8  Design a Lab That Checks the Right Thing

An objective is a claim about the network, written so the simulator can test it.

8.1 The Concept First

A NetSim objective has an id, a student-facing label and a check. The check is passive or active.

Common check types:

  • iface-ip: an interface has an address or an address in a subnet.
  • default-route: a default route exists, optionally via a gateway.
  • forwarding: IP forwarding matches the expected value.
  • fw-policy: a chain policy matches.
  • ping: an ICMP echo succeeds or fails.
  • tcp: a TCP connect is open, refused or filtered.
  • dns: a name resolves or returns NXDOMAIN.
  • http: a GET succeeds, optionally with a required string.

The design question is not “what did I teach?” It is “what evidence would prove the student did it?”

8.2 Understanding Through a Building Inspection

An inspector does not ask whether the builder read the code. The inspector checks the exit, the alarm and the record.

A lab objective is the check. The step is the instruction.

8.3 Discovering Objectives with Your AI Partner

Exploration 1: Turn a Step into a Check

Ask the AI to choose a check.

Step: "Give pc1 the address 192.168.1.10/24 on eth0."
Write the objective and the check. Then write a second objective that
proves the host can reach its gateway.

One check for configuration, one for behaviour.

Exploration 2: Prove the Problem

Ask for a vulnerability-first objective.

A lab asks students to harden a network. Write an objective that passes
while the vulnerability exists, then the objective that passes after the
fix. Explain the order.

The first objective establishes the baseline.

Exploration 3: Find the Weak Check

Ask what a check misses.

A lab checks that tcp/22 is filtered from the internet. What does that
not prove? Write a second check that closes the gap.

A check is evidence for one claim, not all claims.

8.4 From Concept to Practice

An objective record:

Field Entry
ID l6-ip
Label pc1 has an address in 192.168.1.0/24
Check iface-ip
Device pc1
Interface eth0
Subnet 192.168.1.0/24
Type Passive

8.5 Build Task

Write three objectives for one step: one configuration, one behaviour and one failure. Test them in the app.

8.6 Classroom Connection

Objectives make the assessment transparent. The student knows what evidence is required, and the grader can show why it passed or failed.

8.7 Practice Exercises

Exercise 8.1 (Level 1). Name three check types. Exercise 8.2 (Level 2). Write an iface-ip objective. Exercise 8.3 (Level 3). Write a ping objective. Exercise 8.4 (Level 4). Write a vulnerability-first objective. Exercise 8.5 (Level 5). Find a check that can pass while the student has not learned the concept, and fix it.

8.8 Chapter Summary

  • An objective is a testable claim.
  • Passive checks read state; active checks send packets.
  • Use a configuration check and a behaviour check.
  • Prove the problem before the fix.
  • A check is evidence for one claim.

8.9 Reflection

Before moving on, ensure you:

8.10 Your Learning Journal

  1. Which check was strongest?
  2. What did a passive check miss?
  3. Where could a student game the check?

8.11 Next Steps

The objectives are designed. The next chapter writes and distributes the lab.