Incompatible Protocol
Production Risk
Medium — misconfigured ALB protocol settings will block all traffic to the affected target group.
464 is used by AWS Elastic Load Balancer to indicate a protocol incompatibility between the client and the load balancer, typically when the client attempts to use a protocol version or feature that the ALB does not support for that listener.
- 1The client is using a protocol not supported by the ALB listener configuration.
- 2The target group protocol does not match what the client is using.
- 3Attempting to use HTTP/2 features on an HTTP/1.1-only listener.
A client sends a request using a protocol incompatible with the ALB listener.
# Client attempts gRPC over an HTTP listener not configured for it grpc-call → ALB HTTP listener (not gRPC-configured)
expected output
HTTP/1.1 464 (AWS ALB protocol error)
Fix
Match the client protocol to the ALB listener configuration
WHEN Protocol mismatch between client and ALB.
# AWS Console: update ALB listener protocol to match client # or update the target group protocol settings
Why this works
Ensures the ALB listener and target group are configured to handle the client's protocol.
AWS-specific. Not a standard IETF code.
AWS Application Load Balancer documentation
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