Context
An S3 gateway endpoint changes the route used for supported S3 requests from associated subnets. It does not automatically grant access to a bucket, repair a KMS policy, or make every S3 request in the network use that endpoint. Validate connectivity and authorization as separate layers before adding a restrictive bucket condition.
Identify one exact request
Record the caller's role, bucket, object key, AWS Region, SDK endpoint, source subnet, and operation. A list request, an object read, and a multipart upload need different permissions. Include an encrypted object's key dependency when relevant.
Run the test with the workload's real identity. An administrator's successful command from a laptop proves little about a task running in a private subnet. Keep tokens and object contents out of diagnostic output; request IDs and sanitized error codes are usually more useful.
Follow the effective subnet route
A gateway endpoint is associated with route tables. AWS adds a route for the service prefix list to the selected tables. Confirm which table the workload subnet actually uses, including the main-table association when no explicit association exists.
Inspect the destination Region and address family supported by the selected configuration. A route to one regional service destination is not a general substitute for internet or cross-region connectivity. Clients still use S3 service names; a public-looking DNS answer does not by itself prove the request traversed a NAT gateway.
Check security-group egress and network ACL behavior for the flow. Gateway endpoints do not use an interface endpoint's security group model. Mixing those two endpoint types often creates a misleading checklist and an incorrect explanation of where traffic is blocked.
Evaluate the permission layers together
The endpoint policy controls which requests may pass through the endpoint. IAM identity policy, bucket policy, organization controls, and applicable key policy still matter. An explicit deny in an applicable policy can defeat an allow elsewhere.
Treat aws:SourceVpce as a request-context condition, not a new grant. A bucket policy that denies access outside an endpoint can also deny console and administrative access that uses a different path. Map recovery access, deployment jobs, replication, and service integrations before enforcing the boundary.
Use the provider's documented examples as a starting point, then adapt the resource scope and conditions to the real callers. Copying a bucket-wide deny into production without a recovery test can turn a networking improvement into an access incident.
An illustrative partial rollout
A web tier begins using the gateway endpoint successfully, but scheduled workers still fail after the bucket is restricted. The workers run in a different subnet associated with another route table. Their IAM role is correct, yet their request context does not match the bucket condition.
The correction associates the intended route table and verifies a worker request before widening enforcement. A separate deployment job is deliberately kept on an approved alternative access path. The final policy describes those paths explicitly instead of assuming every legitimate caller lives in one subnet.
Quick diagnostic map
| Evidence | Boundary to investigate | Next check |
|---|---|---|
| Only one subnet fails | Route association | Effective subnet route table |
| Network request reaches S3; response is denied | Authorization | Caller, endpoint, bucket, and key policies |
| Console fails after policy change | Context restriction | Source endpoint condition |
| Same role works for some objects | Object or encryption scope | Key, prefix, and KMS permissions |
A focused operator exercise
Create a test bucket and a narrowly scoped workload role. Exercise an allowed object read and a deliberately denied read from each intended subnet. Record request identity, route-table association, operation, and outcome without retaining credentials.
Introduce the intended endpoint restriction in the test policy. Confirm the permitted workload still succeeds and an unapproved route fails. Rehearse the documented recovery procedure using an independently authorized administrator. Only then prepare the equivalent production change and its rollback configuration.
Confirm the operational and cost result
Check request evidence and relevant network usage after the change. A lower NAT byte count can support the routing finding, but it is not a complete authorization test. Likewise, a successful object read does not prove that unrelated requests no longer use NAT.
Retain the endpoint associations and policies in reviewed infrastructure configuration. Add a regression request to migration and subnet-provisioning checks so future workers receive the same intentional access path.
Frequently asked questions
Does an S3 gateway endpoint make a bucket public?
No. Routing and authorization remain separate. Access still depends on applicable policies and the identity making the request.
Can I restrict a bucket to an endpoint immediately?
First inventory legitimate callers and validate recovery access. A condition that excludes their request context can block them even when their IAM permissions previously worked.
Continue the investigation
- Amazon S3 Access Denied: Diagnose the Exact Object Request
- PrivateLink interface endpoints: verify DNS and traffic
Browse the cloud, server and infrastructure guide library for related operational questions.
Explore Clouvenix AWS VPC & Private Connectivity to discuss implementation, validation and operational handover.
Technical reference
Controlling access from VPC endpoints with bucket policies - Amazon Simple Storage Service
Control access to VPC endpoints using endpoint policies - Amazon Virtual Private Cloud