This LFCS practice question tests your understanding of networking. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. After answering, compare your reasoning against the explanation and wrong-answer breakdown below. Once you have made your selection, read the full explanation to reinforce the concept and understand why each distractor is designed to mislead on exam day.
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?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "most likely"
Why it matters: Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
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.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
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.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
Common exam traps
Common exam trap: answer the scenario, not the keyword
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.
Trap categories for this question
Command / output trap
'ss -tuln' shows port 80 is listening, so the service is running.
Detailed technical explanation
How to think about this question
Under the hood, `ss -tuln` reads socket information from the kernel's netlink interface, showing TCP sockets in the LISTEN state (indicated by 'LISTEN' in the output). A firewall typically operates at the network layer (iptables/netfilter) or the cloud hypervisor level, dropping or rejecting packets before they reach the application. In a real-world scenario, a common subtlety is that the firewall might be blocking only specific source IP ranges or using stateful rules that drop SYN packets but allow established connections, which would still prevent a new client from connecting.
KKey Concepts to Remember
Read the scenario before looking for a memorised answer.
Find the constraint that changes the correct option.
Eliminate answers that are true in general but not in this case.
TExam Day Tips
→Watch for words such as best, first, most likely and least administrative effort.
→Review why wrong options are wrong, not only why the correct option is correct.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
How this comes up in practice
A practitioner preparing for the LFCS exam encounters this exact type of scenario on the job. The correct answer here is not the most general option — it is the best answer for the specific constraint described. Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option. Real exam questions reward reading the full scenario before eliminating options, because the constraint defines which answer fits.
What to study next
Got this wrong? Here's your next step.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Networking — This question tests Networking — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: 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.
What should I do if I get this LFCS question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Are there clue words in this question I should notice?
Yes — watch for: "most likely". Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
About these practice questions
Courseiva creates original exam-style practice questions with explanations and wrong-answer analysis. It does not publish real exam questions, exam dumps, or protected exam content. Learn why practice questions differ from exam dumps →
Share a tip, memory trick, or ask about the reasoning behind this question. Do not post real exam questions, leaked content, braindumps, or copyrighted exam material. Comments are moderated and may be removed without notice.
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.
Question Discussion
Share a tip, memory trick, or ask about the reasoning behind this question. Do not post real exam questions, leaked content, braindumps, or copyrighted exam material. Comments are moderated and may be removed without notice.
Sign in to join the discussion.