- A
The default policy DROP on the INPUT chain drops echo replies, but they should be matched by the ESTABLISHED,RELATED rule.
Why wrong: Echo replies are outgoing, so INPUT chain does not process them.
- B
The ICMP rule is placed after the ESTABLISHED,RELATED rule, so it is never evaluated for new ICMP packets.
Why wrong: The ICMP rule is before the DROP rule, so it is evaluated and matches echo requests.
- C
The ACCEPT rule for ICMP only permits echo request (type 8), but ping requires echo reply (type 0) which is not allowed.
Why wrong: Echo reply is outgoing from the server, not incoming.
- D
The ICMP rule only allows incoming echo requests; outgoing echo replies are not covered by the displayed rules and must be allowed by the OUTPUT chain.
Outgoing echo replies require an OUTPUT chain rule to be allowed.
Quick Answer
The correct answer is that the ICMP echo reply is not sent because the OUTPUT chain has a default drop policy and lacks an explicit rule for outgoing echo replies. While the INPUT chain rule correctly allows incoming ICMP echo requests (type 8), the server’s response—an ICMP echo reply (type 0)—is an outgoing packet that must be permitted by the OUTPUT chain. If that chain’s default policy is DROP, the reply is silently discarded, breaking ping even though SSH works fine (SSH uses TCP, which is handled by stateful connection tracking). On the ISC2 Certified in Cybersecurity CC exam, this scenario tests your understanding of iptables chain directionality and the stateless nature of ICMP. A common trap is assuming that allowing incoming traffic automatically permits the corresponding reply; in iptables, each chain is evaluated independently. Memory tip: “Requests in, replies out—check both chains or you’ll be left without a ping.”
ISC2 CC Network Security Practice Question
This CC practice question tests your understanding of network 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.
Refer to the exhibit. A security analyst runs the above iptables command on a Linux server. The server is configured with a default policy of DROP on the INPUT chain. Users report they can SSH to the server but cannot ping it. What is the most likely reason?
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 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 ICMP rule only allows incoming echo requests; outgoing echo replies are not covered by the displayed rules and must be allowed by the OUTPUT chain.
Option D is correct because the displayed iptables rules only govern the INPUT chain, which controls incoming packets. While the rule allows incoming ICMP echo requests (type 8), the server's response—an ICMP echo reply (type 0)—is an outgoing packet that must traverse the OUTPUT chain. If the OUTPUT chain has a default policy of DROP or lacks an explicit ACCEPT rule for ICMP echo replies, the replies are dropped, preventing ping from working even though SSH succeeds (since SSH uses TCP, which is handled differently).
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 default policy DROP on the INPUT chain drops echo replies, but they should be matched by the ESTABLISHED,RELATED rule.
Why it's wrong here
Echo replies are outgoing, so INPUT chain does not process them.
- ✗
The ICMP rule is placed after the ESTABLISHED,RELATED rule, so it is never evaluated for new ICMP packets.
Why it's wrong here
The ICMP rule is before the DROP rule, so it is evaluated and matches echo requests.
- ✗
The ACCEPT rule for ICMP only permits echo request (type 8), but ping requires echo reply (type 0) which is not allowed.
Why it's wrong here
Echo reply is outgoing from the server, not incoming.
- ✓
The ICMP rule only allows incoming echo requests; outgoing echo replies are not covered by the displayed rules and must be allowed by the OUTPUT chain.
Why this is correct
Outgoing echo replies require an OUTPUT chain rule to be allowed.
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
ISC2 often tests the misconception that a single chain (INPUT) controls all traffic to and from the server, leading candidates to overlook the fact that outgoing packets (like ICMP echo replies) are filtered by the OUTPUT chain, not the INPUT chain.
Detailed technical explanation
How to think about this question
In iptables, packet filtering is chain-specific: the INPUT chain processes packets destined for the local system, while the OUTPUT chain processes packets originating from the local system. ICMP echo requests (type 8) arrive at the server and are matched by the INPUT rule, but the kernel generates an echo reply (type 0) as a new packet that must pass through the OUTPUT chain. Even if the OUTPUT chain has a default policy of ACCEPT, a restrictive default (e.g., DROP) or a missing rule will block the reply. This is a common misconfiguration where administrators focus only on inbound rules and forget that outbound traffic for stateless protocols like ICMP requires explicit allowance.
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 SOC analyst notices unusual lateral movement in the network at 2 AM. The IR playbook dictates: identify and contain (isolate the affected machine), then eradicate (remove the malware), then recover (restore from backup), then document. Skipping containment before eradication risks the attacker regaining access. Questions like this test the sequence and rationale of incident response phases.
What to study next
Got this wrong? Here's your next step.
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FAQ
Questions learners often ask
What does this CC question test?
Network Security — This question tests Network Security — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The ICMP rule only allows incoming echo requests; outgoing echo replies are not covered by the displayed rules and must be allowed by the OUTPUT chain. — Option D is correct because the displayed iptables rules only govern the INPUT chain, which controls incoming packets. While the rule allows incoming ICMP echo requests (type 8), the server's response—an ICMP echo reply (type 0)—is an outgoing packet that must traverse the OUTPUT chain. If the OUTPUT chain has a default policy of DROP or lacks an explicit ACCEPT rule for ICMP echo replies, the replies are dropped, preventing ping from working even though SSH succeeds (since SSH uses TCP, which is handled differently).
What should I do if I get this CC 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.
About these practice questions
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Last reviewed: Jun 30, 2026
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