LFCS Networking Practice Question
A system administrator needs to securely transfer files between two Linux servers using port 22. The administrator uses the following command: 'scp file.txt user@remote:/tmp/'. The transfer fails with the error 'Permission denied (publickey)'. What is the most likely cause?
⚠ Common exam trap
Test-takers frequently confuse network-level issues (firewall, service status) with authentication-level errors, but the specific 'Permission denied (publickey)' message directly points to SSH key authentication failure, not connectivity or service availability.
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
✓
The client's public key is not in the remote user's authorized_keys file.
The error 'Permission denied (publickey)' indicates that the SSH key-based authentication failed. SCP uses SSH for transport, and by default, SSH on the remote server checks the client's public key against the remote user's ~/.ssh/authorized_keys file. If the client's public key is not listed there, the SSH server rejects the connection, causing the SCP transfer to fail.
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 client's public key is not in the remote user's authorized_keys file.
Why this is correct
'Permission denied (publickey)' indicates key authentication failed.
- ✗
The remote server does not have SSH installed.
Why it's wrong here
Similar to A, would not get publickey error.
- ✗
The SSH service is not running on the remote server.
Why it's wrong here
Would give 'Connection refused', not 'Permission denied'.
- ✗
The remote server's firewall is blocking port 22.
Why it's wrong here
Would timeout or show 'no route to host'.
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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.