The correct answer is DNS tunneling, as abnormal traffic to an external IP on destination port 53 is the hallmark of this attack. DNS tunneling works by encoding non-DNS data—such as exfiltrated files or command-and-control instructions—within DNS query and response packets, allowing covert communication through firewalls that almost always permit outbound DNS traffic. On the CISSP exam, this scenario tests your understanding of network-based covert channels and the importance of monitoring for anomalous DNS behavior, often appearing in the Communication and Network Security domain. A common trap is to confuse this with a standard DNS amplification attack, but remember that amplification uses spoofed source IPs and targets many victims, whereas tunneling focuses on a single external IP for data exfiltration. Memory tip: think of DNS tunneling as a “secret tunnel” inside a legitimate protocol—if the traffic volume on port 53 spikes to one IP, suspect data is being smuggled out.
CISSP Communication and Network Security Practice Question
This CISSP practice question tests your understanding of communication and network security. Read the scenario carefully and evaluate each option against the stated constraints before committing to an answer. 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.
A security analyst receives an alert that a host in the internal network is sending abnormal amounts of traffic to an external IP. The traffic uses destination port 53. What is the most likely attack?
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
✓
DNS tunneling
The alert describes a host sending abnormal traffic to an external IP on destination port 53, which is the default port for DNS. DNS tunneling exploits the DNS protocol to encapsulate non-DNS data (e.g., commands or exfiltrated files) within DNS queries and responses, allowing covert communication through firewalls that typically allow DNS traffic. The abnormal volume of traffic to a single external IP is a classic indicator of a DNS tunnel, as the compromised host continuously sends encoded data to an external command-and-control server.
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.
✗
DNS cache poisoning
Why it's wrong here
Cache poisons DNS records, not cause high volumes of outgoing traffic.
✗
DNS amplification
Why it's wrong here
Amplification attacks generate large responses from external resolvers to a spoofed victim.
✓
DNS tunneling
Why this is correct
DNS tunneling encapsulates data in DNS queries to exfiltrate information, causing abnormal traffic patterns.
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.
✗
DNS zone transfer
Why it's wrong here
Zone transfer is used between DNS servers, not by a single host.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates confuse the use of port 53 with DNS amplification attacks, but amplification requires a victim IP and open resolvers, not a single internal host sending traffic to an external IP.
Detailed technical explanation
How to think about this question
DNS tunneling often uses tools like dnscat2 or Iodine to encode data in DNS query names (e.g., base32-encoded payloads in subdomains) and receive responses in TXT or NULL records. The traffic pattern shows a high volume of small UDP packets to a single external IP, which differs from the normal bursty DNS traffic pattern. In real-world scenarios, attackers may use DNS tunneling to bypass captive portals or exfiltrate data from air-gapped networks by encoding data in DNS queries that traverse allowed egress paths.
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 team runs a vulnerability scan on a web application and discovers an unpatched SQL injection flaw. The team prioritises remediation by CVSS score — critical flaws are patched within 24 hours, high within 7 days. Questions like this test whether you understand vulnerability management processes, scanning tools, and remediation prioritisation.
Related glossary terms
Concepts from this question explained
These glossary pages explain the core terms tested in this CISSP question in full detail.
Communication and Network Security — This question tests Communication and Network Security — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: DNS tunneling — The alert describes a host sending abnormal traffic to an external IP on destination port 53, which is the default port for DNS. DNS tunneling exploits the DNS protocol to encapsulate non-DNS data (e.g., commands or exfiltrated files) within DNS queries and responses, allowing covert communication through firewalls that typically allow DNS traffic. The abnormal volume of traffic to a single external IP is a classic indicator of a DNS tunnel, as the compromised host continuously sends encoded data to an external command-and-control server.
What should I do if I get this CISSP 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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Question Discussion
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