Troubleshooting S3 Access Failures When Using VPC Endpoints

Amazon S3 is one of the most widely used AWS services, and integrating it with VPC endpoints ensures secure, private connectivity between your VPC and S3. However, misconfigurations or overlooked details can often lead to access failures. In this blog, we’ll dive into common issues with S3 access when using VPC endpoints and provide step-by-step troubleshooting techniques.

What Are VPC Endpoints for S3?

A VPC endpoint for Amazon S3 allows you to privately connect your VPC to S3 without routing traffic through the internet. This is achieved using a gateway endpoint, which ensures all S3 traffic stays within the AWS private network.

Common Symptoms of S3 Access Failures:

1. Access Denied Errors:

- You receive `403 Forbidden` errors when accessing S3 buckets or objects.

- Example: `An error occurred (AccessDenied) when calling the GetObject operation: Access Denied`

2. Timeouts or Connection Refused:

- Requests to S3 hang or fail to connect.

- Example: `Could not resolve host: bucket-name.s3.amazonaws.com`

3. Unexpected Internet Traffic:

- Data transfer costs appear, indicating traffic is routed through the internet instead of the VPC endpoint.

Step-by-Step Troubleshooting Guide:

1. Verify VPC Endpoint Configuration

- Check that the VPC endpoint for S3 is correctly configured:

- Type: Ensure you’ve created a *gateway endpoint* for S3.

- Service Name: The service name should be `com.amazonaws.<region>.s3`.

- Route Tables: Confirm that the endpoint is associated with the correct route table for the subnets accessing S3.

2. Confirm Private DNS Settings

- If you are using a private DNS-enabled endpoint, ensure that:

- DNS resolution is enabled in your VPC.

- The `bucket-name.s3.amazonaws.com` hostname resolves to a private IP address.

3. Validate IAM Permissions

- Ensure the IAM role or user accessing S3 has the necessary permissions:

- Bucket policies should allow access from the principal (IAM role or user) making the request.

- Example policy for allowing access via a VPC endpoint

4. Check Security Group Rules

- Security groups should allow outbound traffic to the S3 endpoint.

- Ensure rules allow traffic to `0.0.0.0/0` on port `443` for HTTPS access.

5. Review Network ACLs

- Network ACLs associated with your subnets should allow:

- Outbound traffic to the S3 endpoint.

- Inbound return traffic on ephemeral ports (1024-65535).

6. Test Connectivity

- Use AWS CLI or curl commands to confirm connectivity:

- If this fails, verify:

- You are using the correct AWS CLI profile with valid credentials.

- The endpoint DNS resolves to a private IP.

7. Inspect S3 Bucket Policy

- Verify that the bucket policy includes the correct `aws:SourceVpce` condition and does not restrict access for your VPC endpoint.

8. Enable Flow Logs

- Use VPC Flow Logs to identify issues with traffic flow:

- Look for rejected traffic to the VPC endpoint’s IP address.

9. Confirm S3 Access Logs

- Check S3 access logs to see if requests are reaching the bucket and identify errors like permission mismatches.

Real-World Example:

Let’s say you’re running an EC2 instance in a private subnet with no internet gateway, and you need it to upload files to S3 via a VPC endpoint. If the EC2 instance returns `403 Access Denied`, check:

- IAM role policies for the EC2 instance profile.

- S3 bucket policy for the `aws:SourceVpce` condition.

- Route table and security group configurations for traffic to the endpoint.

Conclusion:

Misconfigured VPC endpoints can disrupt access to S3, but with a structured troubleshooting approach, you can quickly identify and resolve these issues. By combining AWS CLI commands, flow logs, and policy reviews, you can ensure secure, seamless S3 connectivity.