Port Kerberos auth fixes to 2.1 branch (#40109)

This PR ports some important Kerberos auth fixes from the 3.0 to 2.1 LTS
branch. These fixes help enterprise customers that have complex Kerberos
authentication scenarios that involve cross Windows (Active Directory)
and Linux (Kerberos) domains/realms.

These fixes are from PRs:

* #38465 - Use 'Host' header when calculating SPN for Kerberos auth
* #38377 - Use GSS_C_NT_HOSTBASED_SERVICE format for Linux Kerberos SPN

and are related to issue #36329.

src/Native/Unix/System.Net.Security.Native/pal_gssapi.cpp

@@ -111,7 +111,7 @@ static uint32_t NetSecurityNative_DisplayStatus(uint32_t* minorStatus,
     assert(minorStatus != nullptr);
     assert(outBuffer != nullptr);
 
-    uint32_t messageContext;
+    uint32_t messageContext = 0; // Must initialize to 0 before calling gss_display_status.
     GssBuffer gssBuffer{.length = 0, .value = nullptr};
     uint32_t majorStatus =
         gss_display_status(minorStatus, statusValue, statusType, GSS_C_NO_OID, &messageContext, &gssBuffer);
@@ -155,19 +155,36 @@ extern "C" uint32_t NetSecurityNative_ImportPrincipalName(uint32_t* minorStatus,
     assert(outputName != nullptr);
     assert(*outputName == nullptr);
 
-    gss_OID nameType;
-
-    if (strchr(inputName, '/') != nullptr)
+    // Principal name will usually be in the form SERVICE/HOST. But SPNEGO protocol prefers
+    // GSS_C_NT_HOSTBASED_SERVICE format. That format uses '@' separator instead of '/' between
+    // service name and host name. So convert input string into that format.
+    char* ptrSlash = (char*) memchr(inputName, '/', inputNameLen);
+    char* inputNameCopy = NULL;
+    if (ptrSlash != NULL)
     {
-        nameType = const_cast<gss_OID>(GSS_KRB5_NT_PRINCIPAL_NAME);
+        inputNameCopy = (char*) malloc(inputNameLen);
+        if (inputNameCopy != NULL)
+        {
+            memcpy(inputNameCopy, inputName, inputNameLen);
+            inputNameCopy[ptrSlash - inputName] = '@';
+            inputName = inputNameCopy;
+        }
+        else
+        {
+          *minorStatus = 0;
+          return GSS_S_BAD_NAME;
+        }
     }
-    else
+
+    GssBuffer inputNameBuffer = {.length = inputNameLen, .value = inputName};
+    uint32_t result = gss_import_name(minorStatus, &inputNameBuffer, GSS_C_NT_HOSTBASED_SERVICE, outputName);
+
+    if (inputNameCopy != NULL)
     {
-        nameType = const_cast<gss_OID>(GSS_C_NT_HOSTBASED_SERVICE);
+        free(inputNameCopy);
     }
 
-    GssBuffer inputNameBuffer{.length = inputNameLen, .value = inputName};
-    return gss_import_name(minorStatus, &inputNameBuffer, nameType, outputName);
+    return result;
 }
 
 extern "C" uint32_t NetSecurityNative_InitSecContext(uint32_t* minorStatus,

src/System.Net.Http/src/System.Net.Http.csproj

@@ -592,6 +592,9 @@
     <Compile Include="System\Net\Http\HttpClientHandler.Core.cs" />
     <Compile Include="uap\System\Net\HttpClientHandler.cs" />
   </ItemGroup>
+  <ItemGroup Condition="'$(TargetGroup)' == 'netcoreapp'">
+    <Reference Include="System.Net.NameResolution" />
+  </ItemGroup>
   <ItemGroup Condition="'$(TargetGroup)' == 'netcoreapp' OR '$(TargetGroup)' == 'uap'">
     <Reference Include="Microsoft.Win32.Primitives" />
     <Reference Include="System.Buffers" />

src/System.Net.Http/src/System/Net/Http/SocketsHttpHandler/AuthenticationHelper.NtAuth.cs

@@ -3,6 +3,7 @@
 // See the LICENSE file in the project root for more information.
 
 using System.Collections.Generic;
+using System.Net;
 using System.Net.Http.Headers;
 using System.Threading;
 using System.Threading.Tasks;
@@ -76,7 +77,44 @@ private static async Task<HttpResponseMessage> SendWithNtAuthAsync(HttpRequestMe
 
                         string challengeData = challenge.ChallengeData;
 
-                        string spn = "HTTP/" + authUri.IdnHost;
+                        // Calculate SPN (Service Principal Name) using the host name of the request.
+                        // Use the request's 'Host' header if available. Otherwise, use the request uri.
+                        // Ignore the 'Host' header if this is proxy authentication since we need to use
+                        // the host name of the proxy itself for SPN calculation.
+                        string hostName;
+                        if (!isProxyAuth && request.HasHeaders && request.Headers.Host != null)
+                        {
+                            // Use the host name without any normalization.
+                            hostName = request.Headers.Host;
+                            if (NetEventSource.IsEnabled)
+                            {
+                                NetEventSource.Info(connection, $"Authentication: {challenge.AuthenticationType}, Host: {hostName}");
+                            }
+                        }
+                        else
+                        {
+                            // Need to use FQDN normalized host so that CNAME's are traversed.
+                            // Use DNS to do the forward lookup to an A (host) record.
+                            // But skip DNS lookup on IP literals. Otherwise, we would end up
+                            // doing an unintended reverse DNS lookup.
+                            UriHostNameType hnt = authUri.HostNameType;
+                            if (hnt == UriHostNameType.IPv6 || hnt == UriHostNameType.IPv4)
+                            {
+                                hostName = authUri.IdnHost;
+                            }
+                            else
+                            {
+                                IPHostEntry result = await Dns.GetHostEntryAsync(authUri.IdnHost).ConfigureAwait(false);
+                                hostName = result.HostName;
+                            }
+                        }
+
+                        string spn = "HTTP/" + hostName;
+                        if (NetEventSource.IsEnabled)
+                        {
+                            NetEventSource.Info(connection, $"Authentication: {challenge.AuthenticationType}, SPN: {spn}");
+                        }
+
                         ChannelBinding channelBinding = connection.TransportContext?.GetChannelBinding(ChannelBindingKind.Endpoint);
                         NTAuthentication authContext = new NTAuthentication(isServer:false, challenge.SchemeName, challenge.Credential, spn, ContextFlagsPal.Connection, channelBinding);
                         try