This LFCS practice question tests your understanding of networking. Examine the command output carefully: the correct answer depends on what the output actually shows, not on general recall alone. 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.
A system administrator runs 'ss -tuln' and sees that port 80 is listening. What does the 'u' option represent?
User
Why wrong: There is no -u for user.
Unix sockets
Why wrong: Unix sockets are shown with -x.
UDP
The -u option filters for UDP sockets.
Unicast
Why wrong: Unicast is not a socket type; -u is for UDP.
Answer choices
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
UDP
In the `ss` command, the `-u` option filters output to show only UDP sockets. Since the question shows `ss -tuln`, which combines `-t` (TCP), `-u` (UDP), `-l` (listening), and `-n` (numeric), the `u` specifically represents UDP. This is confirmed by the `ss` man page and standard Linux networking tools.
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.
User
Why it's wrong here
There is no -u for user.
Unix sockets
Why it's wrong here
Unix sockets are shown with -x.
UDP
Why this is correct
The -u option filters for UDP sockets.
Related concept
Read the scenario before looking for a memorised answer.
Unicast
Why it's wrong here
Unicast is not a socket type; -u is for UDP.
Common exam traps
The trap here is that candidates confuse `-u` with 'Unix sockets' (which is `-x`) or 'User' (which is `-p`), because the letter 'u' is commonly associated with 'Unix' or 'user' in other commands, but in `ss` it specifically means UDP.
Trap categories for this question
Command / output trap
Unix sockets are shown with -x.
Detailed technical explanation
The `ss` utility reads socket information from the `/proc/net/` filesystem (e.g., `/proc/net/tcp`, `/proc/net/udp`). When `-u` is specified, `ss` parses `/proc/net/udp` and `/proc/net/udp6` to list UDP sockets. A subtle behavior: `ss -tuln` will show both TCP and UDP listening sockets, but UDP sockets are considered 'listening' by default because UDP is connectionless, so `-l` effectively shows all UDP sockets that have a bound local address.
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: UDP — In the `ss` command, the `-u` option filters output to show only UDP sockets. Since the question shows `ss -tuln`, which combines `-t` (TCP), `-u` (UDP), `-l` (listening), and `-n` (numeric), the `u` specifically represents UDP. This is confirmed by the `ss` man page and standard Linux networking tools.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Read the scenario before looking for a memorised answer.
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Last reviewed: Jun 25, 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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