The answer is that the ACL is restricting DNS to a single server, causing resolution failure when users query a different DNS server. This occurs because the access control list permits UDP port 53 traffic only to host 10.2.2.10, so any DNS query sent to another server is dropped by the firewall. Users can still browse HTTPS websites because that traffic uses TCP port 443, which is not restricted by this ACL. On the CISA exam, this scenario tests your understanding of how overly restrictive ACLs can break specific services while leaving others functional—a common trap is assuming all web traffic fails together. Remember the memory tip: “DNS is a one-server show; if the ACL says no, the names don’t go.”
CISA Protection of Information Assets Practice Question
This CISA practice question tests your understanding of protection of information assets. 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.
Exhibit: Firewall rule excerpt (Cisco ASA)
access-list INSIDE extended permit tcp 10.1.1.0 255.255.255.0 any eq 443
access-list INSIDE extended permit udp 10.1.1.0 255.255.255.0 host 10.2.2.10 eq 53
access-list INSIDE extended deny ip any any
interface GigabitEthernet0/0
nameif INSIDE
security-level 100
ip address 10.1.1.1 255.255.255.0
interface GigabitEthernet0/1
nameif OUTSIDE
security-level 0
ip address 192.168.1.1 255.255.255.0
route OUTSIDE 0.0.0.0 0.0.0.0 192.168.1.254 1
Refer to the exhibit. A security analyst notices that users on the INSIDE network (10.1.1.0/24) can browse HTTPS websites but cannot resolve domain names. 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 ACL only permits DNS traffic to host 10.2.2.10, but users need to query a different DNS server
The exhibit shows an ACL that permits DNS traffic (UDP port 53) only to host 10.2.2.10. Since users can browse HTTPS (TCP/443) but cannot resolve domain names, the ACL is blocking DNS queries to any other DNS server. Option B correctly identifies that the ACL restricts DNS to a single server, and if users are configured to query a different DNS server, resolution fails.
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 ACL denies TCP traffic to port 443
Why it's wrong here
The ACL permits TCP to any destination on port 443.
✓
The ACL only permits DNS traffic to host 10.2.2.10, but users need to query a different DNS server
Why this is correct
The DNS request to an external server is denied because the ACL only allows UDP to 10.2.2.10.
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 DNS server at 10.2.2.10 is unreachable
Why it's wrong here
The ACL permits UDP to 10.2.2.10 port 53, so if that server is reachable, DNS should work.
✗
The OUTSIDE interface has no security-level configured correctly
Why it's wrong here
Security-level 0 is standard for outside interfaces.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates assume DNS resolution failure must be due to a DNS server being unreachable (Option C), but the ACL is actually restricting the destination IP of DNS queries, not the protocol itself.
Detailed technical explanation
How to think about this question
DNS resolution uses UDP port 53 by default, and the ACL in the exhibit explicitly permits UDP port 53 only to host 10.2.2.10. If clients are configured with a different DNS server (e.g., 8.8.8.8 or a corporate DNS at 10.2.2.11), the ACL drops the query, causing resolution failure while HTTPS traffic (permitted by a separate ACE) still works. In Cisco ASA, ACLs are evaluated top-down, and the implicit deny at the end blocks any traffic not explicitly permitted.
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.
Protection of Information Assets — This question tests Protection of Information Assets — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The ACL only permits DNS traffic to host 10.2.2.10, but users need to query a different DNS server — The exhibit shows an ACL that permits DNS traffic (UDP port 53) only to host 10.2.2.10. Since users can browse HTTPS (TCP/443) but cannot resolve domain names, the ACL is blocking DNS queries to any other DNS server. Option B correctly identifies that the ACL restricts DNS to a single server, and if users are configured to query a different DNS server, resolution fails.
What should I do if I get this CISA 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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