The answer is that the iptables rule for port 443 only permits traffic from the 10.0.0.0/8 private subnet, which excludes all external public IP addresses. This is correct because iptables processes rules sequentially, and the explicit rule for HTTPS traffic specifies a source match of 10.0.0.0/8—a private RFC 1918 range—so any packet from an external IP fails that match and is implicitly dropped, even though the default policy is ACCEPT. On the CompTIA Cloud+ CV0-004 exam, this tests your ability to read iptables output and understand how source-based filtering can inadvertently block legitimate external access, a common trap where candidates overlook the source restriction and focus only on the default policy. Remember the mnemonic "Source First" to always check the source IP range in a rule before assuming the default policy controls access.
CV0-004 Security Practice Question
This CV0-004 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.
# iptables -L -n -v
Chain INPUT (policy ACCEPT 0 packets, 0 bytes)
pkts bytes target prot opt in out source destination
100 12000 ACCEPT all -- * * 0.0.0.0/0 0.0.0.0/0 state RELATED,ESTABLISHED
50 6000 ACCEPT tcp -- eth0 * 0.0.0.0/0 0.0.0.0/0 tcp dpt:22
20 2400 ACCEPT tcp -- eth0 * 10.0.0.0/8 0.0.0.0/0 tcp dpt:443
10 1200 DROP tcp -- eth0 * 0.0.0.0/0 0.0.0.0/0 tcp dpt:80
A cloud administrator is troubleshooting connectivity to a web server running on a Linux VM. The web server is configured to listen on ports 80 (HTTP) and 443 (HTTPS). The administrator runs the iptables command shown in the exhibit. Based on the output, what is the MOST likely reason that external users cannot access the web server on port 443?
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.
Refer to the exhibit.
# iptables -L -n -v
Chain INPUT (policy ACCEPT 0 packets, 0 bytes)
pkts bytes target prot opt in out source destination
100 12000 ACCEPT all -- * * 0.0.0.0/0 0.0.0.0/0 state RELATED,ESTABLISHED
50 6000 ACCEPT tcp -- eth0 * 0.0.0.0/0 0.0.0.0/0 tcp dpt:22
20 2400 ACCEPT tcp -- eth0 * 10.0.0.0/8 0.0.0.0/0 tcp dpt:443
10 1200 DROP tcp -- eth0 * 0.0.0.0/0 0.0.0.0/0 tcp dpt:80
A
The web server is not configured to listen on port 443, so iptables rules are irrelevant.
Why wrong: The question states the web server is configured to listen on both ports; the issue is firewall-related.
B
The iptables default policy is ACCEPT, but the rule for port 443 explicitly drops traffic from external sources.
Why wrong: The rule for port 443 does not drop traffic; it accepts from a specific subnet. Traffic from other sources is subject to the default policy.
C
The rule for port 80 is placed before the rule for port 443, causing all HTTPS traffic to be evaluated as HTTP and dropped.
Why wrong: The rules are evaluated in order, but the port numbers are different; HTTPS traffic matches the port 443 rule, not port 80.
D
The iptables rule for port 443 only allows traffic from the 10.0.0.0/8 subnet, which does not include external IP addresses.
The rule for HTTPS only accepts from the private subnet, so external traffic is dropped by the default DROP policy.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The iptables rule for port 443 only allows traffic from the 10.0.0.0/8 subnet, which does not include external IP addresses.
Option D is correct because the iptables rule for port 443 specifies a source IP range of 10.0.0.0/8, which is a private RFC 1918 address space. External users have public IP addresses that do not fall within this subnet, so their HTTPS traffic is implicitly dropped by the rule's match condition. The default policy being ACCEPT does not override the explicit rule that only permits traffic from the 10.0.0.0/8 subnet.
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 web server is not configured to listen on port 443, so iptables rules are irrelevant.
Why it's wrong here
The question states the web server is configured to listen on both ports; the issue is firewall-related.
✗
The iptables default policy is ACCEPT, but the rule for port 443 explicitly drops traffic from external sources.
Why it's wrong here
The rule for port 443 does not drop traffic; it accepts from a specific subnet. Traffic from other sources is subject to the default policy.
✗
The rule for port 80 is placed before the rule for port 443, causing all HTTPS traffic to be evaluated as HTTP and dropped.
Why it's wrong here
The rules are evaluated in order, but the port numbers are different; HTTPS traffic matches the port 443 rule, not port 80.
✓
The iptables rule for port 443 only allows traffic from the 10.0.0.0/8 subnet, which does not include external IP addresses.
Why this is correct
The rule for HTTPS only accepts from the private subnet, so external traffic is 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.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates assume a default ACCEPT policy means all traffic is allowed, overlooking that an explicit rule with a restrictive source match will only permit traffic from that source, effectively denying all others by not matching.
Detailed technical explanation
How to think about this question
In iptables, rules are evaluated in order until a match is found; if no rule matches, the default policy (here ACCEPT) applies. However, the rule for port 443 explicitly matches only source IPs in 10.0.0.0/8, so external traffic (with a different source IP) does not match that rule and falls through to the next rule or default policy. Since the default policy is ACCEPT, external HTTPS traffic would actually be accepted unless a later rule drops it, but the exhibit shows no such drop rule; thus, the most likely cause is that the administrator intended to restrict access but misconfigured the source subnet, inadvertently blocking external users. In practice, this is a common misconfiguration when using iptables for access control lists (ACLs) on cloud VMs.
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 — 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 iptables rule for port 443 only allows traffic from the 10.0.0.0/8 subnet, which does not include external IP addresses. — Option D is correct because the iptables rule for port 443 specifies a source IP range of 10.0.0.0/8, which is a private RFC 1918 address space. External users have public IP addresses that do not fall within this subnet, so their HTTPS traffic is implicitly dropped by the rule's match condition. The default policy being ACCEPT does not override the explicit rule that only permits traffic from the 10.0.0.0/8 subnet.
What should I do if I get this CV0-004 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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