hardMultiple ChoiceObjective-mapped
ISC2 CC Practice Question: Refer to the exhibit
Network Topology
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?
⚠ Common exam trap
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.
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.
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).
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.
Go deeper
Related to this question
Learn chapter
Introduction to Security Principles
Key term
SSH
SSH (Secure Shell) is a cryptographic network protocol that provides secure, encrypted communication and remote administration between two devices over an unsecured network.
Key term
ICMP
ICMP is a network-layer protocol used by network devices to send error messages and operational information about network connectivity.
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JA
Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This CC practice question is part of Courseiva's free ISC2 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 CC exam.