This LFCS practice question tests your understanding of networking. Read the scenario carefully and evaluate each option against the stated constraints before committing to an answer. 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.
Which command displays the listening UDP ports on a Linux system?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "which command"
Why it matters: Tests specific CLI syntax. Recall the exact command and its required context — near-synonyms and partial matches are common distractors.
ss -a
Why wrong: This shows all sockets (listening and established).
ss -tln
Why wrong: This shows TCP listening ports.
ss -uln
-u for UDP, -l for listening, -n for numeric.
netstat -tln
Why wrong: This shows TCP listening ports.
Answer choices
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
ss -uln
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.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
For each option: why learners choose it and why it is or isn't the right answer here.
ss -a
Why it's wrong here
This shows all sockets (listening and established).
ss -tln
Why it's wrong here
This shows TCP listening ports.
ss -uln
Why this is correct
-u for UDP, -l for listening, -n for numeric.
Clue confirmation
The clue word "which command" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
netstat -tln
Why it's wrong here
This shows TCP listening ports.
Common exam traps
The trap here is that candidates often confuse the `-t` (TCP) and `-u` (UDP) flags, or assume that `netstat -tln` or `ss -tln` will show all listening ports, forgetting that UDP requires explicit `-u` filtering.
Trap categories for this question
Command / output trap
This shows all sockets (listening and established).
Detailed technical explanation
Under the hood, `ss` reads socket information from the `/proc/net/` filesystem (e.g., `/proc/net/udp` for UDP sockets), providing faster and more detailed output than `netstat`. In real-world scenarios, using `ss -uln` is essential for verifying that services like DNS (port 53) or DHCP (ports 67/68) are listening on UDP, as these services do not use TCP and would be missed by TCP-only commands.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
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
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FAQ
Networking — This question tests Networking — Read the scenario before looking for a memorised answer..
The correct answer is: ss -uln — 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.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Yes — watch for: "which command". Tests specific CLI syntax. Recall the exact command and its required context — near-synonyms and partial matches are common distractors.
Read the scenario before looking for a memorised answer.
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Last reviewed: Jun 11, 2026
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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