Hands-on Lab·CompTIA Linux+

Publishing a service and the three ways a client fails

A service can be running, its port open in the firewall, and a client still get nowhere. This guide installs a real service on one host and connects to it from another, producing all three failures on purpose - nothing listening, the firewall refusing, and a service bound to loopback - and reads the server-side line that distinguishes them. It ends with a reload that reported success and changed nothing.

Services and Users Guide 22 of 28 Intermediate

PlatformsUbuntu 26.04 LTS + AlmaLinux 10.2
Mandatory access controlAppArmor on Ubuntu, SELinux enforcing on Alma
Firewallufw / nftables 1.1.6
TimeAbout 28 min

The RPM-family host runs the service; the Debian-family host is only ever the client. Two machines, because a client proof made on the server itself proves nothing.
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-B01	192.168.0.74	AlmaLinux 10.2	RPM-family host - dnf, firewalld, NetworkManager, SELinux	2 Core	4 GB	50 GB

This guide includes

Use this when something works on the server and not from anywhere else. This matters because the client reports the same failure for causes that need completely different fixes, and because the bind address is the one people check last after an hour on the firewall.

reading a bind address in ss and knowing what 0.0.0.0, 127.0.0.1 and * each mean
proving a service works locally and fails remotely, in the same minute
telling a firewall refusal from a loopback binding without touching the firewall
opening a port for the running firewall and knowing it is not yet permanent
recognising a reload that reports success and does not apply the change
using curl exit codes as the first triage step from the client side

Before you start

the-firewall-on-both-families-and-what-survives-a-reload
network-configuration-on-both-families

Three services, three meanings of listening
The client, before there is anything to reach
A real service on a port nobody opened
The service is perfect and the client still fails
Opening the port, and one honest client proof
A reload that reports success and changes nothing
The failure the firewall cannot explain
Putting both machines back

Official sources

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Publishing a service and the three ways a client fails

This guide includes

Before you start

Three services, three meanings of listening

The client, before there is anything to reach

A real service on a port nobody opened

The service is perfect and the client still fails

Opening the port, and one honest client proof

A reload that reports success and changes nothing

The failure the firewall cannot explain

Putting both machines back

Official sources