The answer is a firewall blocking UDP port 123, because the `reach` value of 0 in `ntpq -p` output means the NTP client has received no responses from any configured servers. NTP relies entirely on UDP port 123 for both sending requests and receiving replies, so when a firewall blocks this port, the client’s packets never reach the server, and the server’s responses never return, leaving the reach register stuck at zero. On the LPIC-1 exam, this scenario tests your understanding of NTP troubleshooting and network-layer filtering; a common trap is to assume a configuration error or service failure, but when all servers show a reach of 0 simultaneously, the culprit is almost always a network filter rather than a server-side issue. A useful memory tip: “Reach of zero? Port’s a hero—check UDP 123, the firewall’s the blocker.”
LPIC-1 Essential System Services and Networking Practice Question
This LPIC-1 practice question tests your understanding of essential system services and 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 notices that the NTP service on a Linux server is not synchronizing time with external NTP servers. The administrator runs 'ntpq -p' and sees that all servers listed have a 'reach' value of 0. Which of the following is the most likely cause?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "most likely"
Why it matters: Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
A firewall is blocking UDP port 123.
The `reach` value of 0 in `ntpq -p` output indicates that the NTP client has received no responses from any of the configured servers. Since NTP uses UDP port 123 for communication, a firewall blocking this port would prevent the client from sending or receiving NTP packets, resulting in zero reachability. This is the most common cause when all servers show a reach of 0.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
The NTP service is configured to use the local clock.
Why it's wrong here
Local clock would show reach only if configured as server, not client.
✓
A firewall is blocking UDP port 123.
Why this is correct
Reach 0 indicates no response, common when firewall blocks NTP.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
✗
The NTP server is using a different NTP version.
Why it's wrong here
Version mismatch typically causes 'no data' or 'refused' but not reach 0.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates may confuse a reach value of 0 with a stratum value of 16 or a synchronization failure due to timezone misconfiguration, but the reach value specifically indicates network-level communication failure, not configuration or version issues.
Trap categories for this question
Command / output trap
Local clock would show reach only if configured as server, not client.
Detailed technical explanation
How to think about this question
NTP uses UDP port 123 for both client and server communication. The `reach` value is an octal bitmask that tracks the last 8 poll attempts; a value of 0 means no responses were received in any of those attempts. A common real-world scenario is that an iptables or cloud security group rule blocks outbound UDP 123, or a corporate firewall restricts NTP traffic to specific internal servers, causing all external servers to appear unreachable.
KKey Concepts to Remember
Read the scenario before looking for a memorised answer.
Find the constraint that changes the correct option.
Eliminate answers that are true in general but not in this case.
TExam Day Tips
→Watch for words such as best, first, most likely and least administrative effort.
→Review why wrong options are wrong, not only why the correct option is correct.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
How this comes up in practice
A practitioner preparing for the LPIC-1 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
Got this wrong? Here's your next step.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Essential System Services and Networking — This question tests Essential System Services and Networking — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: A firewall is blocking UDP port 123. — The `reach` value of 0 in `ntpq -p` output indicates that the NTP client has received no responses from any of the configured servers. Since NTP uses UDP port 123 for communication, a firewall blocking this port would prevent the client from sending or receiving NTP packets, resulting in zero reachability. This is the most common cause when all servers show a reach of 0.
What should I do if I get this LPIC-1 question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Are there clue words in this question I should notice?
Yes — watch for: "most likely". Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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