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
Timezone does not affect NTP synchronization.
- ✗
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.
- ✗
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.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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