IP Spoofing: Identifying Forged Source IP Addresses on External Interfaces
A security analyst is reviewing firewall logs and sees many incoming packets with a source IP address that matches the internal IP range of the company (10.0.0.0/8) arriving on the external interface. Which type of attack is likely being attempted?
Quick Answer
The detail that identifies this as IP spoofing is where the traffic is arriving versus what address it claims to come from: packets showing an internal 10.0.0.0/8 source address should only ever originate from inside the network, yet these are arriving on the external interface, which is a logical impossibility unless the source address has been forged. Attackers do this deliberately because many firewall rules and internal systems extend a degree of implicit trust to traffic that appears to originate from inside the network, so by faking an internal-looking source address, the attacker hopes to slip past filtering rules that would otherwise block or scrutinize genuine external traffic more closely. This is different from an attack that merely floods a target or manipulates a local Layer 2 mapping, since spoofing specifically targets the trust placed in the IP header's source field rather than exhausting a resource or corrupting a local cache. Firewalls can be configured to catch this by rejecting packets whose source address doesn't logically match the interface it arrived on. Any time an exam scenario shows internal-looking or otherwise implausible source addresses appearing on the wrong interface, that mismatch between claimed origin and actual arrival point is the signature of an IP spoofing attempt.
⚠ Common exam trap
CompTIA often tests the distinction between IP spoofing and other attacks by focusing on the specific packet characteristic (source IP matching internal range on an external interface) rather than the attack's goal or volume, leading candidates to confuse it with a Smurf or SYN flood attack.
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
IP spoofing attack
Packets arriving on the external interface with a source IP address from the internal 10.0.0.0/8 range indicate that the attacker is forging (spoofing) the source address to impersonate an internal host. This is a classic IP spoofing attack, often used to bypass access control lists or to launch further attacks that rely on trust relationships based on source IP.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
Smurf attack
Why it's wrong here
A Smurf attack uses ICMP echo requests with a spoofed source IP to broadcast addresses, causing amplification; the source IP is not necessarily internal.
When this WOULD be correct
A Smurf attack would be correct if the question described the attacker sending ICMP echo requests to a network broadcast address with a spoofed source IP of the victim, causing all hosts on that network to reply to the victim, overwhelming it.
- ✓
IP spoofing attack
Why this is correct
Correct. The attacker is spoofing the source IP address to appear as an internal host, trying to bypass firewall rules that may allow internal traffic without inspection.
- ✗
SYN flood
Why it's wrong here
A SYN flood sends many TCP SYN packets without completing handshakes, but the source IPs are often random, not specifically internal.
When this WOULD be correct
A SYN flood would be correct if the question described a high volume of incomplete TCP handshakes (SYN packets without final ACK) overwhelming a server, causing denial of service, without mention of source IP matching internal ranges.
- ✗
DNS amplification
Why it's wrong here
DNS amplification uses open DNS resolvers to flood a target with large DNS responses; the source IPs are the resolvers, not internal addresses.
When this WOULD be correct
A DNS amplification attack would be correct if the question described a scenario where the attacker sends small queries to open DNS resolvers with a spoofed victim IP, resulting in large responses flooding the victim. For example: 'A security analyst notices a high volume of DNS responses from external servers to a single internal server, causing bandwidth exhaustion.'
Option-by-option analysis
Why each answer is right or wrong
Understanding why wrong answers are wrong — and when they would be correct — is what separates a 750 score from a 900. The N10-009 exam frequently reuses these exact scenarios with slightly different constraints.
✓IP spoofing attackCorrect answer▾
Why this is correct
Correct. The attacker is spoofing the source IP address to appear as an internal host, trying to bypass firewall rules that may allow internal traffic without inspection.
✗Smurf attackWrong answer — click to see why▾
Why this is wrong here
A Smurf attack uses ICMP echo requests with a spoofed source IP to cause a broadcast storm, but the question describes incoming packets with a source IP matching the internal range on the external interface, which is classic IP spoofing, not a Smurf attack.
★ When this WOULD be the correct answer
A Smurf attack would be correct if the question described the attacker sending ICMP echo requests to a network broadcast address with a spoofed source IP of the victim, causing all hosts on that network to reply to the victim, overwhelming it.
Why candidates choose this
Candidates may confuse any attack involving spoofed IPs with a Smurf attack, or they may recall that Smurf attacks use spoofed source addresses, but they forget the specific mechanism of broadcast amplification.
✗SYN floodWrong answer — click to see why▾
Why this is wrong here
A SYN flood attack involves sending many TCP SYN packets to exhaust server resources, but the question describes packets with a spoofed internal source IP arriving on the external interface, which is characteristic of IP spoofing, not SYN flood.
★ When this WOULD be the correct answer
A SYN flood would be correct if the question described a high volume of incomplete TCP handshakes (SYN packets without final ACK) overwhelming a server, causing denial of service, without mention of source IP matching internal ranges.
Why candidates choose this
Candidates may confuse any flood of packets with a SYN flood, especially when the question mentions 'many incoming packets,' leading them to choose a common DoS attack without analyzing the specific IP spoofing indicator.
✗DNS amplificationWrong answer — click to see why▾
Why this is wrong here
DNS amplification attacks use open DNS servers to amplify traffic to a victim, but they do not involve source IP addresses matching the internal range of the target. The question describes packets with internal IPs arriving externally, which is characteristic of IP spoofing, not DNS amplification.
★ When this WOULD be the correct answer
A DNS amplification attack would be correct if the question described a scenario where the attacker sends small queries to open DNS resolvers with a spoofed victim IP, resulting in large responses flooding the victim. For example: 'A security analyst notices a high volume of DNS responses from external servers to a single internal server, causing bandwidth exhaustion.'
Why candidates choose this
Candidates may confuse any attack involving spoofed IPs with amplification attacks, or they might think that the internal IP range in packets indicates a reflection attack, but DNS amplification specifically uses DNS traffic, not arbitrary packets with internal IPs.
Analysis generated from the official N10-009blueprint and verified against question context. The “when correct” sections are what AI assistants cite when candidates ask “what’s the difference between these options?”
Visual reference
Quick reference
IPv4 Address Class Summary
| Class | First Octet Range | Default Mask | Networks | Hosts per Network |
|---|---|---|---|---|
| A | 1–126 | /8 (255.0.0.0) | 126 | 16,777,214 |
| B | 128–191 | /16 (255.255.0.0) | 16,384 | 65,534 |
| C | 192–223 | /24 (255.255.255.0) | 2,097,152 | 254 |
| D | 224–239 | N/A | Multicast groups | — |
| E | 240–255 | N/A | Reserved / experimental | — |
127.x.x.x is reserved for loopback. Modern networks use CIDR (classless) rather than classful addressing.
Go deeper
Related to this question
Learn chapter
Network Access Control (NAC)
Key term
Least privilege
Least privilege is a security principle that means giving users, systems, or programs only the minimum permissions they need to do their job and nothing more.
Key term
IP address
An IP address is a unique numerical label assigned to each device connected to a computer network that uses the Internet Protocol for communication.
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Same concept, more angles
1 more way this is tested on N10-009
These questions test the same concept from different angles. Work through them to make sure you can recognise it however the exam phrases it.
Variation 1. A network security analyst notices that the firewall is logging traffic on the external interface that has a source IP address of 10.0.1.5, which is within the internal network range. This is most likely the result of which type of attack?
medium- A.DNS poisoning
- ✓ B.IP spoofing
- C.ARP poisoning
- D.VLAN hopping
Why B: The firewall is logging traffic on its external interface with a source IP address from the internal RFC 1918 range (10.0.1.5). This indicates the source IP has been forged, because private IP addresses should never appear as source addresses on a public-facing interface. This is the classic signature of an IP spoofing attack, where the attacker modifies the source IP in the packet header to impersonate an internal host.
JA
Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This N10-009 practice question is part of Courseiva's free CompTIA certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the N10-009 exam.