This CAS-004 practice question tests your understanding of security engineering. 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.
```
# iptables -L FORWARD -v -n
Chain FORWARD (policy DROP 0 packets, 0 bytes)
pkts bytes target prot opt in out source destination
0 0 ACCEPT all -- eth0 eth1 10.0.1.0/24 0.0.0.0/0 state NEW,ESTABLISHED
0 0 ACCEPT all -- eth1 eth0 0.0.0.0/0 10.0.1.0/24 state ESTABLISHED
```
A network administrator is troubleshooting connectivity issues. Based on the exhibit, which of the following is true about the iptables rules?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Traffic from 10.0.1.0/24 to the internet is allowed, but return traffic is only allowed if it is part of an established connection.
Option C is correct because the iptables rules show a default FORWARD policy of ACCEPT, but the INPUT chain has a default policy of DROP. The rule '-A FORWARD -s 10.0.1.0/24 -j ACCEPT' allows outbound traffic from the 10.0.1.0/24 subnet to the internet. The rule '-A FORWARD -m state --state ESTABLISHED,RELATED -j ACCEPT' ensures that only return traffic belonging to an existing connection is permitted back, while new inbound connections from the internet are implicitly dropped by the FORWARD chain's default policy.
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.
✗
New connections from the internet to 10.0.1.0/24 are allowed.
Why it's wrong here
Only established connections from internet to 10.0.1.0/24 are allowed; new connections are dropped.
✗
All traffic is allowed by default because the policy is ACCEPT.
Why it's wrong here
The policy is DROP, not ACCEPT.
✓
Traffic from 10.0.1.0/24 to the internet is allowed, but return traffic is only allowed if it is part of an established connection.
Why this is correct
The first rule matches new and established traffic from 10.0.1.0/24; the second matches only established traffic back.
Related concept
Read the scenario before looking for a memorised answer.
✗
The rules apply to incoming traffic on the firewall itself.
Why it's wrong here
The FORWARD chain applies to traffic passing through the firewall, not traffic destined to the firewall.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates often confuse the FORWARD chain with the INPUT chain, assuming that rules in the FORWARD chain apply to traffic destined for the firewall itself, or they overlook that the default policy for the FORWARD chain is DROP (not ACCEPT) unless explicitly set to ACCEPT in the exhibit.
Detailed technical explanation
How to think about this question
In iptables, the FORWARD chain governs traffic that passes through the firewall (i.e., routed traffic), while the INPUT chain handles traffic destined for the firewall host. The state module with --state ESTABLISHED,RELATED leverages connection tracking to permit return packets for existing sessions, which is critical for stateful firewall behavior. A common real-world scenario is a NATgateway where internal hosts initiate outbound connections; the firewall must allow related return traffic while blocking unsolicited inbound packets.
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.
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.
Security Engineering — This question tests Security Engineering — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: Traffic from 10.0.1.0/24 to the internet is allowed, but return traffic is only allowed if it is part of an established connection. — Option C is correct because the iptables rules show a default FORWARD policy of ACCEPT, but the INPUT chain has a default policy of DROP. The rule '-A FORWARD -s 10.0.1.0/24 -j ACCEPT' allows outbound traffic from the 10.0.1.0/24 subnet to the internet. The rule '-A FORWARD -m state --state ESTABLISHED,RELATED -j ACCEPT' ensures that only return traffic belonging to an existing connection is permitted back, while new inbound connections from the internet are implicitly dropped by the FORWARD chain's default policy.
What should I do if I get this CAS-004 question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
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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