LFCS Networking Practice Question
The command 'ss -tuln' shows port 80 is listening on a server, but a remote client cannot connect via HTTP. What is the most likely cause?
⚠ Common exam trap
The trap here is that candidates see 'listening' on port 80 and assume the service is fully accessible, forgetting that a firewall can silently drop incoming packets even when the service is up and listening.
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
A firewall is blocking incoming TCP port 80
The `ss -tuln` command shows that port 80 is in the LISTEN state, which means the HTTP service (e.g., Apache or Nginx) is bound to the port and ready to accept connections. Since the server is listening but the remote client cannot connect, the most likely cause is a firewall (such as iptables, nftables, or a cloud security group) that is blocking incoming TCP SYN packets destined for port 80, preventing the three-way handshake from completing.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
The HTTP service is not running
Why it's wrong here
'ss -tuln' shows port 80 is listening, so the service is running.
- ✗
The client has a misconfigured default gateway
Why it's wrong here
That would affect all connectivity, not just port 80.
- ✗
The server's /etc/hosts file is misconfigured
Why it's wrong here
/etc/hosts does not affect incoming connections.
- ✓
A firewall is blocking incoming TCP port 80
Why this is correct
Firewalls commonly block ports; even if the service listens, external access may be blocked.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This LFCS practice question is part of Courseiva's free Linux Foundation certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the LFCS exam.