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.
Which TWO commands can be used to display listening TCP ports on a Linux system?
ss -tln
ss -tln lists listening TCP ports (t for TCP, l for listening, n for numeric).
netstat -tln
netstat -tln also lists listening TCP ports with numeric addresses.
nmap -sT localhost
Why wrong: nmap is a scanner, not a tool to display current listening ports.
iptables -L
Why wrong: iptables -L lists firewall rules, not listening ports.
lsof -i TCP
Why wrong: lsof -i TCP shows all TCP connections, not only 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 -tln
The `ss -tln` command displays listening TCP sockets by using the `-t` flag for TCP, `-l` for listening sockets, and `-n` to show numeric addresses and ports without resolving service names. It reads socket information directly from the kernel's netlink interface, making it the modern replacement for netstat on Linux systems.
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 -tln
Why this is correct
ss -tln lists listening TCP ports (t for TCP, l for listening, n for numeric).
Related concept
Read the scenario before looking for a memorised answer.
netstat -tln
Why this is correct
netstat -tln also lists listening TCP ports with numeric addresses.
Related concept
Read the scenario before looking for a memorised answer.
nmap -sT localhost
Why it's wrong here
nmap is a scanner, not a tool to display current listening ports.
iptables -L
Why it's wrong here
iptables -L lists firewall rules, not listening ports.
lsof -i TCP
Why it's wrong here
lsof -i TCP shows all TCP connections, not only listening ports.
Common exam traps
The trap here is that candidates often assume `lsof -i TCP` shows only listening ports, but without the `-sTCP:LISTEN` filter it displays all TCP sockets, including established connections, making it incorrect for the specific requirement of listing only listening TCP ports.
Trap categories for this question
Command / output trap
lsof -i TCP shows all TCP connections, not only listening ports.
Detailed technical explanation
The `ss` command uses the netlink socket API (NETLINK_SOCK_DIAG) to query the kernel's socket hash tables directly, providing faster and more detailed output than the older /proc/net/tcp parsing used by netstat. In real-world scenarios, `ss -tln` is preferred for performance monitoring on busy servers because it avoids the overhead of reading and parsing multiple proc files, and it can show additional details like socket memory usage and TCP congestion algorithm.
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 -tln — The `ss -tln` command displays listening TCP sockets by using the `-t` flag for TCP, `-l` for listening sockets, and `-n` to show numeric addresses and ports without resolving service names. It reads socket information directly from the kernel's netlink interface, making it the modern replacement for netstat on Linux systems.
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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