Agent connection methods (SSH, JNLP) in Jenkins - Practice Problems & Coding Challenges

Challenge - 5 Problems

🧠 Conceptual

intermediate

Understanding Jenkins Agent Connection Methods

Which statement correctly describes the difference between SSH and JNLP agent connection methods in Jenkins?

A JNLP agents require Jenkins to have SSH access to the agent machine, but SSH agents do not require any network connection.

B SSH agents use a Java Web Start protocol, while JNLP agents use direct SSH commands to connect.

C SSH agents connect by Jenkins initiating an SSH session to the agent machine, while JNLP agents connect by the agent machine initiating a connection to Jenkins.

D Both SSH and JNLP agents require the agent machine to open port 22 for Jenkins to connect.

💡 Hint

Think about who starts the connection in each method.

SSH agents connect by Jenkins initiating an SSH session to the agent machine, requiring Jenkins to access the agent's SSH port. JNLP agents run a Java process on the agent machine that connects back to Jenkins, so the agent initiates the connection.

💻 Command Output

intermediate

Output of Starting a JNLP Agent Manually

What is the expected output when running the following command on the agent machine to start a JNLP agent?

Jenkins

java -jar agent.jar -jnlpUrl http://jenkins.example.com/computer/agent1/slave-agent.jnlp -secret 1234567890abcdef

A

INFO: Connecting to Jenkins at http://jenkins.example.com
INFO: Connected
INFO: Agent started successfully

B

ERROR: Could not find agent.jar file
Exception in thread "main" java.lang.NoClassDefFoundError

C

INFO: SSH connection established
INFO: Agent ready

D

ERROR: Invalid secret key
Connection refused

💡 Hint

JNLP agents connect using Java and a secret key.

The command runs the Java agent jar with the JNLP URL and secret key. It connects to Jenkins and starts the agent process, showing connection info messages.

❓ Configuration

advanced

Configuring SSH Agent in Jenkins

Which SSH configuration snippet correctly sets up a Jenkins agent to connect via SSH with a private key located at /home/jenkins/.ssh/id_rsa?

A

Host agent1
  HostName agent1.example.com
  User jenkins
  IdentityFile /home/jenkins/.ssh/id_rsa
  StrictHostKeyChecking no

B

Host agent1
  HostName agent1.example.com
  User jenkins
  PasswordAuthentication yes
  IdentityFile /home/jenkins/.ssh/id_rsa

C

Host agent1
  HostName agent1.example.com
  User root
  IdentityFile /root/.ssh/id_rsa
  StrictHostKeyChecking no

D

Host agent1
  HostName agent1.example.com
  User jenkins
  IdentityFile /home/jenkins/.ssh/id_rsa
  Port 8080

💡 Hint

Check the user, key file path, and SSH options for security.

Option A correctly uses the Jenkins user, the correct private key path, and disables strict host key checking for easier connection. Option A incorrectly enables password authentication which is not needed. Option A uses root user and wrong key path. Option A sets a wrong port for SSH.

❓ Troubleshoot

advanced

Troubleshooting SSH Agent Connection Failure

A Jenkins SSH agent fails to connect with the error: Permission denied (publickey). Which is the most likely cause?

A The Jenkins master is not running the Java agent jar.

B The agent machine's firewall is blocking port 8080.

C The JNLP secret key is incorrect.

D The private key on Jenkins master does not match the public key on the agent machine.

💡 Hint

Think about SSH authentication errors.

Permission denied (publickey) means SSH authentication failed because the private key Jenkins uses does not match any authorized public key on the agent machine. Firewall blocking port 8080 or JNLP secret key issues relate to JNLP agents, not SSH. The Java agent jar is unrelated to SSH connection.

✅ Best Practice

expert

Choosing Agent Connection Method for Security and Scalability

For a Jenkins setup with many dynamic cloud agents behind firewalls, which agent connection method is best practice and why?

A Use JNLP agents because they require Jenkins to open SSH ports on all agents.

B Use JNLP agents because they initiate the connection to Jenkins, avoiding firewall inbound rules.

C Use SSH agents because Jenkins can directly connect to all agents via SSH without extra setup.

D Use SSH agents because they are easier to configure on cloud providers.

💡 Hint

Consider firewall and network restrictions for cloud agents.

JNLP agents connect by the agent initiating the connection to Jenkins, which works well behind firewalls and NATs common in cloud environments. SSH agents require Jenkins to connect inbound to agents, which is often blocked. Thus, JNLP is preferred for dynamic cloud agents.