LFCS Networking Practice Question
A system administrator runs 'ss -tuln' and sees that port 80 is listening. What does the 'u' option represent?
⚠ Common exam trap
It's easy for candidates to 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.
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
✓
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.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
User
Why it's wrong here
The 'u' flag selects UDP sockets, so it filters output to UDP listeners rather than showing TCP. It is tempting because 'u' commonly abbreviates 'user' in other command contexts, but within ss the user-oriented option is '-p', which displays the owning process.
- ✗
Unix sockets
Why it's wrong here
The 'u' flag selects UDP sockets; Unix domain sockets are selected with the '-x' option instead. It is tempting because 'u' appears in 'Unix', but ss reserves '-x' for AF_UNIX sockets, and combining '-tuln' shows TCP and UDP listeners only.
- ✓
UDP
Why this is correct
In the ss command, the 'u' flag restricts output to UDP sockets, complementing 't' for TCP. Combined with 'l' and 'n', it lists listening UDP ports numerically, confirming which transport protocol is bound to port 80.
- ✗
Unicast
Why it's wrong here
The 'u' flag selects UDP sockets, not unicast traffic; unicast is a delivery method, not a socket type ss filters on. It is tempting because UDP is often described as connectionless unicast transport, but the flag's actual function is protocol selection.
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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.