Why gRPC Breaks in a VPC Private Subnet Proxy Setup

The gRPC calls kept failing, and nobody knew why.

Deployment logs looked clean. Health checks passed. Yet every client inside the VPC private subnet hit an invisible wall when routing through the proxy. The error was stubborn, persistent, and cryptic: unavailable, transport is closing.

This is the kind of problem that chews up hours but has one root cause: missing awareness of how gRPC, VPC networking, and proxies interact.

Why gRPC Breaks in a VPC Private Subnet Proxy Setup

When you deploy inside a private subnet and route traffic through a proxy, gRPC depends on HTTP/2 streams staying alive across the connection. Many proxies strip or alter headers, mishandle TCP keep-alives, or downgrade protocols. That kills the long-lived connections gRPC needs.

A VPC private subnet adds a second layer of complexity: no direct internet outbound, NAT gateways in the path, and tight security groups. If DNS resolution is inconsistent or SSL handshakes fail across hops, gRPC sees the connection as broken before the app logic even starts.

Common Failure Points

• Proxy Idle Timeouts: gRPC calls require persistent HTTP/2 streams. Proxies with low idle timeouts close the connection mid-stream.
• Header or Protocol Downgrading: Some proxies terminate and reinitiate HTTP over HTTP/1.1, which breaks gRPC.
• DNS in Private Subnets: Misconfigured resolvers or split-horizon DNS can make gRPC clients think the endpoint is unreachable.
• Security Group Rules: Outbound rules that block ephemeral ports can silently drop gRPC streams.
• Load Balancer HTTP/2 Settings: Even ALB or NLB misconfigurations can downgrade or reject connections.

How to Fix It

1. Enable Full HTTP/2 Support Across the Path: Make sure every hop — proxy, load balancer, service — advertises and uses HTTP/2.
2. Raise Idle Timeouts: Set proxy and load balancer idle timeouts higher than the longest possible request duration.
3. Keep-Alive Tuning: Enable gRPC keep-alives at a shorter interval than proxy idle timeouts.
4. Check DNS Resolution in the VPC Context: Use VPC DNS or a known reachable resolver.
5. Audit Network ACLs and Security Group Rules: Confirm ports 443 and required ephemeral ranges are open for outbound traffic.
6. TLS and Certificate Validity: Ensure the proxy passes through TLS handshake data without terminating unless configured for it.

Deployment Patterns That Work

The most reliable pattern is to keep gRPC services communicating within the same protocol tier end-to-end. If a proxy is required, use one that supports HTTP/2 pass-through. For cross-VPC or public-bound traffic, terminate TLS at the final service endpoint instead of the proxy. Layer in monitoring on idle connection drops to catch early regressions.

When deploying in production inside a private subnet, test gRPC calls in an environment with identical networking before going live. Simulate load. Inspect packet flows with tcpdump. Any hint of a downgrade or timeout during that inspection means tuning is still needed.

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