Day 75: Jenkins Slave Nodes | 100 Days of DevOps
Content
Today I worked on configuring Jenkins SSH build agents (slave nodes) for all application servers in the environment. The goal was to allow Jenkins to execute jobs directly on the application servers through SSH connections. During the process, I also encountered and resolved a Java version compatibility issue between the Jenkins controller and the remote agents.
🔹 What I Learned
- Jenkins Controller and Agent Communication
- Installing and Managing Jenkins Plugins
- Creating Jenkins Credentials
- Configuring SSH Build Agents
- Managing Labels and Node Restrictions
- Troubleshooting Jenkins Agent Connection Issues
- Understanding Java Version Compatibility
- Running Jobs on Specific Jenkins Agents
- Verifying Agent Connectivity and Functionality
🔹 Task Requirement
The Nautilus DevOps team required all application servers to be configured as Jenkins SSH build agents so that Jenkins could execute jobs directly on those servers.
Jenkins Access Details
| Field | Value |
|---|---|
| Username | admin |
| Password | Adm!n321 |
Application Server Details
| Server | Hostname | User | Password |
|---|---|---|---|
| Application Server 1 | stapp01 | tony | Ir0nM@n |
| Application Server 2 | stapp02 | steve | Am3ric@ |
| Application Server 3 | stapp03 | banner | BigGr33n |
Node Configuration Requirements
| Node Name | Label | Remote Root Directory |
|---|---|---|
| App_server_1 | stapp01 | /home/tony/jenkins |
| App_server_2 | stapp02 | /home/steve/jenkins |
| App_server_3 | stapp03 | /home/banner/jenkins |
🔹 Steps I Followed
1. Verified Java Installation on All Application Servers
Before configuring Jenkins agents, I connected to each application server and verified whether Java was installed.
App Server 1
ssh tony@stapp01
sudo su -
java --version
Output:
openjdk 11.0.20.1
App Server 2
ssh steve@stapp02
java --version
Output:
openjdk 11.0.20.1
App Server 3
ssh banner@stapp03
java --version
Output:
openjdk 11.0.20.1
This confirmed that Java was already installed on all servers.
2. Accessed Jenkins
Clicked the Jenkins button from the lab navigation bar.
This opened the Jenkins login page.
3. Logged into Jenkins
Used the administrator credentials:
Username
admin
Password
Adm!n321
Successfully logged into the Jenkins Dashboard.
4. Installed Required Plugins
Navigated to:
Manage Jenkins
→ Plugins
Installed the following plugins:
- SSH
- SSH Credentials
- SSH Build Agents
After installation completed, Jenkins was restarted.
Logged back in using the admin credentials.
5. Created Jenkins Credentials
Navigated to:
Manage Jenkins
→ Credentials
→ System
→ Global Credentials
→ Add Credentials
Credential Type:
Username with Password
Credential for App Server 1
| Field | Value |
|---|---|
| Username | tony |
| Password | Ir0nM@n |
| ID | stapp01 |
Saved the credential.
Credential for App Server 2
| Field | Value |
|---|---|
| Username | steve |
| Password | Am3ric@ |
| ID | stapp02 |
Saved the credential.
Credential for App Server 3
| Field | Value |
|---|---|
| Username | banner |
| Password | BigGr33n |
| ID | stapp03 |
Saved the credential.
6. Created Jenkins Agent Node for App_server_1
Navigated to:
Manage Jenkins
→ Nodes
→ New Node
Entered:
Node Name:
App_server_1
Selected:
Permanent Agent
Configured:
| Field | Value |
|---|---|
| Remote Root Directory | /home/tony/jenkins |
| Labels | stapp01 |
| Usage | Use this node as much as possible |
| Launch Method | Launch agents via SSH |
| Host | stapp01 |
| Credentials | stapp01 |
| Host Key Verification | Non verifying Verification Strategy |
Clicked Save.
7. Created Jenkins Agent Node for App_server_2
Configured:
| Field | Value |
|---|---|
| Node Name | App_server_2 |
| Remote Root Directory | /home/steve/jenkins |
| Labels | stapp02 |
| Launch Method | Launch agents via SSH |
| Host | stapp02 |
| Credentials | stapp02 |
| Host Key Verification | Non verifying Verification Strategy |
Clicked Save.
8. Created Jenkins Agent Node for App_server_3
Configured:
| Field | Value |
|---|---|
| Node Name | App_server_3 |
| Remote Root Directory | /home/banner/jenkins |
| Labels | stapp03 |
| Launch Method | Launch agents via SSH |
| Host | stapp03 |
| Credentials | stapp03 |
| Host Key Verification | Non verifying Verification Strategy |
Clicked Save.
9. Encountered Agent Connection Failure
After creating the nodes, Jenkins failed to connect to the agents.
Error message:
UnsupportedClassVersionError
hudson/remoting/Launcher has been compiled by a more recent version of the Java Runtime (class file version 61.0)
this version of the Java Runtime only recognizes class file versions up to 55.0
Understanding the Error
| Class Version | Java Version |
|---|---|
| 61 | Java 17 |
| 55 | Java 11 |
This indicated that the Jenkins controller was using Java 17 while the remote agents were still running Java 11.
Because Jenkins Remoting requires a compatible Java version, the agents could not start.
10. Installed Java 17 on App Server 1
Connected to the server:
ssh tony@stapp01
sudo su -
Installed Java 17:
yum install -y java-17-openjdk
Verified:
java --version
Output:
openjdk 17.0.18
11. Installed Java 17 on App Server 2
Connected:
ssh steve@stapp02
sudo su -
Installed Java 17:
yum install -y java-17-openjdk
Verified:
java --version
Output:
openjdk 17.0.18
12. Installed Java 17 on App Server 3
Connected:
ssh banner@stapp03
sudo su -
Installed Java 17:
yum install -y java-17-openjdk
Verified:
java --version
Output:
openjdk 17.0.18
13. Verified Jenkins Agent Connectivity
After upgrading Java on all application servers, Jenkins automatically reconnected to the agents.
All nodes displayed:
Online
Verified nodes:
App_server_1
App_server_2
App_server_3
All agents were successfully connected.
14. Created a Test Job
To verify the agent functionality, I created a Freestyle Project.
Job Name
testingNode
Selected:
Freestyle Project
15. Restricted Job to App_server_1
Under General:
Enabled:
Restrict where this project can be run
Label Expression:
stapp01
This ensured the job would execute only on App_server_1.
16. Added Build Step
Added:
Build
→ Execute Shell
Script:
echo "Testing App_server_1"
hostname
whoami
pwd
java -version
Saved the job.
17. Triggered the Build
Opened:
testingNode
→ Build Now
Jenkins started executing the job on App_server_1.
18. Verified Build Success
Opened:
Build History
→ #1
→ Console Output
Output:
Started by user admin
Running as SYSTEM
Building remotely on App_server_1 (stapp01)
workspace:
/home/tony/jenkins/workspace/testingNode
Testing App_server_1
hostname
stapp01
whoami
tony
pwd
/home/tony/jenkins/workspace/testingNode
java -version
openjdk version "17.0.18"
Finished: SUCCESS
This confirmed that Jenkins successfully executed the job on the remote agent.
🔹 Simple Explanation of Jenkins Components Used
Jenkins Agents (Slave Nodes)
Jenkins agents are remote machines that execute build jobs on behalf of the Jenkins controller.
Benefits:
- Distribute workloads
- Improve scalability
- Run jobs on specific servers
- Isolate build environments
In this task:
App_server_1
App_server_2
App_server_3
were configured as SSH agents.
Jenkins Credentials
Credentials provide a secure way to store authentication details.
In this task:
tony / Ir0nM@n
steve / Am3ric@
banner / BigGr33n
were stored securely and used by Jenkins to connect via SSH.
SSH Build Agents Plugin
This plugin enables Jenkins to:
- Connect to remote servers
- Transfer the Jenkins agent files
- Launch agents remotely
- Execute jobs through SSH
Without this plugin, Jenkins cannot manage SSH-based build agents.
Labels
Labels help direct jobs to specific agents.
Examples:
stapp01
stapp02
stapp03
A job can be restricted to a particular server by specifying its label.
Remote Root Directory
This is the working directory used by Jenkins on the agent.
Examples:
/home/tony/jenkins
/home/steve/jenkins
/home/banner/jenkins
Jenkins stores:
- Workspace files
- Build artifacts
- Temporary build data
inside these directories.
Java Requirement for Jenkins Agents
Jenkins agents require a compatible Java version to run the Jenkins Remoting process.
In this task:
Java 11
caused the connection failure because Jenkins was expecting:
Java 17
Upgrading the agents to Java 17 resolved the issue.
🔹 My Understanding
This task gave me practical experience with Jenkins distributed build architecture. I learned how Jenkins controllers communicate with remote agents over SSH, how credentials and labels are used to manage agent execution, and how Java compatibility plays a critical role in Jenkins agent connectivity.
🔹 What I Found Interesting
What I found most interesting was how Jenkins automatically deploys and manages its remoting agent on remote servers using SSH. The troubleshooting aspect was particularly valuable because the agent connection initially failed due to a Java version mismatch. By understanding the class version error and upgrading Java from version 11 to version 17 on all application servers, I was able to restore connectivity and successfully execute jobs remotely. This demonstrated how important environment compatibility is when managing distributed Jenkins infrastructure.
📌 Full notes: GitHub link