200-201 Network Intrusion Analysis Practice Question
An analyst examining a PCAP sees an internal host sending ICMP echo requests where the payload length is consistently 1,100 bytes and the payload bytes change on every packet, while the destination is an external IP that returns echo replies of normal size. The host has no monitoring tool installed and no legitimate reason to send large ICMP. Which technique is most likely being used?
⚠ Common exam trap
The trap here is treating any large ICMP packet as a denial-of-service or MTU issue, when varying payload contents inside echo requests reveal a covert data channel instead.
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
✓
ICMP tunneling used to exfiltrate or relay data inside echo request payloads.
Normal ICMP echo traffic uses small, fixed payloads for reachability. Large echo requests whose payload changes on every packet indicate the ICMP payload is being used as a transport for data, a covert channel known as ICMP tunneling. The external endpoint returning ordinary replies supports a request-carries-data-out model. Flood, MTU discovery, and smurf do not produce varying large payloads from a single internal host.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
A ping flood denial-of-service attack against the external host.
Why it's wrong here
A ping flood sends a high volume of echo requests to overwhelm a target, but it uses many sources or rapid repetition against the victim, and the payload is typically small or default-sized. Here the emphasis is on large, varying payloads from one internal host, not saturation volume. A flood would also not require the payload contents to change on every packet, since reachability is the only goal.
- ✗
Path MTU discovery using oversized ICMP packets to find fragmentation limits.
Why it's wrong here
Path MTU discovery relies on ICMP fragmentation-needed messages and DF-bit probing, not on sending large echo requests with unique payloads to an external host. The technique involves receiving specific ICMP error types about packet size, and the payloads are not meaningful data. The observed behavior of changing payload content is unrelated to MTU probing.
- ✓
ICMP tunneling used to exfiltrate or relay data inside echo request payloads.
Why this is correct
Large, consistently sized ICMP echo requests with payloads that change on every packet indicate data is being carried inside the ICMP payload rather than standard reachability testing. Normal ping payloads are small and static, often a fixed pattern. The external host returning normal-sized replies fits a covert channel where the request carries the data outbound. This is a classic ICMP tunneling signature that warrants payload inspection and host isolation.
- ✗
A smurf attack reflecting ICMP echo requests off the external host to a broadcast address.
Why it's wrong here
A smurf attack spoofs the victim's source address and sends echo requests to a network broadcast, causing many hosts to reply to the victim. In this scenario the internal host uses its own address and receives normal replies, with no broadcast amplification occurring. The changing payload size is also inconsistent with smurf, which depends on reflection volume rather than embedded data.
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JA
Written and reviewed by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
Last reviewed September 2026 · checked against the official Cisco exam blueprint
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