Module 16: Jenkins Master/Slave Set-up

How to configure Jenkins master/slave nodes in Ubuntu | Jenkins master and slave setup on Ubuntu

Jenkins has powerful feature of master slave architecture which enables distributed builds. This article we will learn how to establish Jenkins Master and slave nodes on Ubuntu machines.

 Few info on Jenkins master and slave setup:

Jenkins Master

Your main Jenkins server is the Master. The Master’s job is to handle:

• Scheduling build jobs.
• Dispatching builds to the slaves for the actual execution.
• Monitor the slaves (possibly taking them online and offline as required).
• Recording and presenting the build results.
• A Master instance of Jenkins can also execute build jobs directly.

Jenkins Slave

A Slave is a Java executable that runs on a remote machine. Following are the characteristics of Jenkins Slaves:

• It hears requests from the Jenkins Master instance.
• Slaves can run on a variety of operating systems.
• The job of a Slave is to do as they are told to, which involves executing build jobs dispatched by the Master.
• You can configure a project to always run on a particular Slave machine, or a particular type of Slave machine, or simply let Jenkins pick the next available Slave.

Lets see how to configure both Jenkins master and slave nodes on Ubuntu EC2.

Step 1 - Jenkins master node configuration

(If you have already Jenkins up and running, this step # 1 is not required, Go to next step # 2, slave node configuration)

Install first Java by following below steps. Make sure port 8080 is opened in security group.

sudo apt-get update
sudo apt install openjdk-11-jdk

Jenkins (make sure you open port number)

wget -q -O - https://pkg.jenkins.io/debian/jenkins.io.key | sudo apt-key add -
echo deb http://pkg.jenkins.io/debian-stable binary/ | sudo tee /etc/apt/sources.list.d/jenkins.list
sudo apt-get update
sudo apt-get install jenkins -y
now to go to browser --> http://server_ip_address:8080/
Copy the password from this location
By entering
sudo cat /var/lib/jenkins/secrets/initialAdminPassword
Paste the password and click on install suggested plug-ins.

Create SSH keys in master node by executing below command:

ssh-keygen (no need to enter any password, just enter three times)

Step # 2 Slave node configuration(you need new micro Ubuntu 18.0.4 instance for this slave)
only port 22 needs to be open

Install Java

sudo apt-get update
sudo apt install openjdk-11-jdk

Install Maven
sudo apt-get install maven -y

Create User as Jenkins

sudo useradd -m jenkins

sudo -u jenkins mkdir /home/jenkins/.ssh

Add SSH Keys from Master to Slave 
Execute the below command in Jenkins master Ec2 to print the ssh keys by ssh-keygen command
sudo cat ~/.ssh/id_rsa.pub
If you do not have keys, create using ssh-keygen command.
Copy the output of the above command:

Now go to Slave node and execute the below command
sudo -u jenkins vi /home/jenkins/.ssh/authorized_keys

This will be empty file, now copy the public keys from master into above file.
Once you pasted the public keys in the above file in Slave, come out of the file by entering wq!

Now go into master node
ssh jenkins@slave_node_ip

this is to make sure master is able to connect slave node. once you are successfully logged into slave, type exit to come out of slave.

Now copy the SSH keys into /var/lib/jenkins/.ssh by executing below command in master(make sure you exited from slave by typing exit command:

sudo cp ~/.ssh/known_hosts  /var/lib/jenkins/.ssh

Because jenkins master will look keys from the above folder.

Step # 3 Register slave node in Jenkins:
Now to go Jenkins Master, manage jenkins, manage nodes.

Click on new node. give name and check permanent agent. Click Ok to create the slave node.
in the next screen
give name and no of executors as 1. enter /home/jenkins as remote directory.
select launch method as Launch slaves nodes via SSH.
enter Slave node ip address as Host.

click on credentials. Enter user name as jenkins. Make jenkins lowercase as it is shown.
 Kind as SSH username with private key. enter private key of master node directly by executing below command:

sudo cat ~/.ssh/id_rsa
(Make sure you copy the whole key including the below without missing anything)
-----BEGIN RSA PRIVATE KEY-----

-----END RSA PRIVATE KEY-----

click Save.
select Host key verification strategy as "manually trusted key verification strategy".

Click Save. Now click on Log in drop down like shown below.

It should connect and show like this...no red marked error..

Now you can kick start building the jobs, you will see Jenkins master runs jobs in slave nodes.