Add support for socks5h proxy scheme

src/libraries/System.Net.Http/src/Resources/Strings.resx

@@ -367,7 +367,7 @@
     <value>Cannot access a closed stream.</value>
   </data>
   <data name="net_http_invalid_proxy_scheme" xml:space="preserve">
-    <value>Only the 'http', 'https', 'socks4', 'socks4a' and 'socks5' schemes are allowed for proxies.</value>
+    <value>Only the 'http', 'https', 'socks4', 'socks4a', 'socks5', and 'socks5h' schemes are allowed for proxies.</value>
   </data>
   <data name="net_http_request_invalid_char_encoding" xml:space="preserve">
     <value>Request headers must contain only ASCII characters.</value>

src/libraries/System.Net.Http/src/System/Net/Http/SocketsHttpHandler/HttpEnvironmentProxy.cs

@@ -146,6 +146,11 @@ private HttpEnvironmentProxy(Uri? httpProxy, Uri? httpsProxy, string? bypassList
                 hostIndex = 9;
                 protocol = "socks5";
             }
+            else if (value.StartsWith("socks5h://", StringComparison.OrdinalIgnoreCase))
+            {
+                hostIndex = 10;
+                protocol = "socks5h";
+            }
             else if (value.StartsWith("socks4a://", StringComparison.OrdinalIgnoreCase))
             {
                 hostIndex = 10;

src/libraries/System.Net.Http/src/System/Net/Http/SocketsHttpHandler/HttpUtilities.SocketsHttpHandler.cs

@@ -26,6 +26,7 @@ internal static bool IsSupportedProxyScheme(string scheme) =>
 
         internal static bool IsSocksScheme(string scheme) =>
             string.Equals(scheme, "socks5", StringComparison.OrdinalIgnoreCase) ||
+            string.Equals(scheme, "socks5h", StringComparison.OrdinalIgnoreCase) ||
             string.Equals(scheme, "socks4a", StringComparison.OrdinalIgnoreCase) ||
             string.Equals(scheme, "socks4", StringComparison.OrdinalIgnoreCase);
     }

src/libraries/System.Net.Http/src/System/Net/Http/SocketsHttpHandler/SocksHelper.cs

@@ -37,7 +37,8 @@ public static async ValueTask EstablishSocksTunnelAsync(Stream stream, string ho
                 {
                     NetworkCredential? credentials = proxyCredentials?.GetCredential(proxyUri, proxyUri.Scheme);
 
-                    if (string.Equals(proxyUri.Scheme, "socks5", StringComparison.OrdinalIgnoreCase))
+                    if (string.Equals(proxyUri.Scheme, "socks5", StringComparison.OrdinalIgnoreCase) ||
+                        string.Equals(proxyUri.Scheme, "socks5h", StringComparison.OrdinalIgnoreCase))
                     {
                         await EstablishSocks5TunnelAsync(stream, host, port, credentials, async).ConfigureAwait(false);
                     }

src/libraries/System.Net.Http/tests/FunctionalTests/SocksProxyTest.cs

@@ -15,12 +15,12 @@ public abstract class SocksProxyTest : HttpClientHandlerTestBase
     {
         public SocksProxyTest(ITestOutputHelper helper) : base(helper) { }
 
-        private static string[] Hosts(string socksScheme) => socksScheme == "socks5"
+        private static string[] Hosts(string socksScheme) => socksScheme == "socks5" || socksScheme == "socks5h"
             ? new[] { "localhost", "127.0.0.1", "::1" }
             : new[] { "localhost", "127.0.0.1" };
 
         public static IEnumerable<object[]> TestLoopbackAsync_MemberData() =>
-            from scheme in new[] { "socks4", "socks4a", "socks5" }
+            from scheme in new[] { "socks4", "socks4a", "socks5", "socks5h" }
             from useSsl in BoolValues
             from useAuth in BoolValues
             from host in Hosts(scheme)
@@ -77,7 +77,7 @@ public static IEnumerable<object[]> TestExceptionalAsync_MemberData()
 
             yield return new object[] { "socks4", new string('a', 256), false, null, "Failed to resolve the destination host to an IPv4 address." };
 
-            foreach (string scheme in new[] { "socks4a", "socks5" })
+            foreach (string scheme in new[] { "socks4a", "socks5", "socks5h" })
             {
                 yield return new object[] { scheme, new string('a', 256), false, null, "Encoding the host took more than the maximum of 255 bytes." };
             }
@@ -86,6 +86,10 @@ public static IEnumerable<object[]> TestExceptionalAsync_MemberData()
             yield return new object[] { "socks5", "localhost", true, new NetworkCredential("bad_username", "bad_password"), "Failed to authenticate with the SOCKS server." };
             yield return new object[] { "socks5", "localhost", true, new NetworkCredential(new string('a', 256), "foo"), "Encoding the UserName took more than the maximum of 255 bytes." };
             yield return new object[] { "socks5", "localhost", true, new NetworkCredential("foo", new string('a', 256)), "Encoding the Password took more than the maximum of 255 bytes." };
+            yield return new object[] { "socks5h", "localhost", true, null, "SOCKS server did not return a suitable authentication method." };
+            yield return new object[] { "socks5h", "localhost", true, new NetworkCredential("bad_username", "bad_password"), "Failed to authenticate with the SOCKS server." };
+            yield return new object[] { "socks5h", "localhost", true, new NetworkCredential(new string('a', 256), "foo"), "Encoding the UserName took more than the maximum of 255 bytes." };
+            yield return new object[] { "socks5h", "localhost", true, new NetworkCredential("foo", new string('a', 256)), "Encoding the Password took more than the maximum of 255 bytes." };
         }
 
         [Theory]

src/libraries/System.Net.Http/tests/UnitTests/HttpEnvironmentProxyTest.cs

@@ -149,6 +149,7 @@ await RemoteExecutor.Invoke(() =>
         [InlineData("socks4://1.2.3.4:8888/foo", "1.2.3.4", "8888", null, null)]
         [InlineData("socks4a://1.2.3.4:8888/foo", "1.2.3.4", "8888", null, null)]
         [InlineData("socks5://1.2.3.4:8888/foo", "1.2.3.4", "8888", null, null)]
+        [InlineData("socks5h://1.2.3.4:8888/foo", "1.2.3.4", "8888", null, null)]
         [InlineData("https://1.1.1.5:3005", "1.1.1.5", "3005", null, null)]
         [InlineData("https://1.1.1.5", "1.1.1.5", "443", null, null)]
         // Everything before the last '@' is considered as user info (unlike regular Uri parsing).