The correct answer is IP spoofing, as this scenario is a textbook indicator of an IP spoofing attack detection scenario. When a network device logs packets with an internal source IP address arriving on its external interface, it violates the principle of ingress filtering, which dictates that traffic from outside your network should never carry an internal source address. An attacker forges the source IP in the packet header to impersonate a trusted host, often to bypass ACLs or launch reflection-based attacks. On the CompTIA Network+ N10-009 exam, this concept tests your understanding of network boundary security and common attack signatures; a frequent trap is confusing this with a DDoS or man-in-the-middle attack, but the key clue is the mismatch between the source address and the interface direction. Remember the memory tip: “External interface, internal source — that’s a spoofed course.”
N10-009 Network Security Practice Question
This N10-009 practice question tests your understanding of 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 is investigating a potential breach. A network device shows logs indicating that it received packets with a source IP address belonging to the internal network range on its external (internet-facing) interface. This is a classic indication of which type of attack?
Why wrong: A DoS attack aims to overwhelm a system, but the symptom described (internal source IP on external interface) is specifically characteristic of spoofing.
B
IP spoofing
IP spoofing is when an attacker forges the source IP address to appear as a trusted host. Seeing an internal IP on an external interface is a clear sign of spoofing.
C
Man-in-the-middle (MITM) attack
Why wrong: MITM attacks involve intercepting communications between two parties, usually on the same local network. This scenario shows spoofed packets from outside.
D
ARP poisoning
Why wrong: ARP poisoning manipulates MAC-IP mappings within a local network and would not produce logs of internal source IPs on an external interface.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
IP spoofing
B is correct because receiving packets with a source IP address from the internal network range on an external (internet-facing) interface is a classic sign of IP spoofing. In IP spoofing, an attacker forges the source IP address in packet headers to impersonate a trusted internal host, often to bypass access controls or launch reflection attacks. This violates the expected behavior of ingress filtering, where external interfaces should never see internal source addresses.
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.
✗
Denial-of-service (DoS) attack
Why it's wrong here
A DoS attack aims to overwhelm a system, but the symptom described (internal source IP on external interface) is specifically characteristic of spoofing.
✓
IP spoofing
Why this is correct
IP spoofing is when an attacker forges the source IP address to appear as a trusted host. Seeing an internal IP on an external interface is a clear sign of spoofing.
Related concept
Read the scenario before looking for a memorised answer.
✗
Man-in-the-middle (MITM) attack
Why it's wrong here
MITM attacks involve intercepting communications between two parties, usually on the same local network. This scenario shows spoofed packets from outside.
✗
ARP poisoning
Why it's wrong here
ARP poisoning manipulates MAC-IP mappings within a local network and would not produce logs of internal source IPs on an external interface.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates may confuse IP spoofing with a DoS attack because spoofing is often used in DDoS amplification, but the question's specific clue—internal source IP on an external interface—directly points to spoofing, not the volumetric nature of a DoS.
Trap categories for this question
Command / output trap
MITM attacks involve intercepting communications between two parties, usually on the same local network. This scenario shows spoofed packets from outside.
Scenario analysis trap
MITM attacks involve intercepting communications between two parties, usually on the same local network. This scenario shows spoofed packets from outside.
Detailed technical explanation
How to think about this question
Under the hood, IP spoofing exploits the lack of authentication in the IP protocol (RFC 791), where the source address field can be arbitrarily set. In a real-world scenario, an attacker might spoof an internal IP to bypass firewall rules that permit traffic from trusted subnets, or to amplify a DDoS attack via DNS reflection (e.g., sending queries with a spoofed victim IP). Network administrators can mitigate this with ingress/egress filtering (BCP 38), which drops packets with source addresses that do not match the expected interface subnet.
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 small business has 20 workstations on the 192.168.1.0/24 network and one public IP from its ISP. The router uses PAT (NAT overload) so all 20 devices share one public address using different source ports. NAT questions test whether you understand the four address terms and which direction each translation applies.
Related glossary terms
Concepts from this question explained
These glossary pages explain the core terms tested in this N10-009 question in full detail.
Network Security — This question tests Network Security — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: IP spoofing — B is correct because receiving packets with a source IP address from the internal network range on an external (internet-facing) interface is a classic sign of IP spoofing. In IP spoofing, an attacker forges the source IP address in packet headers to impersonate a trusted internal host, often to bypass access controls or launch reflection attacks. This violates the expected behavior of ingress filtering, where external interfaces should never see internal source addresses.
What should I do if I get this N10-009 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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