Moving workloads into private subnets does not automatically make their service dependencies private. Applications may still need to reach Amazon S3, DynamoDB, Secrets Manager, Systems Manager or other AWS APIs. The design question is not simply whether traffic can avoid the public internet. It is how that traffic is routed, authorised, observed and recovered when something fails.
Choose the right endpoint type
Gateway endpoints provide a route-table-based path to S3 and DynamoDB from resources within the VPC. Interface endpoints use AWS PrivateLink and place private network interfaces in selected subnets for supported services. The distinction matters: interface endpoints introduce security-group, DNS, availability-zone and hourly/data-processing cost considerations. A gateway endpoint does not behave like a general-purpose private proxy for connected on-premises networks.
Map dependencies before changing routes
Start with an inventory of application calls, service names, source subnets, regions and DNS resolution paths. Include build agents, patching processes, monitoring agents and operational access, not just the application’s primary transactions. Identify which calls currently traverse a NAT gateway or another egress path and confirm that the proposed endpoint supports each required API and access pattern.
For interface endpoints, decide which availability zones need local access, whether private DNS should be enabled and how clients using custom DNS resolvers will obtain the correct answers. Check routing and name resolution from every relevant network segment rather than assuming that a successful test from one instance proves the design.
Keep authorisation separate from connectivity
A private network path is not an access-control decision. Review IAM identity policies, resource policies and, where supported, VPC endpoint policies together. Restrict access to the intended principals and resources, but avoid conditions that inadvertently break administrative tooling or cross-account integrations. Test both permitted and denied requests.
Prove operations and economics
Before removing NAT or internet egress, exercise the real workflows: application startup, secret retrieval, object access, patching, deployment and incident recovery. Capture DNS results, connection outcomes and relevant logs. Document how endpoint or zone failure affects each dependency and how teams will diagnose it.
Finally, compare interface endpoint charges and data processing with the existing egress pattern. Multiple endpoints across several zones can be justified for security and resilience, but they should be an explicit architectural choice rather than an assumed saving.
Practical takeaway: Treat VPC endpoints as part of a documented service-access design. Private routing, least-privilege permissions, DNS, resilience and cost all need to agree before the change is production-ready.