Enable Http/2 by default

src/Kestrel.Core/CoreStrings.resx

@@ -354,9 +354,6 @@
   <data name="EndPointRequiresAtLeastOneProtocol" xml:space="preserve">
     <value>An endpoint must be configured to serve at least one protocol.</value>
   </data>
-  <data name="EndPointRequiresTlsForHttp1AndHttp2" xml:space="preserve">
-    <value>Using both HTTP/1.x and HTTP/2 on the same endpoint requires the use of TLS.</value>
-  </data>
   <data name="EndPointHttp2NotNegotiated" xml:space="preserve">
     <value>HTTP/2 over TLS was not negotiated on an HTTP/2-only endpoint.</value>
   </data>

src/Kestrel.Core/Internal/HttpConnection.cs

@@ -363,11 +363,6 @@ private HttpProtocols SelectProtocol()
                 error = CoreStrings.EndPointRequiresAtLeastOneProtocol;
             }
 
-            if (!hasTls && http1Enabled && http2Enabled)
-            {
-                error = CoreStrings.EndPointRequiresTlsForHttp1AndHttp2;
-            }
-
             if (!http1Enabled && http2Enabled && hasTls && !Http2Id.Span.SequenceEqual(applicationProtocol.Span))
             {
                 error = CoreStrings.EndPointHttp2NotNegotiated;
@@ -379,6 +374,12 @@ private HttpProtocols SelectProtocol()
                 return HttpProtocols.None;
             }
 
+            if (!hasTls && http1Enabled)
+            {
+                // Even if Http2 was enabled, default to Http1 because it's ambiguous without ALPN.
+                return HttpProtocols.Http1;
+            }
+
             return http2Enabled && (!hasTls || Http2Id.Span.SequenceEqual(applicationProtocol.Span)) ? HttpProtocols.Http2 : HttpProtocols.Http1;
         }
 

src/Kestrel.Core/ListenOptions.cs

@@ -126,8 +126,8 @@ public FileHandleType HandleType
         /// <summary>
         /// The protocols enabled on this endpoint.
         /// </summary>
-        /// <remarks>Defaults to HTTP/1.x only.</remarks>
-        public HttpProtocols Protocols { get; set; } = HttpProtocols.Http1;
+        /// <remarks>Defaults to HTTP/1.x and HTTP/2.</remarks>
+        public HttpProtocols Protocols { get; set; } = HttpProtocols.Http1AndHttp2;
 
         /// <summary>
         /// Gets the <see cref="List{IConnectionAdapter}"/> that allows each connection <see cref="System.IO.Stream"/>

test/Kestrel.Core.Tests/ListenOptionsTests.cs

@@ -17,7 +17,7 @@ public class ListenOptionsTests
         public void ProtocolsDefault()
         {
             var listenOptions = new ListenOptions(new IPEndPoint(IPAddress.Loopback, 0));
-            Assert.Equal(HttpProtocols.Http1, listenOptions.Protocols);
+            Assert.Equal(HttpProtocols.Http1AndHttp2, listenOptions.Protocols);
         }
 
         [Fact]

test/Kestrel.FunctionalTests/HttpProtocolSelectionTests.cs

@@ -25,9 +25,9 @@ public Task Server_NoProtocols_Error()
         }
 
         [Fact]
-        public Task Server_Http1AndHttp2_Cleartext_Error()
+        public Task Server_Http1AndHttp2_Cleartext_Http1Default()
         {
-            return TestError<InvalidOperationException>(HttpProtocols.Http1AndHttp2, CoreStrings.EndPointRequiresTlsForHttp1AndHttp2);
+            return TestSuccess(HttpProtocols.Http1AndHttp2, "GET / HTTP/1.1\r\nHost:\r\n\r\n", "HTTP/1.1 200 OK");
         }
 
         [Fact]