Hands-on Lab·Linux Foundation Certified System Administrator
When the network does not work
Not a made-up scenario: ssh lfcs-b01 genuinely fails on this host, and this guide works out why in five commands. The method is worth more than the answer - start at the bottom, change one layer at a time, and watch for the error message to move.
Networking Guide 29 of 38 Intermediate
- OSUbuntu 26.04 LTS
- LVM2.03.31 (Ubuntu) / 2.03.36 (AlmaLinux)
- nftables1.1.6 (Ubuntu) / 1.1.5 (AlmaLinux)
- TimeAbout 14 min
Every command here works on both families. The failure is a real gap in this lab's configuration, not a fault introduced for the page.
- Firewallufw 0.36.2 enabled but reporting inactive / firewalld active
- Network confignetplan + systemd-networkd / NetworkManager 1.56.0
| Server Name | IP Address | OS | Roles | CPU | RAM | HDD |
|---|---|---|---|---|---|---|
| LFCS-A01 | 192.168.0.70 | Ubuntu 26.04 LTS | Primary host - most guides run only here | 2 Core | 4 GB | 50 GB |
This guide includes
Use this for a failure that is real on this host rather than invented. This matters because the error moving is the proof - when connecting by IP fails differently, you have found which layer is broken.
- starting from the failure exactly as reported
- asking whether the machine is there at all
- checking each place the answer could have come from
- watching the error move, which is the proof
- fixing only the layer that is broken, then putting it back
Before you start
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The failure, as reported
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Is the machine there at all
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Which resolution source is missing it
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The error moves, which is the proof
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Fix the layer that is broken, and only that one
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Put it back