Agent connection methods (SSH, JNLP) in Jenkins - Step-by-Step Execution

Process Flow - Agent connection methods (SSH, JNLP)

Start Jenkins Master

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Choose Agent Connection Method

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SSH Agent

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Master connects

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Agent ready for jobs

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Run Jobs

Jenkins master and agent connect using either SSH where master initiates connection, or JNLP where agent initiates connection, enabling job execution.

Execution Sample

Jenkins

ssh-agent setup:
ssh user@agent-host

jnlp-agent setup:
java -jar agent.jar -jnlpUrl http://master/computer/agent/slave-agent.jnlp

Shows commands to connect Jenkins agent via SSH and JNLP methods.

Process Table

Step	Action	Connection Method	Result	Next Step
1	Jenkins master starts	N/A	Master ready	Choose agent connection method
2	Select SSH method	SSH	Master prepares SSH connection	Master connects to agent via SSH
3	Master runs ssh user@agent-host	SSH	SSH session established	Agent ready for jobs
4	Agent accepts SSH connection	SSH	Agent connected	Run jobs on agent
5	Select JNLP method	JNLP	Agent downloads agent.jar	Agent runs java -jar agent.jar
6	Agent runs java -jar with JNLP URL	JNLP	Agent connects to master	Agent ready for jobs
7	Agent connected via JNLP	JNLP	Agent connected	Run jobs on agent
8	Jobs dispatched	SSH or JNLP	Jobs run on agent	Execution complete

💡 Jobs run on agent; connection established either by SSH initiated by master or JNLP initiated by agent.

Status Tracker

Variable	Start	After SSH Connect	After JNLP Connect	Final
Connection Status	Disconnected	Connected (SSH)	Connected (JNLP)	Connected
Agent Ready	No	Yes (SSH)	Yes (JNLP)	Yes

Key Moments - 2 Insights

Why does SSH connection start from the master but JNLP connection starts from the agent?

SSH requires master to initiate connection to agent's SSH server (see execution_table step 3), while JNLP agent runs on the agent machine and connects back to master (see step 6).

What happens if the agent machine blocks incoming SSH connections?

SSH connection will fail at step 3 in execution_table, but JNLP can still work because agent initiates connection outbound to master (step 6).

Visual Quiz - 3 Questions

Look at the execution table, at which step does the agent initiate the connection to the master?

A Step 3

B Step 6

C Step 4

D Step 2

💡 Hint

Check the 'Action' and 'Connection Method' columns for JNLP where agent runs java -jar to connect.

According to the variable tracker, what is the connection status after SSH connect?

A Disconnected

B Connected (JNLP)

C Connected (SSH)

D Connecting

💡 Hint

Look at the 'Connection Status' row under 'After SSH Connect' column.

If the agent blocks incoming SSH but allows outbound connections, which method will succeed?

A JNLP only

B Both SSH and JNLP

C SSH only

D Neither

💡 Hint

Refer to key_moments explanation about connection initiation direction.

Concept Snapshot

Jenkins agents connect to master via two main methods:
- SSH: Master initiates connection to agent's SSH server.
- JNLP: Agent runs a Java client that connects back to master.
SSH requires agent to accept incoming connections.
JNLP works well when agent cannot accept inbound connections.
Both enable agents to run jobs dispatched by master.

Full Transcript

This visual execution shows how Jenkins agents connect to the master using SSH or JNLP methods. The master starts and chooses the connection method. For SSH, the master initiates an SSH session to the agent machine, which must accept incoming SSH connections. For JNLP, the agent machine runs a Java client that connects back to the master, useful when the agent cannot accept inbound connections. The execution table traces each step from starting the master, selecting connection method, establishing connection, to running jobs. The variable tracker shows connection status and agent readiness changing from disconnected to connected. Key moments clarify why connection initiation direction matters and what happens if SSH is blocked. The quiz tests understanding of connection initiation steps and conditions for success. The snapshot summarizes the two methods and their key differences.