Linux for network engineers lab 13 of 14
Troubleshooting tickets: routing, DNS, firewall and MTU
Four tickets wait in the queue at Grimsby, and each hides a fault in routing, DNS, the firewall or the MTU, on Linux and on the router. Find each cause with evidence, fix it, and explain it.
Run this lab
It needs Lite: its 5 nodes are set to 5 vCPU and 3,072 MB in total. That fits the free demo too.
In your workspace: Shared, then Linux for network engineers guided labs, then 13 - Troubleshooting tickets - routing DNS firewall and MTU
- Level
- Advanced
- Time
- About 90 minutes
- Nodes need
- 5 vCPU, 3,072 MB
- Smallest plan
- Lite
The scenario
Your colleague has gone on leave and left you the Grimsby queue. GW is the site's Linux router and firewall, NS its name server, and R1 the link to the far site, where the portal server SRV lives. You're sitting at WS, the PC that raised the tickets. There are four, and nobody has touched them. Work them in order, because each fault hides the next. Prove every cause before you change anything.
| Device | Interface | Address | Role |
|---|---|---|---|
| WS | eth0 | 10.13.10.50/24 | the PC reporting the faults; dig, curl, mtr |
| GW | eth0, eth1, eth2 | 10.13.10.1/24, 10.13.20.1/24, 10.13.1.2/30 | LAN, services, link to R1 |
| NS | eth0 | 10.13.20.53/24 | DNS for kestrel.lab |
| R1 | Gi0/0, Gi0/1 | 10.13.1.1/30, 10.13.30.1/24 | far-site router; Gi0/0 carries a tunnel with a 1400-byte MTU |
| SRV | eth0 | 10.13.30.30/24 | the portal: TCP 80 and 8443, plus report.bin |
| Device | Runs | How to log in |
|---|---|---|
| WS | Ubuntu 22.04 desktop container | Its console is a remote desktop: root / eve. Open a terminal there. |
| NS, SRV | Alpine Linux 3.20 | root, no password. |
| R1 | Cisco IOSv 15.9 | The console opens at >; enable has no password. |
| GW | Alpine Linux 3.16 with iptables and iproute2 | Opens on a menu: press 5, then Enter, for a root shell. |
What you’ll practise
- Trace a path hop by hop and test both directions
- Separate DNS reachability from wrong answers
- Find a shadowed firewall rule from its counters
- Recognise an MTU fault from its symptoms and a capture
- Fix faults on Linux and IOS and record why they happened
The tasks
- Start the Linux nodes
- Ticket 1: Accounts can't reach the portal
- Ticket 2: "The portal name doesn't work"
- Ticket 3: "Port 8443 times out"
- Ticket 4: "The report hangs"
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.
Every link, as text
- WS
eth0to GWeth0 - GW
eth1to NSeth0 - GW
eth2to R1Gi0/0 - R1
Gi0/1to SRVeth0
| Node | Device | Image in Add Node | Set to |
|---|---|---|---|
| WS | EVE-NG GUI server (Docker) | eve-gui-server:latest | 1 vCPU, 1,024 MB |
| NS | Alpine Linux | clbalpinebase-alpine-base-3-20-3 | 1 vCPU, 512 MB |
| SRV | Alpine Linux | clbalpinebase-alpine-base-3-20-3 | 1 vCPU, 512 MB |
| R1 | Cisco IOSv | vios-adventerprisek9-m.spa.159-3.m9 | 1 vCPU, 512 MB |
| GW | Alpine WAN emulator | clbalpinewanem-alpine-3-16-2-wanem | 1 vCPU, 512 MB |
How to open this lab
Sign in to your workspace at
lab.cloudlabbox.com.In File Manager, open Shared, then the folder
Linux for network engineers guided labs.Open
13 - Troubleshooting tickets - routing DNS firewall and MTUand 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 Cisco, CompTIA, the Linux Foundation and the Linux Professional Institute. The lab follows exam topics and isn’t vendor training material; vendor names identify the exams and the images in the workspace.