Quick Answer
The answer is the custom format with the 'action' field. VPC Flow Logs by default capture only the standard fields like source and destination IPs, ports, and protocol, but they omit the 'action' field, which indicates whether traffic was ACCEPTED or REJECTED by security groups or network ACLs. To specifically capture rejected traffic, you must define a custom flow log format that explicitly includes the 'action' field, as this is the only way to filter or analyze rejections. On the AWS Certified Advanced Networking Specialty ANS-C01 exam, this concept tests your understanding of flow log granularity and the distinction between default and custom formats—a common trap is assuming the default log includes all metadata. Remember, the default format is like a basic snapshot; the custom format is your configurable lens. Memory tip: "No action, no rejection—custom format is your rejection detection."
This ANS-C01 practice question tests your understanding of network management and operations. Match the stated requirement to the specific cloud service, access model, or configuration option — many options are valid in isolation but not for this scenario. 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 network engineer is monitoring network traffic using VPC Flow Logs. The engineer wants to capture traffic that is rejected by security groups and network ACLs. Which flow log format should be used?
Default format
Why wrong: Default format does not include the action field.
Flow logs are delivered to CloudWatch Logs with DNS query logs
Why wrong: DNS query logs are separate.
Custom format with 'srcaddr' and 'dstaddr' only
Why wrong: Missing the action field.
Custom format with 'action' field
The action field shows ACCEPT or REJECT.
Answer choices
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
Custom format with 'action' field
Option D is correct because the default format includes only the first 20-30 bytes of the packet, not the action field. To capture rejections, the custom format must include 'action'. Option A is incorrect because the default format omits action. Option B is incorrect because the default format does not include it. Option C is incorrect because DNS is not involved.
Key principle: ACLs process entries top to bottom and stop at the first match. Entry order and interface direction matter as much as the permit or deny statement.
Answer analysis
For each option: why learners choose it and why it is or isn't the right answer here.
Default format
Why it's wrong here
Default format does not include the action field.
Flow logs are delivered to CloudWatch Logs with DNS query logs
Why it's wrong here
DNS query logs are separate.
Custom format with 'srcaddr' and 'dstaddr' only
Why it's wrong here
Missing the action field.
Custom format with 'action' field
Why this is correct
The action field shows ACCEPT or REJECT.
Related concept
Standard ACLs match source addresses.
Common exam traps
ACLs are processed top to bottom. The first matching entry wins, and an implicit deny usually exists at the end.
Detailed technical explanation
ACL questions test precision: source, destination, protocol, port and direction. A generally correct ACL can still fail if it is applied on the wrong interface or in the wrong direction.
Key takeaway
ACLs process entries top to bottom and stop at the first match. Entry order and interface direction matter as much as the permit or deny statement.
Real-world example
A healthcare organisation deploys an application with a public-facing web tier and a private database tier. The database subnet has no public IP and only accepts connections from the web tier's security group. Questions like this test whether you can design cloud network isolation using VNets/VPCs, subnets, and security group rules.
What to study next
Review ACL processing order, placement rules (standard near destination, extended near source), and inbound vs outbound direction. Study wildcard masks and implicit deny. Then practise related ANS-C01 ACL questions on filtering logic and placement.
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FAQ
Network Management and Operations — This question tests Network Management and Operations — Standard ACLs match source addresses..
The correct answer is: Custom format with 'action' field — Option D is correct because the default format includes only the first 20-30 bytes of the packet, not the action field. To capture rejections, the custom format must include 'action'. Option A is incorrect because the default format omits action. Option B is incorrect because the default format does not include it. Option C is incorrect because DNS is not involved.
Review ACL processing order, placement rules (standard near destination, extended near source), and inbound vs outbound direction. Study wildcard masks and implicit deny. Then practise related ANS-C01 ACL questions on filtering logic and placement.
Standard ACLs match source addresses.
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Last reviewed: Jun 20, 2026
This ANS-C01 practice question is part of Courseiva's free Amazon Web Services 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 ANS-C01 exam.
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