LFCS Networking Practice Question
Which command displays the listening UDP ports on a Linux system?
⚠ Common exam trap
It's easy for candidates to 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.
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
✓
ss -uln
`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.
Answer analysis
Option-by-option breakdown
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
Omitting the -u flag means ss -a lists both TCP and UDP sockets, so listening UDP ports are not isolated. It is tempting because -a does show all sockets, and it would be correct when you genuinely want a combined TCP and UDP listing rather than UDP alone.
- ✗
ss -tln
Why it's wrong here
The -t flag restricts ss to TCP sockets, so listening UDP ports are omitted entirely. It is tempting because ss -tln is the standard way to list listening TCP ports, and would be correct if the question asked about TCP rather than UDP.
- ✓
ss -uln
Why this is correct
-u for UDP, -l for listening, -n for numeric.
- ✗
netstat -tln
Why it's wrong here
The -t flag limits netstat to TCP sockets, so UDP listeners are never shown. It tempts because -l and -n correctly filter listening sockets and numeric ports, but the protocol selector is wrong; -u is needed for UDP.
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JA
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.