fix(core): support hostnames whose last label starts with a digit

core/src/main/java/io/grpc/internal/DnsNameResolver.java

@@ -163,14 +163,32 @@ protected DnsNameResolver(
     // Must prepend a "//" to the name when constructing a URI, otherwise it will be treated as an
     // opaque URI, thus the authority and host of the resulted URI would be null.
     URI nameUri = URI.create("//" + checkNotNull(name, "name"));
-    Preconditions.checkArgument(nameUri.getHost() != null, "Invalid DNS name: %s", name);
     authority = Preconditions.checkNotNull(nameUri.getAuthority(),
         "nameUri (%s) doesn't have an authority", nameUri);
-    host = nameUri.getHost();
-    if (nameUri.getPort() == -1) {
-      port = args.getDefaultPort();
+    if (nameUri.getHost() != null) {
+      host = nameUri.getHost();
+      if (nameUri.getPort() == -1) {
+        port = args.getDefaultPort();
+      } else {
+        port = nameUri.getPort();
+      }
     } else {
-      port = nameUri.getPort();
+      // JDK-8188305: java.net.URI.getHost() returns null when the last label of a hostname
+      // starts with a digit (e.g. otlp.1234-k8s-namespace). Parse host and port from authority.
+      Preconditions.checkArgument(authority.indexOf('@') == -1, "Invalid DNS name: %s", name);
+      int colonIndex = authority.lastIndexOf(':');
+      if (colonIndex != -1 && (authority.indexOf(']') == -1 || colonIndex > authority.lastIndexOf(']'))) {
+        host = authority.substring(0, colonIndex);
+        try {
+          port = Integer.parseInt(authority.substring(colonIndex + 1));
+        } catch (NumberFormatException e) {
+          throw new IllegalArgumentException("Invalid DNS name: " + name, e);
+        }
+      } else {
+        host = authority;
+        port = args.getDefaultPort();
+      }
+      Preconditions.checkArgument(!host.isEmpty(), "Invalid DNS name: %s", name);
     }
     this.proxyDetector = checkNotNull(args.getProxyDetector(), "proxyDetector");
     Executor offloadExecutor = args.getOffloadExecutor();

core/src/main/java/io/grpc/internal/GrpcUtil.java

@@ -547,7 +547,15 @@ public static String authorityFromHostAndPort(String host, int port) {
     try {
       return new URI(null, null, host, port, null, null, null).getAuthority();
     } catch (URISyntaxException ex) {
-      throw new IllegalArgumentException("Invalid host or port: " + host + " " + port, ex);
+      // JDK-8188305: java.net.URI rejects valid RFC 1123 hostnames whose last label starts with
+      // a digit (e.g. otlp.1234-k8s-namespace). Fall back to manual combination and checkAuthority.
+      String formattedHost = (host.contains(":") && !host.startsWith("[")) ? "[" + host + "]" : host;
+      String authority = formattedHost + ":" + port;
+      try {
+        return checkAuthority(authority);
+      } catch (IllegalArgumentException e) {
+        throw new IllegalArgumentException("Invalid host or port: " + host + " " + port, ex);
+      }
     }
   }
 

core/src/test/java/io/grpc/internal/DnsNameResolverProviderTest.java

@@ -79,6 +79,16 @@ public void newNameResolver_acceptsHostAndPort() {
     assertThat(((DnsNameResolver) nameResolver).getPort()).isEqualTo(443);
   }
 
+  @Test
+  public void newNameResolver_acceptsHostStartingWithDigit() {
+    NameResolver nameResolver = newNameResolver("dns:///otlp.1234-k8s-namespace:4317", args);
+    assertThat(nameResolver).isNotNull();
+    assertThat(nameResolver.getClass()).isSameInstanceAs(DnsNameResolver.class);
+    assertThat(nameResolver.getServiceAuthority()).isEqualTo("otlp.1234-k8s-namespace:4317");
+    assertThat(((DnsNameResolver) nameResolver).getHost()).isEqualTo("otlp.1234-k8s-namespace");
+    assertThat(((DnsNameResolver) nameResolver).getPort()).isEqualTo(4317);
+  }
+
   @Test
   public void newNameResolver_acceptsRootless() {
     assume().that(enableRfc3986UrisParam).isTrue();

core/src/test/java/io/grpc/internal/GrpcUtilTest.java

@@ -202,6 +202,12 @@ public void checkAuthority_failsOnNull() {
     assertThrows(NullPointerException.class, () -> GrpcUtil.checkAuthority(null));
   }
 
+  @Test
+  public void authorityFromHostAndPort_succeedsOnHostStartingWithDigit() {
+    String actual = GrpcUtil.authorityFromHostAndPort("otlp.1234-k8s-namespace", 4317);
+    assertEquals("otlp.1234-k8s-namespace:4317", actual);
+  }
+
   @Test
   public void checkAuthority_succeedsOnHostAndPort() {
     String actual = GrpcUtil.checkAuthority("valid:1234");