Can IAM be used to authenticate between RDS Proxy and RDS Instance?

Question

I might be missing something obvious, but after going over the documentation, there doesn't seem to be anything that answers my question.

I'm trying to move my existing infrastructure, which consists of a VPC with lambda functions connecting directly to RDS using password authentication.

The password is stored in SecretsManager and I've set up all the roles and permissions necessary to provision this, together with the internet gateway needed to access SecretsManager from Lambdas in the VPC. I have two problems with this approach.

1. The connection pool of the RDS instance fills up quickly under load and future connections are rejected. This is to be expected and for that reason I've decided to add the RDS Proxy between the Lambdas and the RDS instance.

2. I have to fetch the credentials from the SecretsManager every time my Lambda runs, which is costly because my Lambdas run within a VPC and I need an Internet Gateway to connect to the SecretsManager.

To solve these problems, I'm trying to set up a proxy that uses IAM authentication both between the Lambda and the RDS Proxy, and between the RDS Proxy and RDS Instance. As I understand it from the current documentation, IAM between the Lambda and RDS Proxy should be a fairly common use case that's easy enough to set up. I don't however know whether it's possible to set up an IAM-authenticated connection between the RDS Proxy and RDS Instance.

Furthermore, I'm struggling with configuring the CloudFormation template for this setup (maybe because it's impossible?) when it comes to defining the AuthScheme for the AWS::RDS::DBProxy. It requires "SECRETS" to always be the AuthScheme, even when using IAM. Does that mean I need to store the actual username and password credentials for the database and use those when authenticating with the RDS from RDS Proxy?

UPDATE 2024-09-25 12:06

It seems that the only relevant entry describing the authentication between the RDS Proxy and RDS Instance is here:

The connection from RDS Proxy to the underlying database doesn't go through IAM.

Here is an excerpt from my current template:

Database:
  Type: AWS::RDS::DBInstance
  Properties:
    DBInstanceIdentifier: ${self:service}-database-${self:provider.stage} # This is pulled from the other parts of the config and work correctly.
    Engine: postgres
    DBName: postgres
    DBParameterGroupName: ${self:service}-database-${self:provider.stage}-parameter-group # This is pulled from the other parts of the config and work correctly.
    AllocatedStorage: 20
    EngineVersion: 16.3
    DBInstanceClass: db.t3.micro
    MasterUsername: ${self:custom.config.database.user} # This is pulled from the other parts of the config and work correctly.
    MasterUserPassword:
      Fn::Join:
        - ''
        - - '{{resolve:secretsmanager:'
          - Ref: DatabaseSecret
          - ':SecretString:password}}'  # This is pulled from the other parts of the config and work correctly.
    DBSubnetGroupName:
      Ref: DbSubnetGroup
    VPCSecurityGroups:
      - Ref: DbSecurityGroup
    EnableIAMDatabaseAuthentication: true
  DeletionPolicy: ${self:custom.config.cloudformation.databaseDeletionPolicy}  # This is pulled from the other parts of the config and work correctly.


DatabaseProxyRole:
  Type: AWS::IAM::Role
  Properties:
    AssumeRolePolicyDocument:
      Version: '2012-10-17'
      Statement:
        - Effect: Allow
          Principal:
            Service:
              - rds.amazonaws.com
          Action: sts:AssumeRole
    Path: /
    Policies:
      - PolicyName: AllowDatabaseSecretAccess
        PolicyDocument:
          Version: '2012-10-17'
          Statement:
            - Effect: Allow
              Action:
                - secretsmanager:DescribeSecret
                - secretsmanager:GetSecretValue
              Resource:
                Ref: DatabaseSecret
      - PolicyName: AllowRDSConnectionViaIAM
        PolicyDocument:
          Version: '2012-10-17'
          Statement:
            - Effect: Allow
              Action:
                - rds-db:connect
              Resource:
                Fn::Join:
                  - ':'
                  - - arn:aws:rds-db
                    - Ref: AWS::Region
                    - Ref: AWS::AccountId
                    - dbuser
                    - Fn::Join:
                        - '/'
                        - - Fn::GetAtt:
                              - Database
                              - DbiResourceId
                          - ${self:custom.config.database.proxyUser}  # This is pulled from the other parts of the config and work correctly.


DatabaseProxy:
  Type: AWS::RDS::DBProxy
  Properties:
    DBProxyName: my-db-proxy
    Auth:                         #   \
      - AuthScheme: SECRETS       #    |
        IAMAuth: REQUIRED         #    | - This is questionable
        SecretArn:                #    |
          Ref: DatabaseSecret     #   /
    EngineFamily: POSTGRESQL
    IdleClientTimeout: 1800
    RequireTLS: true
    RoleArn:
      Fn::GetAtt:
        - DatabaseProxyRole
        - Arn
    VpcSecurityGroupIds:
      - ${cf:${self:custom.config.stackName.prerequisites}.GeneralSecurityGroupId} # This is pulled from a different CloudFormation stack and work correctly.
    VpcSubnetIds:
      - ${cf:${self:custom.config.stackName.prerequisites}.PrivateSubnet1} # This is pulled from a different CloudFormation stack and work correctly.
      - ${cf:${self:custom.config.stackName.prerequisites}.PrivateSubnet2} # This is pulled from a different CloudFormation stack and work correctly.
      - ${cf:${self:custom.config.stackName.prerequisites}.PrivateSubnet3} # This is pulled from a different CloudFormation stack and work correctly.

DatabaseProxyTargetGroup:
  Type: AWS::RDS::DBProxyTargetGroup
  Properties:
    DBProxyName:
      Ref: DatabaseProxy
    DBInstanceIdentifiers:
      - Ref: Database
    TargetGroupName: default

DatabaseSecretAttachment:
  Type: AWS::SecretsManager::SecretTargetAttachment
  Properties:
    SecretId:
      Ref: DatabaseSecret
    TargetId:
      Ref: Database
    TargetType: AWS::RDS::DBInstance