Troubleshooting·CompTIA Linux+

Faults, and the order that finds them

Domain 5 is 22% of XK0-006 and it is a method rather than a topic. This guide stages three faults - a process pinned to a core, a filesystem whose free space is held by a deleted file, and a service reachable only from itself - and finds each one with the command that produces a fact instead of a suspicion.

Troubleshooting Guide 7 of 28 Intermediate

OSUbuntu 26.04 LTS
Kernel7.0.0-30-generic
systemd259
Mandatory access controlAppArmor on Ubuntu, SELinux enforcing on Alma
TimeAbout 24 min

Firewallufw / nftables 1.1.6 - firewalld / nftables 1.1.5

Two hosts, because a service that has only ever been tested from itself is the third fault on this page.
Server Name	IP Address	OS	Roles	CPU	RAM	HDD
LPLUS-A01	192.168.0.73	Ubuntu 26.04 LTS	Debian-family host - apt, ufw, netplan, AppArmor	2 Core	4 GB	50 GB
LPLUS-C01	192.168.0.75	Ubuntu 26.04 LTS	Second Debian host - so networking and ssh have two ends	2 Core	4 GB	50 GB

This guide includes

Use this when something is wrong and you do not yet know which layer to look at. This matters because the fastest route is elimination in a fixed order, and because two of these faults look like something they are not: a full disk that du says is empty, and a refused connection that has nothing to do with the network.

ruling out three domains in three commands, then reading pressure rather than averages
finding the process on a core with ps and top, and stopping it by PID
meeting a filesystem where df and du disagree, and naming the holder with lsof +L1
diagnosing a refused connection from the listener's bind address
putting every staged fault back and confirming the machine is clean

Before you start

a-service-an-account-and-the-permission-between-them

The questions to ask before any theory
A process eating a core, and finding it
A filesystem that fills, and the space df cannot see
And once nothing holds it open
A service that answers itself and nothing else
The same service, bound where it can be reached
Putting the machine back

Official sources

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Faults, and the order that finds them

This guide includes

Before you start

The questions to ask before any theory

A process eating a core, and finding it

A filesystem that fills, and the space df cannot see

And once nothing holds it open

A service that answers itself and nothing else

The same service, bound where it can be reached

Putting the machine back

Official sources