AWS DMS IAM Permissions: Missing DescribeReplicationInstances
Exhibit
Refer to the exhibit.
```
{
"Version": "2012-10-17",
"Statement": [
{
"Effect": "Allow",
"Action": [
"dms:CreateReplicationTask",
"dms:StartReplicationTask",
"dms:DescribeReplicationTasks"
],
"Resource": "*"
},
{
"Effect": "Allow",
"Action": [
"rds:DescribeDBInstances",
"rds:ModifyDBInstance"
],
"Resource": "arn:aws:rds:us-east-1:123456789012:db:targetdb"
}
]
}
```Refer to the exhibit. An IAM policy is attached to an IAM user that performs database migrations. When the user tries to start an AWS DMS replication task that writes to the RDS instance 'targetdb', the task fails with an access denied error. Which additional permission is required?
Quick Answer
The correct answer is dms:DescribeReplicationInstances because, while the IAM policy grants dms:StartReplicationTask, AWS DMS internally calls DescribeReplicationInstances to verify the target replication instance exists and is in a valid state before launching the task; without this read permission, the service cannot confirm the instance’s availability and returns an access denied error. On the AWS Certified Database Specialty DBS-C01 exam, this scenario tests your understanding that DMS actions often require supporting describe permissions even when the primary action is allowed—a common trap is assuming only write permissions like dms:CreateReplicationInstance are needed, but if the instance already exists, the missing describe call is the blocker. To remember this, think of DMS as needing to “see” before it can “do”: every StartReplicationTask first requires a DescribeReplicationInstances to locate the instance, just like you need to find a car before you can drive it.
⚠ Common exam trap
Watch out — candidates often assume the error is due to missing permissions for the target database (RDS) or for creating resources, when in fact DMS requires read-only describe permissions on the replication instance to validate its state before starting a task.
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
✓
dms:DescribeReplicationInstances
The DMS replication task requires permission to describe the replication instance that is being used to run the task. The `dms:DescribeReplicationInstances` action allows the IAM user to retrieve metadata about the replication instance, which DMS needs to validate the instance state and configuration before starting the task. Without this permission, the start task operation fails with an access denied error even if the user has permissions to start the task itself.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
dms:CreateEndpoint
Why it's wrong here
If endpoints already exist, not needed.
- ✗
dms:CreateReplicationInstance
Why it's wrong here
If instance already exists, this permission is not needed.
- ✓
dms:DescribeReplicationInstances
Why this is correct
The user may need to describe replication instances to select one for the task.
- ✗
rds:ModifyDBInstance on the source database
Why it's wrong here
Source not specified, and ModifyDBInstance is already allowed on target.
Go deeper
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Same concept, more angles
1 more way this is tested on DBS-C01
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 company is migrating a database using AWS DMS. The IAM policy shown is attached to the DMS replication instance role. When the DMS replication task is created in the us-west-2 region, it fails. What is the most likely cause?
medium- A.The IAM policy does not allow the 'dms:CreateReplicationTask' action.
- B.The IAM policy does not specify the VPC where the replication instance is deployed.
- C.The IAM policy does not specify the resource ARN for the replication instance.
- ✓ D.The IAM policy restricts the region to us-east-1, but the task is created in us-west-2.
Why D: The IAM policy explicitly restricts the region to us-east-1 using the `aws:RequestedRegion` condition key. When the DMS replication task is created in us-west-2, the policy denies the `dms:CreateReplicationTask` action because the region condition fails, causing the task creation to fail. DMS replication tasks require the IAM role to have permissions in the same region where the task is being created.
JA
Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This DBS-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 DBS-C01 exam.