The answer is to obtain the missing public key and add it permanently using `apt-key add` or by piping the key with `wget -O- | apt-key add -`. This is correct because the GPG NO_PUBKEY error occurs when APT encounters a signed repository’s Release file but cannot verify its authenticity due to the absence of the corresponding public key in the system’s trusted keyring; without this key, APT refuses to trust the repository metadata, causing update failures. On the LPIC-2 exam, this scenario tests your understanding of secure APT repository management and GPG signature verification, often appearing in questions about adding third-party repositories or troubleshooting `apt update` errors. A common trap is assuming the key is already present or that a temporary workaround like disabling signature checking is acceptable—neither is correct for a permanent fix. Memory tip: think “NO_PUBKEY = No Public Key, so go get the key and add it.”
LPIC-2 System Security Practice Question
This LPIC-2 practice question tests your understanding of system security. 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.
Network Topology
Given the exhibit, what is the most likely reason for the GPG error, and what is the correct way to fix it permanently?
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 public key is missing; obtain and add it with 'apt-key add' or 'wget -O- | apt-key add -'
The GPG error indicates that the repository's Release file is signed but the system lacks the corresponding public key to verify the signature. This is a common issue when adding third-party repositories. The correct permanent fix is to obtain the missing public key and add it to the APT keyring using 'apt-key add' or by piping the key with 'wget -O- | apt-key add -', which allows APT to authenticate the repository's metadata.
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 repository is not signed; use '--allow-unauthenticated' permanently in /etc/apt/apt.conf.d/
Why it's wrong here
This bypasses security permanently, not recommended.
✗
The repository URL is incorrect; change 'http://deb.example.com' to 'https://deb.example.com'
The InRelease file is corrupted; remove it and run 'apt-get update' again
Why it's wrong here
The file is likely fine; the key is missing.
✓
The public key is missing; obtain and add it with 'apt-key add' or 'wget -O- | apt-key add -'
Why this is correct
Adding the correct public key resolves the error.
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.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates often confuse a missing GPG key with a corrupted file or an incorrect repository URL, but the GPG error message explicitly mentions 'NO_PUBKEY', which directly points to a missing public key.
Detailed technical explanation
How to think about this question
APT uses GPG signatures to verify the integrity and authenticity of repository metadata (Release files). The public key is stored in the trusted keyring under /etc/apt/trusted.gpg.d/ or /etc/apt/trusted.gpg. When a key is missing, APT reports a 'NO_PUBKEY' error. Adding the key with 'apt-key add' imports it into the keyring, but note that 'apt-key' is deprecated in newer Debian/Ubuntu versions in favor of placing keys in /etc/apt/trusted.gpg.d/ as individual .asc files. A real-world scenario is adding the Docker or Microsoft repository, where the key must be fetched and added before the repository can be used securely.
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 small business has 20 workstations on the 192.168.1.0/24 network and one public IP from its ISP. The router uses PAT (NAT overload) so all 20 devices share one public address using different source ports. NAT questions test whether you understand the four address terms and which direction each translation applies.
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.
System Security — This question tests System Security — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The public key is missing; obtain and add it with 'apt-key add' or 'wget -O- | apt-key add -' — The GPG error indicates that the repository's Release file is signed but the system lacks the corresponding public key to verify the signature. This is a common issue when adding third-party repositories. The correct permanent fix is to obtain the missing public key and add it to the APT keyring using 'apt-key add' or by piping the key with 'wget -O- | apt-key add -', which allows APT to authenticate the repository's metadata.
What should I do if I get this LPIC-2 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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