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FortiGate lab 23 of 25

Troubleshooting: policy, NAT and routing faults

A weekend change left Harbourside's office offline and its web site unreachable. Five realistic faults are layered on top of each other in DHCP, policy order, NAT, a virtual IP and a stale route; find each one with the right tool and fix it without breaking anything else.

Run this lab

It needs Lite: its 5 nodes are set to 5 vCPU and 6,656 MB in total. That fits the free demo too.

In your workspace: Shared, then FortiGate NSE 4 guided labs, then 23 - Troubleshooting - policy NAT and routing faults

Try it free for 30 minutesRun it on Lite from £3 a day

The free demo needs no card and no email. Every lab plan includes every guided lab.

Level
Advanced
Time
About 75 minutes
Nodes need
5 vCPU, 6,656 MB
Smallest plan
Lite

The scenario

It's Monday morning at Harbourside. Over the weekend a contractor "tidied up" FGT1, and now nothing works: staff can't browse or even resolve names, and customers can't reach the public web site at 203.0.113.80, which is SRV1 in the DMZ. The contractor's notes are lost. The design is simple and hasn't changed: the LAN uses DNS 198.51.100.53 and browses the internet with source NAT; the web site is published with a VIP; the internet is reached only through ISP-A. Find and fix every fault. Some faults hide others, so re-test after each fix.

DeviceInterfaceAddressConnects to
FGT1port1 (ISP-A)203.0.113.1/24INET, gateway 203.0.113.254
FGT1port2 (LAN)10.1.10.1/24PC1 10.1.10.10, H1 by DHCP
FGT1port3 (DMZ)10.1.30.1/24SRV1 10.1.30.10 (published as 203.0.113.80)

What you’ll practise

  • Work a fault methodically: reproduce, observe, form a hypothesis, test, fix, re-test
  • Find DHCP option, policy-order, NAT, VIP and routing faults with debug flow, the sniffer and the session table
  • Recognise symptoms that hide other faults, and keep going after the first fix
  • Document each fault as symptom, evidence, root cause and fix

The tasks

  1. Start the lab
  2. Reproduce and collect symptoms
  3. Names don't resolve
  4. Nothing reaches the internet
  5. Allowed, but still no answer
  6. The public web site
  7. Write the incident report

The Lab Tasks pane has the scenario, these tasks, the checks to run, hints and the full solution.

Topology and devices

The network as it opens in EVE-NG, drawn from the lab’s own canvas, and the image each node runs.

Topology of Troubleshooting - policy NAT and routing faults5 nodes and 1 network, 5 links. Every link is listed as text below the diagram.FGT1INETPC1H1SRV1LANeth0eth0port2port1eth0port3eth0
5 nodes and 1 network, with the interface names each link uses. On a narrow screen, scroll the diagram sideways.
Every link, as text
  • FGT1 port1 to INET eth0
  • FGT1 port2 to LAN (network)
  • PC1 eth0 to LAN (network)
  • H1 eth0 to LAN (network)
  • FGT1 port3 to SRV1 eth0
Nodes in Troubleshooting: policy, NAT and routing faults, with the image each runs and the vCPU and RAM it’s set to
NodeDeviceImage in Add NodeSet to
FGT1Fortinet FortiGatefortinet-FGT-v7.0.17.M-build06821 vCPU, 2,048 MB
INETEVE-NG GUI server (Docker)eve-gui-server:latest1 vCPU, 1,024 MB
PC1EVE-NG GUI server (Docker)eve-gui-server:latest1 vCPU, 2,048 MB
H1Alpine Linuxclbalpinebase-alpine-base-3-20-31 vCPU, 512 MB
SRV1EVE-NG GUI server (Docker)eve-gui-server:latest1 vCPU, 1,024 MB

How to open this lab

  1. Sign in to your workspace at lab.cloudlabbox.com.

  2. In File Manager, open Shared, then the folder FortiGate NSE 4 guided labs.

  3. Open 23 - Troubleshooting - policy NAT and routing faults and start its nodes, then open Lab Tasks for the scenario and the steps.

The lab is read-only. You can start the nodes and configure the devices, but changes to the topology can’t be saved. To change it, rebuild it in your own folder.

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CloudLabBox is independent of Fortinet. The lab follows exam topics and isn’t vendor training material; vendor names identify the exams and the images in the workspace.