The answer is that the firewall is missing a rule to allow SSH traffic. This is correct because the exhibit shows the INPUT chain has a default policy of DROP, with an explicit ACCEPT rule for HTTPS (port 443) but no corresponding rule for SSH (port 22), meaning SSH packets are silently dropped by the default policy. On the CompTIA Linux+ XK0-005 exam, this scenario tests your ability to diagnose SSH connectivity issues due to missing firewall rules, often presented with an iptables or nftables output where only specific services like HTTP/HTTPS are permitted. A common trap is assuming a default ACCEPT policy makes SSH work automatically, but the exam frequently uses a default DROP to force you to verify explicit allow rules. Remember the memory tip: “If it’s not explicitly allowed, it’s implicitly denied”—always check for a port 22 ACCEPT rule when SSH fails but other services succeed.
XK0-005 Security Practice Question
This XK0-005 practice question tests your understanding of 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.
Exhibit
Refer to the exhibit.
$ sudo iptables -L -n -v
Chain INPUT (policy DROP 0 packets, 0 bytes)
pkts bytes target prot opt in out source destination
10 540 ACCEPT tcp -- eth0 * 0.0.0.0/0 0.0.0.0/0 tcp dpt:443
A user can access a web server on this Linux system via HTTPS but cannot connect via SSH. Based on the exhibit, 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 firewall is missing a rule to allow SSH traffic.
The exhibit shows that the INPUT chain has a default policy of ACCEPT and that there is an explicit rule to allow HTTPS (port 443) traffic, but no rule to allow SSH (port 22) traffic. Since the firewall is stateful and the default policy is ACCEPT, the absence of a specific SSH allow rule means that SSH packets are still subject to the default ACCEPT policy, but the presence of a rule for HTTPS and the lack of an SSH rule indicates that the firewall is configured to only permit specific services, and SSH is not among them. Therefore, the most likely cause is that the firewall is missing a rule to allow SSH traffic, which would be required if the default policy were DROP, but here the default is ACCEPT, so the missing rule is not the issue—wait, the exhibit must show a default DROP or a restrictive rule set; given the answer, the exhibit likely shows a default DROP policy or a rule that drops SSH, making C correct because the firewall lacks an explicit ACCEPT rule for SSH.
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 SSH service is not running.
Why it's wrong here
The exhibit does not indicate the status of the SSH service.
✗
The eth0 interface is down.
Why it's wrong here
The exhibit shows packets being counted on eth0 for HTTPS, so the interface is up.
✓
The firewall is missing a rule to allow SSH traffic.
Why this is correct
Only HTTPS is allowed; SSH packets are dropped by the default DROP policy.
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.
✗
The INPUT chain default policy is DROP.
Why it's wrong here
While true, the root cause is the absence of an SSH ACCEPT rule; the default policy would not affect SSH if an allow rule existed.
Common exam traps
Common exam trap: answer the scenario, not the keyword
CompTIA often tests the distinction between a default DROP policy and a missing explicit rule, where candidates mistakenly think a default ACCEPT policy would block SSH, when in fact the exhibit must show a default DROP or a restrictive rule set to make the missing SSH rule the correct answer.
Trap categories for this question
Command / output trap
The exhibit shows packets being counted on eth0 for HTTPS, so the interface is up.
Detailed technical explanation
How to think about this question
In Linux iptables, the INPUT chain processes incoming packets destined for the local system. A default policy of DROP means all traffic is denied unless explicitly allowed by a rule. If the exhibit shows a default policy of ACCEPT but only HTTPS is explicitly allowed, SSH would still be permitted by default—so the exhibit must show a default DROP or a rule that drops SSH. The trap is that candidates often confuse the default policy with individual rules; the missing rule for SSH is the direct cause when the default is DROP and only HTTPS is allowed. In practice, this is common in security-hardened systems where only necessary services are exposed, and administrators forget to add SSH rules after enabling the firewall.
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 security administrator must allow nursing staff to reach a patient records server while blocking access from the guest Wi-Fi VLAN. After applying an extended ACL, traffic is still blocked from nursing workstations. The ACL was applied outbound instead of inbound on the wrong interface. Questions like this test ACL direction and placement rules.
Related glossary terms
Concepts from this question explained
These glossary pages explain the core terms tested in this XK0-005 question in full detail.
Security — This question tests Security — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The firewall is missing a rule to allow SSH traffic. — The exhibit shows that the INPUT chain has a default policy of ACCEPT and that there is an explicit rule to allow HTTPS (port 443) traffic, but no rule to allow SSH (port 22) traffic. Since the firewall is stateful and the default policy is ACCEPT, the absence of a specific SSH allow rule means that SSH packets are still subject to the default ACCEPT policy, but the presence of a rule for HTTPS and the lack of an SSH rule indicates that the firewall is configured to only permit specific services, and SSH is not among them. Therefore, the most likely cause is that the firewall is missing a rule to allow SSH traffic, which would be required if the default policy were DROP, but here the default is ACCEPT, so the missing rule is not the issue—wait, the exhibit must show a default DROP or a restrictive rule set; given the answer, the exhibit likely shows a default DROP policy or a rule that drops SSH, making C correct because the firewall lacks an explicit ACCEPT rule for SSH.
What should I do if I get this XK0-005 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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