Hostnames whose last label starts with a digit are rejected (java.net.URI, JDK-8188305)

[hitkall]

What version of gRPC-Java are you using?

1.84.0 (grpc-netty-shaded)

What is your environment?

JDK 11 / 17 / 21, reproducible on macOS and Linux.

What did you expect to see?

A channel can be built for valid RFC 1123 hostnames whose last label starts with a digit. These are common in Kubernetes, where a service in namespace 1234-k8s-namespace is reachable as otlp.1234-k8s-namespace.

What did you see instead?

Call	Result
ManagedChannelBuilder.forAddress("otlp.1234-k8s-namespace", 4317)	IllegalArgumentException: Invalid host or port
ManagedChannelBuilder.forTarget("otlp.1234-k8s-namespace:4317")	IllegalArgumentException: Invalid DNS name
ManagedChannelBuilder.forTarget("dns:///otlp.1234-k8s-namespace:4317")	IllegalArgumentException: Invalid DNS name
ManagedChannelBuilder.forTarget("localhost:4317")	OK

GrpcUtil.authorityFromHostAndPort and DnsNameResolver both validate the host with java.net.URI. URI follows RFC 2396, where the last label of a hostname must start with a letter, so URI.getHost() returns null for these names (JDK-8188305). RFC 1123 allows it, and DNS resolves them fine.

The only workaround I found is resolving the address yourself and passing a fixed SocketAddress, which loses re-resolution and load balancing.

Steps to reproduce the bug

ManagedChannelBuilder.forTarget("dns:///otlp.1234-k8s-namespace:4317").usePlaintext().build();
// java.lang.IllegalArgumentException: Invalid DNS name: otlp.1234-k8s-namespace

Downstream context: open-telemetry/opentelemetry-java#8745, where the OTLP exporter's grpc-java sender can't be used with these endpoints.