LPIC-1 Essential System Services and Networking Practice Question
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?
⚠ Common exam trap
Candidates often 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.
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
✓
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.
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 system timezone is incorrectly set.
Why it's wrong here
A wrong timezone shifts displayed local time but leaves NTP packet exchange and the reach register unaffected, so reach would not be zero. It is tempting because timezone misconfiguration does cause apparent clock errors, but it is the correct diagnosis when times look wrong yet synchronisation itself succeeds.
- ✗
The NTP service is configured to use the local clock.
Why it's wrong here
Configuring the local clock as a reference makes the server serve time rather than poll upstream servers, yet the question describes configured external servers showing zero reach, indicating no replies. It is tempting because local-clock configuration is correct for an isolated stratum-1 server with no external sources.
- ✓
A firewall is blocking UDP port 123.
Why this is correct
A reach value of 0 means no NTP reply packets have been received in the last eight poll intervals. Blocked UDP port 123 prevents the server responses from arriving, so the client cannot synchronise despite the daemon running.
- ✗
The NTP server is using a different NTP version.
Why it's wrong here
Version mismatch between NTP peers still permits packet exchange, so reach would increment rather than stay at zero. A reach of 0 means no reply to the last eight polls, indicating blocked UDP port 123 or unreachable servers. Version differences matter when configuring authentication or interop with legacy stratum-1 appliances.
Visual reference
Go deeper
Related to this question
About these practice questions
One of 402 original LPIC-1 practice questions on Courseiva, each with a full explanation and wrong-answer analysis — not exam dumps or protected exam content. Learn why practice questions differ from exam dumps →
JA
Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This LPIC-1 practice question is part of Courseiva's free LPI 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 LPIC-1 exam.