DevOps - How to use a VM as a Jenkins agents

Background

I'm testing a standalone application that needs to be installed, and when it comes up on http://localhost:8080, I need to test various aspects of it. To run it automatically on Jenkins, I create a Dockerfile in an online Git repository that creates a Red Hat Enterprise Linux (RHEL) or Fedora image and install my application on it. I use Podman or Docker in a virtual machine (VM) to install the required container image. Once the application is installed and running in a container, I run my automated tests.

Jenkins and agents

Jenkins runs its jobs on agents, choosing them based on availability. You can add a Jenkins agent manually, and Jenkins doesn't know or care whether an agent is a physical or virtual machine.

To set this up, you must configure a VM before adding it as a Jenkins agent.

First, add a new user named jenkins to the VM, and then log in as that user:

$ sudo useradd jenkin
$ su - jenkins        

Create a directory named /var/lib/jenkins on the VM:

$ mkdir /var/lib/jenkins        

Generate private and public SSH keys:

$ ssh-keygen -b 2048 -t rsa        

This command generates two keys. Copy the contents of id_rsa.pub to the ~/.ssh/authorized_keys file:

$ cat ~/.ssh/id_rsa.pub >> ~/.ssh/authorized_keys        

The contents of the private key (~/.ssh/id_rsa) are required when adding credentials in Jenkins.

Add a VM as a Jenkins agent

To add your VM as a node in Jenkins, go to the Manage Jenkins panel and select Manage Nodes.

Provide a name for the node, select Permanent Agent, and then click OK.

For the Remote root directory, provide /var/lib/jenkins. This is the default directory where Jenkins creates its workspace. Make sure that this directory exists on the VM.

For Launch method, select Launch agent agents via SSH.

For Host, provide the IP address or hostname of the VM.

Under Host Key Verification Strategy, select Manually trusted key Verification Strategy.

Finally, add the credentials for the VM. Click on Add credentials and select SSH Username with the private key. Give the key a name, and add the contents of ~/.ssh/id_rsa as the Private Key value.

Set the node as an agent

You are almost done! You need to tell Jenkins to use this node as a Jenkins agent. Edit the Jenkins job, and under Source Code Management, select Restrict where this project can be run, and provide the node name in the Label Expression field. Save the job.

Start your build

With the virtual agent configured and ready to go, you can switch back to your VM host (your laptop or workstation running the VM, not the VM itself).

In your local instance of Jenkins, start a build. Jenkins executes all commands on your VM.