy app Pod via manifest in Jenkinsfile, always pulling container images from registry.