This LFCS practice question tests your understanding of networking. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. 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 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?
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
✓
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.
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 client's public key is not in the remote user's authorized_keys file.
The clue word "most likely" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
✗
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'.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates often 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.
Trap categories for this question
Similar concept trap
Similar to A, would not get publickey error.
Command / output trap
Would timeout or show 'no route to host'.
Detailed technical explanation
How to think about this question
SCP relies on SSH for authentication and encryption. The SSH server's authentication process first checks for publickey authentication; if the client's key is not in authorized_keys, it falls back to password authentication only if PasswordAuthentication is enabled in sshd_config. In many enterprise environments, password authentication is disabled for security, so the only way to authenticate is via a valid public key. The 'Permission denied (publickey)' error specifically means the server rejected all public keys offered by the client and no other authentication methods succeeded.
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 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
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.
Networking — This question tests Networking — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: 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.
What should I do if I get this LFCS 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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Question Discussion
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