Install Jenkins on Ubuntu and Set up a Master-Slave Configuration

Part 1: Install Jenkins on Ubuntu

Step 1: Update System Packages

sudo apt update && sudo apt upgrade -y

Step 2: Install Java

Jenkins requires Java to run. Install OpenJDK:

sudo apt install -y openjdk-11-jdk

Verify Java installation:

java -version

Step 3: Add Jenkins Repository

Add the Jenkins key and repository:

curl -fsSL https://pkg.jenkins.io/debian-stable/jenkins.io-2023.key | sudo tee \
  /usr/share/keyrings/jenkins-keyring.asc > /dev/null
echo deb [signed-by=/usr/share/keyrings/jenkins-keyring.asc] \
  https://pkg.jenkins.io/debian-stable binary/ | sudo tee \
  /etc/apt/sources.list.d/jenkins.list > /dev/null

Update the package list:

sudo apt update

Step 4: Install Jenkins

sudo apt install -y jenkins

Start and enable Jenkins:

sudo systemctl start jenkins
sudo systemctl enable jenkins

Step 5: Access Jenkins Web Interface

Open your browser and go to:
```
http://<server-ip>:8080
```

Retrieve the initial admin password:

sudo cat /var/lib/jenkins/secrets/initialAdminPassword

Use this password to log in and complete the setup wizard by installing recommended plugins.

Part 2: Setup Jenkins Master-Slave Configuration

Step 1: Add a Slave Node

Log in to the Jenkins dashboard.
Navigate to Manage Jenkins → Manage Nodes and Clouds.
Click New Node.
Enter a name for the node (e.g., slave-node-1) and select Permanent Agent.
Configure the node:
- Remote root directory: Directory on the slave where Jenkins files will be stored (e.g., /home/jenkins).
- Labels: Add labels to identify this node for specific jobs.
- Save the configuration.

Step 2: Prepare the Slave Node

Install Java on the slave machine:

sudo apt update && sudo apt install -y openjdk-11-jdk
java -version

Create a directory for Jenkins:
```
mkdir /home/jenkins
```

Step 3: Configure SSH Access

Generate SSH keys on the master server:
```
ssh-keygen -t rsa -b 4096
```
Copy the public key to the slave machine:
```
ssh-copy-id user@slave-ip
```
Test SSH access:
```
ssh user@slave-ip
```

Step 4: Connect the Slave Node

On the Jenkins master dashboard, edit the node configuration:
- Select Launch agents via SSH.
- Enter the slave's IP address, credentials (username/password or private key).
- Test the connection to ensure successful setup.
Save the configuration.

Step 5: Verify Node Connection

Go to Manage Jenkins → Manage Nodes and Clouds.
Check the status of the slave node. It should show as online.

Step 6: Assign Jobs to the Slave Node

Open a Jenkins job configuration.
Scroll to the "Restrict where this project can be run" section.
Enter the label of the slave node.
Save the configuration.

Troubleshooting

SSH Errors: Ensure the slave node allows SSH connections and that the user has the correct permissions.
Agent Not Starting: Check logs on the slave node or /var/log/jenkins/jenkins.log on the master.
Firewall Rules: Ensure port 22 (SSH) and 8080 (Jenkins) are open on the respective machines.