per discussion

Author: twsouthwick

src/Microsoft.Data.SqlClient/netcore/src/Microsoft.Data.SqlClient.csproj

@@ -730,6 +730,7 @@
     </Compile>
     <Compile Include="Microsoft\Data\Common\DbConnectionOptions.cs" />
     <Compile Include="Microsoft\Data\SqlClient\SNI\ConcurrentQueueSemaphore.cs" />
+    <Compile Include="Microsoft\Data\SqlClient\SNI\ResolvedServerSpn.cs" />
     <Compile Include="Microsoft\Data\SqlClient\SNI\SNIError.cs" />
     <Compile Include="Microsoft\Data\SqlClient\SNI\SNICommon.cs" />
     <Compile Include="Microsoft\Data\SqlClient\SNI\SNIHandle.cs" />

src/Microsoft.Data.SqlClient/netcore/src/Microsoft/Data/SqlClient/SNI/ResolvedServerSpn.cs

@@ -0,0 +1,43 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
+
+#nullable enable
+
+namespace Microsoft.Data.SqlClient.SNI
+{
+    /// <summary>
+    /// This is used to hold the ServerSpn for a given connection. Most connection types have a single format, although TCP connections may allow
+    /// with and without a port. Depending on how the SPN is registered on the server, either one may be the correct name.
+    /// </summary>
+    /// <see href="https://learn.microsoft.com/sql/database-engine/configure-windows/register-a-service-principal-name-for-kerberos-connections?view=sql-server-ver17#spn-formats"/>
+    /// <param name="primary"></param>
+    /// <param name="secondary"></param>
+    /// <remarks>
+    /// <para>SQL Server SPN format follows these patterns:</para>
+    /// <list type="bullet">
+    ///   <item>
+    ///     <term>Default instance, no port (primary):</term>
+    ///     <description>MSSQLSvc/fully-qualified-domain-name</description>
+    ///   </item>
+    ///   <item>
+    ///     <term>Default instance, default port (secondary):</term>
+    ///     <description>MSSQLSvc/fully-qualified-domain-name:1433</description>
+    ///   </item>
+    ///   <item>
+    ///     <term>Named instance or custom port:</term>
+    ///     <description>MSSQLSvc/fully-qualified-domain-name:port_or_instance_name</description>
+    ///   </item>
+    /// </list>
+    /// <para>For TCP connections to named instances, the port number is used in SPN.</para>
+    /// <para>For Named Pipe connections to named instances, the instance name is used in SPN.</para>
+    /// <para>When hostname resolution fails, the user-provided hostname is used instead of FQDN.</para>
+    /// <para>For default instances with TCP protocol, both forms (with and without port) may be returned.</para>
+    /// </remarks>
+    internal readonly struct ResolvedServerSpn(string primary, string? secondary = null)
+    {
+        public string Primary => primary;
+
+        public string? Secondary => secondary;
+    }
+}

src/Microsoft.Data.SqlClient/netcore/src/Microsoft/Data/SqlClient/SNI/SNIProxy.cs

@@ -3,12 +3,9 @@
 // See the LICENSE file in the project root for more information.
 
 using System;
-using System.Buffers;
-using System.Collections.Generic;
 using System.Diagnostics;
 using System.IO;
 using System.Net;
-using System.Net.Security;
 using System.Net.Sockets;
 using System.Text;
 using Microsoft.Data.ProviderBase;
@@ -51,7 +48,7 @@ internal static SNIHandle CreateConnectionHandle(
             string fullServerName,
             TimeoutTimer timeout,
             out byte[] instanceName,
-            out string resolvedSpn,
+            out ResolvedServerSpn resolvedSpn,
             string serverSPN,
             bool flushCache,
             bool async,
@@ -116,12 +113,12 @@ internal static SNIHandle CreateConnectionHandle(
             return sniHandle;
         }
 
-        private static string GetSqlServerSPNs(DataSource dataSource, string serverSPN)
+        private static ResolvedServerSpn GetSqlServerSPNs(DataSource dataSource, string serverSPN)
         {
             Debug.Assert(!string.IsNullOrWhiteSpace(dataSource.ServerName));
             if (!string.IsNullOrWhiteSpace(serverSPN))
             {
-                return serverSPN;
+                return new(serverSPN);
             }
 
             string hostName = dataSource.ServerName;
@@ -139,7 +136,7 @@ private static string GetSqlServerSPNs(DataSource dataSource, string serverSPN)
             return GetSqlServerSPNs(hostName, postfix, dataSource.ResolvedProtocol);
         }
 
-        private static string GetSqlServerSPNs(string hostNameOrAddress, string portOrInstanceName, DataSource.Protocol protocol)
+        private static ResolvedServerSpn GetSqlServerSPNs(string hostNameOrAddress, string portOrInstanceName, DataSource.Protocol protocol)
         {
             Debug.Assert(!string.IsNullOrWhiteSpace(hostNameOrAddress));
             IPHostEntry hostEntry = null;
@@ -170,12 +167,12 @@ private static string GetSqlServerSPNs(string hostNameOrAddress, string portOrIn
                 string serverSpnWithDefaultPort = serverSpn + $":{DefaultSqlServerPort}";
                 // Set both SPNs with and without Port as Port is optional for default instance
                 SqlClientEventSource.Log.TryAdvancedTraceEvent("SNIProxy.GetSqlServerSPN | Info | ServerSPNs {0} and {1}", serverSpn, serverSpnWithDefaultPort);
-                return serverSpnWithDefaultPort;
+                return new(serverSpn, serverSpnWithDefaultPort);
             }
             // else Named Pipes do not need to valid port
 
             SqlClientEventSource.Log.TryAdvancedTraceEvent("SNIProxy.GetSqlServerSPN | Info | ServerSPN {0}", serverSpn);
-            return serverSpn;
+            return new(serverSpn);
         }
 
         /// <summary>

src/Microsoft.Data.SqlClient/netcore/src/Microsoft/Data/SqlClient/TdsParser.cs

@@ -442,7 +442,7 @@ internal void Connect(ServerInfo serverInfo,
                 serverInfo.ExtendedServerName,
                 timeout,
                 out instanceName,
-                out var serverSpn,
+                out var resolvedServerSpn,
                 false,
                 true,
                 fParallel,
@@ -540,7 +540,7 @@ internal void Connect(ServerInfo serverInfo,
                     serverInfo.ExtendedServerName,
                     timeout,
                     out instanceName,
-                    out serverSpn,
+                    out resolvedServerSpn,
                     true,
                     true,
                     fParallel,
@@ -591,9 +591,9 @@ internal void Connect(ServerInfo serverInfo,
             }
             SqlClientEventSource.Log.TryTraceEvent("<sc.TdsParser.Connect|SEC> Prelogin handshake successful");
 
-            if (_authenticationProvider is { } && serverSpn is { })
+            if (_authenticationProvider is { })
             {
-                _authenticationProvider.Initialize(serverInfo, _physicalStateObj, this, serverSpn);
+                _authenticationProvider.Initialize(serverInfo, _physicalStateObj, this, resolvedServerSpn.Primary, resolvedServerSpn.Secondary);
             }
 
             if (_fMARS && marsCapable)

src/Microsoft.Data.SqlClient/netcore/src/Microsoft/Data/SqlClient/TdsParserStateObject.netcore.cs

@@ -11,6 +11,7 @@
 using System.Threading.Tasks;
 using Microsoft.Data.Common;
 using Microsoft.Data.ProviderBase;
+using Microsoft.Data.SqlClient.SNI;
 
 namespace Microsoft.Data.SqlClient
 {
@@ -71,7 +72,7 @@ internal abstract void CreatePhysicalSNIHandle(
             string serverName,
             TimeoutTimer timeout,
             out byte[] instanceName,
-            out string resolvedSpn,
+            out ResolvedServerSpn resolvedSpn,
             bool flushCache,
             bool async,
             bool fParallel,

src/Microsoft.Data.SqlClient/netcore/src/Microsoft/Data/SqlClient/TdsParserStateObjectManaged.cs

@@ -81,7 +81,7 @@ internal override void CreatePhysicalSNIHandle(
             string serverName,
             TimeoutTimer timeout,
             out byte[] instanceName,
-            out string resolvedSpn,
+            out ResolvedServerSpn resolvedSpn,
             bool flushCache,
             bool async,
             bool parallel,

src/Microsoft.Data.SqlClient/netcore/src/Microsoft/Data/SqlClient/TdsParserStateObjectNative.cs

@@ -13,6 +13,7 @@
 using Interop.Windows.Sni;
 using Microsoft.Data.Common;
 using Microsoft.Data.ProviderBase;
+using Microsoft.Data.SqlClient.SNI;
 
 namespace Microsoft.Data.SqlClient
 {
@@ -144,7 +145,7 @@ internal override void CreatePhysicalSNIHandle(
             string serverName,
             TimeoutTimer timeout,
             out byte[] instanceName,
-            out string resolvedSpn,
+            out ResolvedServerSpn resolvedSpn,
             bool flushCache,
             bool async,
             bool fParallel,
@@ -178,7 +179,7 @@ internal override void CreatePhysicalSNIHandle(
 
             _sessionHandle = new SNIHandle(myInfo, serverName, ref serverSPN, timeout.MillisecondsRemainingInt, out instanceName,
                 flushCache, !async, fParallel, ipPreference, cachedDNSInfo, hostNameInCertificate);
-            resolvedSpn = serverSPN.TrimEnd();
+            resolvedSpn = new(serverSPN.TrimEnd());
         }
 
         protected override uint SniPacketGetData(PacketHandle packet, byte[] _inBuff, ref uint dataSize)

src/Microsoft.Data.SqlClient/src/Microsoft/Data/SqlClient/SSPI/NegotiateSspiContextProvider.cs

@@ -9,15 +9,15 @@
 
 namespace Microsoft.Data.SqlClient
 {
-    internal sealed class NegotiateSspiContextProvider : SspiContextProvider
+    internal sealed class NegotiateSspiContextProvider : SspiContextProvider, IDisposable
     {
         private NegotiateAuthentication? _negotiateAuth;
 
         protected override bool GenerateSspiClientContext(ReadOnlySpan<byte> incomingBlob, IBufferWriter<byte> outgoingBlobWriter, SspiAuthenticationParameters authParams)
         {
             NegotiateAuthenticationStatusCode statusCode = NegotiateAuthenticationStatusCode.UnknownCredentials;
 
-            _negotiateAuth ??= new(new NegotiateAuthenticationClientOptions { Package = "Negotiate", TargetName = authParams.Resource });
+            _negotiateAuth = GetNegotiateAuthenticationForParams(authParams);
 
             var sendBuff = _negotiateAuth.GetOutgoingBlob(incomingBlob, out statusCode)!;
 
@@ -33,6 +33,27 @@ protected override bool GenerateSspiClientContext(ReadOnlySpan<byte> incomingBlo
 
             return false;
         }
+
+        public void Dispose()
+        {
+            _negotiateAuth?.Dispose();
+        }
+
+        private NegotiateAuthentication GetNegotiateAuthenticationForParams(SspiAuthenticationParameters authParams)
+        {
+            if (_negotiateAuth is { })
+            {
+                if (string.Equals(_negotiateAuth.TargetName, authParams.Resource, StringComparison.Ordinal))
+                {
+                    return _negotiateAuth;
+                }
+
+                // Dispose of it since we're not going to use it now
+                _negotiateAuth?.Dispose();
+            }
+
+            return _negotiateAuth = new(new NegotiateAuthenticationClientOptions { Package = "Negotiate", TargetName = authParams.Resource });
+        }
     }
 }
 #endif

src/Microsoft.Data.SqlClient/src/Microsoft/Data/SqlClient/SSPI/SspiContextProvider.cs

@@ -11,15 +11,22 @@ internal abstract class SspiContextProvider
         private TdsParser _parser = null!;
         private ServerInfo _serverInfo = null!;
 
-        private SspiAuthenticationParameters? _authParam;
+        private SspiAuthenticationParameters? _primaryAuthParams;
+        private SspiAuthenticationParameters? _secondaryAuthParams;
 
         private protected TdsParserStateObject _physicalStateObj = null!;
 
+#if NET
+        /// <remarks>
+        /// <see cref="SNI.ResolvedServerSpn"/> for details as to what <paramref name="primaryServerSpn"/> and <paramref name="secondaryServerSpn"/> means and why there are two.
+        /// </remarks>
+#endif
         internal void Initialize(
             ServerInfo serverInfo,
             TdsParserStateObject physicalStateObj,
             TdsParser parser,
-            string serverSpn
+            string primaryServerSpn,
+            string? secondaryServerSpn = null
             )
         {
             _parser = parser;
@@ -28,16 +35,23 @@ string serverSpn
 
             var options = parser.Connection.ConnectionOptions;
 
-            SqlClientEventSource.Log.StateDumpEvent("<SspiContextProvider> Initializing provider {0} with SPN={1}", GetType().FullName, serverSpn);
+            SqlClientEventSource.Log.StateDumpEvent("<SspiContextProvider> Initializing provider {0} with SPN={1} and alternate={2}", GetType().FullName, primaryServerSpn, secondaryServerSpn);
 
-            _authParam = new SspiAuthenticationParameters(options.DataSource, serverSpn)
+            _primaryAuthParams = CreateAuthParams(options, primaryServerSpn);
+
+            if (secondaryServerSpn is { })
             {
-                DatabaseName = options.InitialCatalog,
-                UserId = options.UserID,
-                Password = options.Password,
-            };
+                _secondaryAuthParams = CreateAuthParams(options, secondaryServerSpn);
+            }
 
             Initialize();
+
+            static SspiAuthenticationParameters CreateAuthParams(SqlConnectionString connString, string serverSpn) => new(connString.DataSource, serverSpn)
+            {
+                DatabaseName = connString.InitialCatalog,
+                UserId = connString.UserID,
+                Password = connString.Password,
+            };
         }
 
         private protected virtual void Initialize()
@@ -50,11 +64,30 @@ internal void WriteSSPIContext(ReadOnlySpan<byte> receivedBuff, IBufferWriter<by
         {
             using var _ = TrySNIEventScope.Create(nameof(SspiContextProvider));
 
-            if (!(_authParam is { } && RunGenerateSspiClientContext(receivedBuff, outgoingBlobWriter, _authParam)))
+            if (_primaryAuthParams is { })
             {
-                // If we've hit here, the SSPI context provider implementation failed to generate the SSPI context.
-                SSPIError(SQLMessage.SSPIGenerateError(), TdsEnums.GEN_CLIENT_CONTEXT);
+                if (RunGenerateSspiClientContext(receivedBuff, outgoingBlobWriter, _primaryAuthParams))
+                {
+                    return;
+                }
+
+                // remove _primaryAuth from future attempts as it failed
+                _primaryAuthParams = null;
+            }
+
+            if (_secondaryAuthParams is { })
+            {
+                if (RunGenerateSspiClientContext(receivedBuff, outgoingBlobWriter, _secondaryAuthParams))
+                {
+                    return;
+                }
+
+                // remove _secondaryAuthParams from future attempts as it failed
+                _secondaryAuthParams = null;
             }
+
+            // If we've hit here, the SSPI context provider implementation failed to generate the SSPI context.
+            SSPIError(SQLMessage.SSPIGenerateError(), TdsEnums.GEN_CLIENT_CONTEXT);
         }
 
         private bool RunGenerateSspiClientContext(ReadOnlySpan<byte> incomingBlob, IBufferWriter<byte> outgoingBlobWriter, SspiAuthenticationParameters authParams)

src/Microsoft.Data.SqlClient/src/Microsoft/Data/SqlClient/SqlClientEventSource.cs

@@ -893,6 +893,12 @@ internal void StateDumpEvent<T0, T1>(string message, T0 args0, T1 args1)
         {
             StateDump(string.Format(message, args0?.ToString() ?? NullStr, args1?.ToString() ?? NullStr));
         }
+
+        [NonEvent]
+        internal void StateDumpEvent<T0, T1, T2>(string message, T0 args0, T1 args1, T2 args2)
+        {
+            StateDump(string.Format(message, args0?.ToString() ?? NullStr, args1?.ToString() ?? NullStr, args2?.ToString()));
+        }
         #endregion
 
         #region SNI Trace