How to add Jenkins Slave with Jenkins Master

Last Updated on April 14, 2023 by cscontents

Table of Contents

Introduction

Jenkins is a popular pipeline tool. It is extensively used for creating CI (continuous integration) pipelines. Not only CI pipeline, Jenkins can be used for CD (Continuous Delivery) pipeline also. Together we can say Jenkins is used for creating CI/CD pipelines.

In this tutorial, we will discuss Jenkins master-slave architecture. We will see how we can add a slave machine to Jenkins master. We can add multiple slave machines. For example, for each environment, we can have one slave machine.

It is advisable to have Jenkins master-slave architecture if your project is big and has many applications. It gives many flexibilities like –

We can easily separate out Jenkins job executor for the different environments using slave/agent for each env.
Adding slave machines increases the number of job executors.
It reduces the load from Jenkins master. In master-slave architecture, the master server orchestrates the slave machines, etc.

Prerequisite

We should have a Jenkins master server where Jenkins should be up & running. Here we assume that Jenkins is running with the Linux user ‘jenkins’. And Jenkins dashboard should be accessible.
We should have another machine that will work as a Jenkins slave.
Go through the below document to get a basic understanding of how to connect with a Linux machine using SSH key-pair https://cscontents.com/how-to-do-ssh-to-remote-linux-machine-using-ssh-key/

Steps to add Jenkins slave

Below is a step-by-step guide to adding a Jenkins slave machine with Jenkins master.

Step 1: Open port 22 (SSH)

Allow network connectivity over port 22 (SSH) between the Jenkins master & slave node.

Step 2: Install JDK

Install JDK in the slave machine. Follow the below article for installing JDK.

https://cscontents.com/java-installation-on-ubuntu-rhel-centos/

Step 3: Create a Linux user in the slave machine

Create a dedicated user in the slave node that will be used by the Jenkins master to connect with the slave. For example, we will create a user ‘jenkins’.

Run the below command in the slave machine.

sudo useradd jenkins

Step 4: Identify a directory on the slave for workspace

Identify a directory in the slave node that will be used by the Jenkins master. That directory will be known as the “remote directory”. You need to enter this “remote directory” in step 9.

Step 5: Create SSH key-pair in Jenkins master machine

Create SSH key-par in Jenkins Master (public key & private key). To create the SSH key pair in the Jenkins master node, first switch to the jenkins user which is used by the Jenkins service. Run the below command to switch to jenkins user.

sudo su -s /bin/bash jenkins

Now, run the below command.

ssh-keygen

Keep on pressing enter, by default, it will create the private key in the id_rsa file & public key in the id_rsa.pub file.

total 12
-rw-------. 1 jenkins jenkins 2610 Apr  5 14:25 id_rsa
-rw-r--r--. 1 jenkins jenkins  575 Apr  5 14:25 id_rsa.pub
-rw-r--r--. 1 jenkins jenkins  175 Apr  5 15:06 known_hosts

Step 6: Copy the public key into the slave machine

In step-3 we created a user (jenkins) in the slave node, and with that, it automatically created /home/jenkins directory in the slave machine. Under /home/jenkins directory create a .ssh folder and a file with the name authorized_keys.

Run the below commands one by one.

cd /home/jenkins

mkdir .ssh

touch authorized_keys

Copy the public key (id_rsa.pub file which was generated in step-5) from the master node and paste it into the /home/jenkins/.ssh/authorized_keys file of the slave node.

Note: Here owner & group of the authorized_keys file & .ssh folder should be jenkins

Step 7: Create credentials in the Jenkins dashboard

Create a credential of type “SSH Username with private key” in the dashboard of Jenkins master. Here use the username created in step-3 and use the private key that was generated in step-5.

Step 8: Update the known_hosts file in Jenkins master

Log into the Jenkins master machine & switch to the ‘jenkins‘ user which is used by the Jenkins service. Now, SSH into the Jenkins slave node. It will add the fingerprint to the /home/jenkins/.ssh/known_hosts file of the Jenkins master node.

Run the below commands.

sudo su -s /bin/bash jenkins

If you are unable to switch to the ‘jenkins’ user, please use any other user, no issue with that. Here, we just want to add the fingerprint of the slave machine to the Jenkins master machine. For example, you are using a user ‘linux-user’ and as ‘linux-user‘ you run the below command, in that which will add the fingerprint to the /home/linux-user/.ssh/known_hosts file. In that case, you need to copy the content of home/linux-user/.ssh/known_hosts file and paste it into /home/jenkins/.ssh/known_hosts file of the Jenkins master server.

ssh linux-user@28.88.130.44

On the above command replace the ‘linux-user’ with any user which is created in the slave machine. And replace the IP with your slave machine’s IP.

Once we execute the above command it will ask for adding the fingerprint of the slave machine into the master server. Please type ‘yes’.

Step 9: Add the slave node from the Jenkins dashboard

Open the Jenkins Master dashboard & go to Manage Jenkins -> Manage nodes and clouds.

And click on the “New Node”.

This will open up to enter a new node name & node type.

After entering the node name & selecting the type we need to click on “Create”. Once we click on “create” it will open up for detailed configuration details of the slave node.

Once we enter all the configuration details, we need to click on the “save” button which is at the bottom.

Step 10: Launch the slave node

Once we save the configuration in step 9, it starts connecting with the slave machine. If needed we can relaunch the slave node again.

We can check the logs by clicking on the “Log” option shown in the left menu. If there is any issue or error while connecting with the slave machine it will be shown in the logs.

Software packages need to be installed on Jenkins slave

A few software packages need to be installed in the Jenkins slave machine before it is ready for use.

Java – we have already installed JDK in the previous steps.
Git – we need to install git so that it can clone the repo from the source code repository.

Thank You.

If you are interested in learning DevOps, please have a look at the below articles, which will help you greatly.