When deploying AWS RDS in a private subnet within a VPC, the networking requirements differ significantly from EC2 instances. The fundamental question revolves around whether RDS instances require outbound internet access for their core operations.

AWS RDS handles maintenance and updates through AWS's internal service network, not via your VPC's internet gateway. The key points:

• Database engine updates are delivered through AWS's backend systems
• OS patches are applied through internal AWS channels
• No need for public internet access for standard maintenance

While RDS itself doesn't need NAT for maintenance, there are edge cases where you might need it:

# Example CloudFormation snippet showing NAT Gateway association
Resources:
  RDSSecurityGroup:
    Type: AWS::EC2::SecurityGroup
    Properties:
      GroupDescription: "Allow outbound to NAT"
      SecurityGroupEgress:
        - IpProtocol: tcp
          FromPort: 443
          ToPort: 443
          CidrIp: 0.0.0.0/0

Specific scenarios requiring NAT:

1. When using RDS Proxy with IAM authentication
2. If your application needs to fetch external data via RDS functions
3. When implementing custom Lambda functions inside VPC that interact with RDS

For most RDS deployments, these security group rules suffice:

# Terraform example for minimal RDS security group
resource "aws_security_group" "rds_sg" {
  vpc_id = aws_vpc.main.id
  
  ingress {
    from_port   = 3306
    to_port     = 3306
    protocol    = "tcp"
    cidr_blocks = [aws_vpc.main.cidr_block]
  }
  
  egress {
    from_port   = 0
    to_port     = 0
    protocol    = "-1"
    cidr_blocks = ["0.0.0.0/0"]
  }
}

Based on your described architecture (RDS in private subnet, EC2 in public subnets), you can safely:

• Terminate the NAT instance if no other private resources need internet access
• Remove NAT Gateway if provisioned
• Verify through AWS Cost Explorer that no services are unexpectedly using NAT

Execute these AWS CLI commands to confirm no dependencies:

# Check VPC flow logs for NAT usage
aws ec2 describe-flow-logs --filter Name=resource-id,Values=vpc-12345678

# Verify route table associations
aws ec2 describe-route-tables --filters Name=vpc-id,Values=vpc-12345678

# Check RDS network configuration
aws rds describe-db-instances --query 'DBInstances[*].DBSubnetGroup'

When deploying AWS RDS in a private subnet within a VPC, the database instance inherently doesn't require outbound internet access for normal operations. However, there are specific scenarios where internet connectivity becomes relevant:

# Example VPC setup with private/public subnets
resource "aws_vpc" "main" {
  cidr_block = "10.0.0.0/16"
  
  tags = {
    Name = "RDS-VPC"
  }
}

resource "aws_subnet" "private" {
  vpc_id     = aws_vpc.main.id
  cidr_block = "10.0.1.0/24"
  availability_zone = "us-east-1a"

  tags = {
    Name = "RDS-Private"
  }
}

AWS RDS doesn't use NAT gateways for:

• Regular database operations
• Client connections from within the VPC
• Cross-AZ replication

However, these maintenance operations might require internet access:

# Security group allowing RDS maintenance traffic
resource "aws_security_group" "rds_maintenance" {
  name        = "rds-maintenance"
  description = "Allow RDS maintenance traffic"
  vpc_id      = aws_vpc.main.id

  egress {
    from_port   = 443
    to_port     = 443
    protocol    = "tcp"
    cidr_blocks = ["0.0.0.0/0"]
  }
}

Case 1: Minor version updates

When AWS pushes minor version updates, they're delivered through the internal AWS network - no NAT required.

Case 2: Major version upgrades

For major version upgrades where you initiate the process through the AWS Console/CLI, the request goes through AWS's control plane, not your NAT.

You can eliminate the NAT gateway if:

1. Your RDS instance doesn't need to access external services
2. You're not using features like RDS Proxy that might require internet access
3. You're comfortable with AWS handling all maintenance through their backend systems

Before removing NAT, monitor these CloudWatch metrics:

aws cloudwatch get-metric-statistics \
  --namespace AWS/RDS \
  --metric-name NetworkThroughput \
  --dimensions Name=DBInstanceIdentifier,Value=your-rds-instance \
  --start-time $(date -u +"%Y-%m-%dT%H:%M:%S" --date="-7 days") \
  --end-time $(date -u +"%Y-%m-%dT%H:%M:%S") \
  --period 3600 \
  --statistics Average

Check for any unexpected outbound traffic patterns that might indicate dependencies on external services.

For complete isolation without NAT, consider implementing VPC endpoints for these services:

resource "aws_vpc_endpoint" "s3" {
  vpc_id       = aws_vpc.main.id
  service_name = "com.amazonaws.${var.region}.s3"
}