Use reflection to set host header when forwarding

src/Microsoft.AspNetCore.SystemWebAdapters.FrameworkServices/ProxyHeaderModule.cs

@@ -1,9 +1,9 @@
 // Licensed to the .NET Foundation under one or more agreements.
 // The .NET Foundation licenses this file to you under the MIT license.
 
-using System;
 using System.Collections.Specialized;
 using System.Globalization;
+using System.Reflection;
 using System.Web;
 using Microsoft.Extensions.Options;
 
@@ -27,6 +27,8 @@ internal class ProxyHeaderModule : IHttpModule
 
     private readonly IOptions<ProxyOptions> _options;
 
+    private ReflectionHelper? _reflectionHelper;
+
     public ProxyHeaderModule(IOptions<ProxyOptions> options)
     {
         _options = options ?? throw new ArgumentNullException(nameof(options));
@@ -42,10 +44,12 @@ public void Init(HttpApplication context)
 
         if (options.UseForwardedHeaders)
         {
+            _reflectionHelper = new ReflectionHelper();
+
             context.BeginRequest += (s, e) =>
             {
                 var request = ((HttpApplication)s).Context.Request;
-                UseHeaders(request.Headers, request.ServerVariables);
+                UseHeaders(request.Headers, request.ServerVariables, request);
             };
         }
         else
@@ -55,12 +59,12 @@ public void Init(HttpApplication context)
             context.BeginRequest += (s, e) =>
             {
                 var request = ((HttpApplication)s).Context.Request;
-                UseOptions(values, request.Headers, request.ServerVariables);
+                UseOptions(values, request.Headers, request.ServerVariables, request);
             };
         }
     }
 
-    public void UseHeaders(NameValueCollection requestHeaders, NameValueCollection serverVariables)
+    public void UseHeaders(NameValueCollection requestHeaders, NameValueCollection serverVariables, HttpRequest? request = null)
     {
         UseForwardedFor(requestHeaders, serverVariables);
 
@@ -80,19 +84,22 @@ public void UseHeaders(NameValueCollection requestHeaders, NameValueCollection s
             serverVariables.Set(ServerHttps, value.IsSecure ? On : Off);
 
             requestHeaders[Host] = host;
+
+            _reflectionHelper?.SetHostHeader(request, host);
         }
     }
 
-    private static void UseOptions(ServerValues values, NameValueCollection requestHeaders, NameValueCollection serverVariables)
+    private void UseOptions(ServerValues values, NameValueCollection requestHeaders, NameValueCollection serverVariables, HttpRequest? request = null)
     {
         UseForwardedFor(requestHeaders, serverVariables);
 
         serverVariables.Set(ServerName, values.Name);
         serverVariables.Set(ServerPort, values.Port);
         serverVariables.Set(ServerHttps, values.Https);
         requestHeaders[Host] = values.Host;
-    }
 
+        _reflectionHelper?.SetHostHeader(request, values.Host);
+    }
 
     private static void UseForwardedFor(NameValueCollection requestHeaders, NameValueCollection serverVariables)
     {
@@ -103,6 +110,31 @@ private static void UseForwardedFor(NameValueCollection requestHeaders, NameValu
         }
     }
 
+    private class ReflectionHelper
+    {
+        private readonly FieldInfo _wrField;
+        private readonly Type _iis7WorkerRequestType;
+        private readonly FieldInfo _knownRequestHeadersField;
+
+        public ReflectionHelper()
+        {
+            _wrField = typeof(HttpRequest).GetField("_wr", BindingFlags.NonPublic | BindingFlags.Instance);
+            _iis7WorkerRequestType = Assembly.GetAssembly(typeof(HttpRequest)).GetType("System.Web.Hosting.IIS7WorkerRequest");
+            _knownRequestHeadersField = _iis7WorkerRequestType.GetField("_knownRequestHeaders", BindingFlags.NonPublic | BindingFlags.Instance);
+        }
+        public void SetHostHeader(HttpRequest? request, string host)
+        {
+            // Need to use reflection to force HttpRequest to update the known headers
+            if (request != null)
+            {
+                var workerRequest = _wrField.GetValue(request);
+                var knownRequestHeaders = (string[])_knownRequestHeadersField.GetValue(workerRequest);
+                // This is the Host index
+                knownRequestHeaders[HttpWorkerRequest.HeaderHost] = host;
+            }
+        }
+    }
+
     private class ServerValues
     {
         public ServerValues(ProxyOptions options)