The answer is that the IAM policy is missing permissions for the source database endpoint. AWS DMS requires explicit IAM permissions to create, describe, and test connections to both source and target endpoints, even when the source is on-premises. Without actions like dms:CreateEndpoint and dms:TestConnection, the replication instance cannot validate or access the source endpoint, leading to an access denied error when starting the task. On the AWS Certified Database Specialty DBS-C01 exam, this scenario tests your understanding that DMS IAM policies must cover endpoint operations, not just replication task actions—a common trap where candidates overlook endpoint-specific permissions. Remember that DMS treats endpoints as separate resources requiring their own allow statements. A helpful memory tip: "Endpoints first, tasks second"—always ensure your policy grants dms:CreateEndpoint and dms:TestConnection before worrying about task-level actions.
DBS-C01 Deployment and Migration Practice Question
This DBS-C01 practice question tests your understanding of deployment and migration. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. 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.
Refer to the exhibit. A database administrator is trying to use AWS DMS to migrate data from an on-premises Oracle database to Amazon RDS for Oracle. The DMS replication instance is in the same VPC as the RDS instance. The administrator receives an access denied error when starting the replication task. The IAM policy attached to the DMS replication instance is shown. What is the MOST likely cause of the error?
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
✓
The IAM policy does not include permissions for the source database endpoint
The IAM policy allows dms:CreateReplicationTask and dms:StartReplicationTask on replication tasks, and rds:DescribeDBInstances. However, DMS needs permissions to describe RDS instances (rds:DescribeDBInstances is allowed) and to access the source endpoint (on-premises). But the error is likely due to missing permissions on the source endpoint or network connectivity. However, the most common issue is that the DMS replication instance needs permissions to access the source database, but the policy only allows actions on replication tasks, not on endpoints. DMS also needs dms:CreateEndpoint and dms:TestConnection. However, the policy shown is too restrictive. The question asks for the MOST likely cause given the error: the IAM policy does not include permissions for dms:CreateEndpoint or dms:TestConnection. But the error is when starting the task, which may be due to missing permissions on the source endpoint. However, the best answer is that the IAM policy does not include the necessary permissions for the source endpoint. Option C is correct because DMS needs permissions to describe endpoints and test connections.
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
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
✓
The IAM policy does not include permissions for the source database endpoint
Why this is correct
DMS needs permissions to create and test endpoints, which are missing.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Standard ACLs match source addresses.
✗
The IAM policy does not allow the dms:CreateReplicationInstance action
Why it's wrong here
The error occurs when starting the task, not creating the instance.
✗
The RDS instance is not in the same VPC as the DMS replication instance
Why it's wrong here
The problem states they are in the same VPC.
✗
The DMS replication instance is not associated with the IAM role
Why it's wrong here
The policy is attached to the replication instance, but the instance needs a role with the policy.
Common exam traps
Common exam trap: ACLs stop at the first match
ACLs are processed top to bottom. The first matching entry wins, and an implicit deny usually exists at the end.
Detailed technical explanation
How to think about this question
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.
KKey Concepts to Remember
Standard ACLs match source addresses.
Extended ACLs can match source, destination, protocol and ports.
The first matching ACL entry is used.
There is usually an implicit deny at the end.
TExam Day Tips
→Check inbound versus outbound direction.
→Read the ACL from top to bottom.
→Look for a broader permit or deny above the intended line.
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
How this comes up in practice
A company's IT admin needs to give a contractor read-only access to production logs without sharing account credentials. Using role-based access control (RBAC) and temporary scoped permissions — not a permanent shared password — is the correct pattern. Questions like this test whether you can apply least-privilege access across cloud identity services.
What to study next
Got this wrong? Here's your next step.
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 DBS-C01 ACL questions on filtering logic and placement.
Deployment and Migration — This question tests Deployment and Migration — Standard ACLs match source addresses..
What is the correct answer to this question?
The correct answer is: The IAM policy does not include permissions for the source database endpoint — The IAM policy allows dms:CreateReplicationTask and dms:StartReplicationTask on replication tasks, and rds:DescribeDBInstances. However, DMS needs permissions to describe RDS instances (rds:DescribeDBInstances is allowed) and to access the source endpoint (on-premises). But the error is likely due to missing permissions on the source endpoint or network connectivity. However, the most common issue is that the DMS replication instance needs permissions to access the source database, but the policy only allows actions on replication tasks, not on endpoints. DMS also needs dms:CreateEndpoint and dms:TestConnection. However, the policy shown is too restrictive. The question asks for the MOST likely cause given the error: the IAM policy does not include permissions for dms:CreateEndpoint or dms:TestConnection. But the error is when starting the task, which may be due to missing permissions on the source endpoint. However, the best answer is that the IAM policy does not include the necessary permissions for the source endpoint. Option C is correct because DMS needs permissions to describe endpoints and test connections.
What should I do if I get this DBS-C01 question wrong?
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 DBS-C01 ACL questions on filtering logic and placement.
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?
Standard ACLs match source addresses.
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