The answer is port 53, which is listening only on the loopback interface. This is correct because the `ss -tlnp` output shows the service bound to `127.0.0.1:53`, meaning the DNS server is configured to accept connections exclusively on the loopback address, not on any external network interface. On the Linux Foundation Certified System Administrator LFCS exam, this concept tests your ability to interpret socket statistics and understand how services bind to specific interfaces—a common trap is assuming all listening ports are externally accessible. When you see an address like `127.0.0.1` or `::1` in the output, it signals a loopback-only service, which is critical for security and troubleshooting. A helpful memory tip: think of “127.0.0.1” as the “local-only” lane—if a port lives there, it’s not taking any outside traffic.
LFCS Networking Practice Question
This LFCS practice question tests your understanding of networking. This is a configuration task: choose the command set that satisfies every stated requirement. Small differences — like 'secret' vs 'password' or 'transport input ssh' vs 'all' — change whether the answer is correct. 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.
Exhibit
Refer to the exhibit.
[root@server ~]# ss -tln
State Recv-Q Send-Q Local Address:Port Peer Address:Port
LISTEN 0 128 0.0.0.0:22 0.0.0.0:*
LISTEN 0 128 *:80 *:*
LISTEN 0 128 *:443 *:*
LISTEN 0 128 127.0.0.1:53 0.0.0.0:*
LISTEN 0 128 0.0.0.0:8080 0.0.0.0:*
Based on the exhibit, which port is listening only on the loopback interface?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
53
Option A is correct because port 53 (DNS) is configured to listen only on the loopback interface (127.0.0.1) as shown in the exhibit's output of `ss -tlnp`. The listening address `127.0.0.1:53` indicates the service is bound exclusively to the loopback interface, meaning it is not accessible from external network interfaces.
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.
✓
53
Why this is correct
Listening on 127.0.0.1, which is the loopback interface only.
Related concept
Read the scenario before looking for a memorised answer.
✗
22
Why it's wrong here
Listening on 0.0.0.0, meaning all interfaces.
✗
443
Why it's wrong here
Listening on * (all interfaces).
✗
80
Why it's wrong here
Listening on * (all interfaces).
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates often confuse 'listening on all interfaces' (0.0.0.0) with 'listening only on loopback' (127.0.0.1), and may incorrectly assume that common services like SSH or HTTP are loopback-only when they are typically bound to all interfaces.
Detailed technical explanation
How to think about this question
The `ss -tlnp` command shows TCP listening sockets with numeric addresses; an address of `127.0.0.1` restricts the service to the loopback interface, while `0.0.0.0` binds to all available interfaces. This distinction is critical for security—binding DNS to loopback prevents external queries, often used for local caching resolvers. In real-world scenarios, misconfiguring a service to listen on `0.0.0.0` when it should be loopback-only can expose sensitive services to the network.
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: 53 — Option A is correct because port 53 (DNS) is configured to listen only on the loopback interface (127.0.0.1) as shown in the exhibit's output of `ss -tlnp`. The listening address `127.0.0.1:53` indicates the service is bound exclusively to the loopback interface, meaning it is not accessible from external network interfaces.
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.
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 →
These questions test the same concept from different angles. Work through them to make sure you can recognise it however the exam phrases it.
Variation 1. Which command displays the listening UDP ports on a Linux system?
easy
A.ss -a
B.ss -tln
✓ C.ss -uln
D.netstat -tln
Why C: Option C is correct because `ss -uln` specifically displays listening UDP sockets. The `-u` flag filters for UDP, `-l` shows only listening sockets, and `-n` displays numeric addresses and ports (avoiding DNS resolution). This is the most precise command for listing listening UDP ports.
Last reviewed: Jun 11, 2026
Question Discussion
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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.
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