Getting a failure to connect to SQL Server in process request

I have an ASP.NET application that is providing a logon page for an application. It's written in C#.

The application is using forms authentication and Active Directory to do the authentication.

When I click on the logon button, it goes back through the page_load method and then throws the following after timing out trying to connect.

> 2019-09-19 12:55:50.9438 An unhandled error was received in xxxxx
> System.Web.HttpException (0x80004005): Unable to connect to SQL Server database. 
>
> System.Web.HttpException (0x80004005): Unable to connect to SQL Server database. 
>
> System.Data.SqlClient.SqlException (0x80131904): A network-related or instance-specific error occurred while establishing a connection to SQL Server. The server was not found or was not accessible. Verify that the instance name is correct and that SQL Server is configured to allow remote connections. (provider: SQL Network Interfaces, error: 26 - Error Locating Server/Instance Specified)
>
> at System.Data.SqlClient.SqlInternalConnectionTds..ctor(DbConnectionPoolIdentity identity, SqlConnectionString connectionOptions, SqlCredential credential, Object providerInfo, String newPassword, SecureString newSecurePassword, Boolean redirectedUserInstance, SqlConnectionString userConnectionOptions, SessionData reconnectSessionData, DbConnectionPool pool, String accessToken, Boolean applyTransientFaultHandling, SqlAuthenticationProviderManager sqlAuthProviderManager)  
> at System.Data.SqlClient.SqlConnectionFactory.CreateConnection(DbConnectionOptions options, DbConnectionPoolKey poolKey, Object poolGroupProviderInfo, DbConnectionPool pool, DbConnection owningConnection, DbConnectionOptions userOptions)  
> at System.Data.ProviderBase.DbConnectionFactory.CreateNonPooledConnection(DbConnection owningConnection, DbConnectionPoolGroup poolGroup, DbConnectionOptions userOptions)  
> at System.Data.ProviderBase.DbConnectionFactory.TryGetConnection(DbConnection owningConnection, TaskCompletionSource`1 retry, DbConnectionOptions userOptions, DbConnectionInternal oldConnection, DbConnectionInternal& connection)  
> at System.Data.ProviderBase.DbConnectionInternal.TryOpenConnectionInternal(DbConnection outerConnection, DbConnectionFactory connectionFactory, TaskCompletionSource`1 retry, DbConnectionOptions userOptions)  
> at System.Data.ProviderBase.DbConnectionClosed.TryOpenConnection(DbConnection outerConnection, DbConnectionFactory connectionFactory, TaskCompletionSource`1 retry, DbConnectionOptions userOptions)  
> at System.Data.SqlClient.SqlConnection.TryOpenInner(TaskCompletionSource`1 retry)  
> at System.Data.SqlClient.SqlConnection.TryOpen(TaskCompletionSource`1 retry)  
> at System.Data.SqlClient.SqlConnection.Open()  
> at System.Web.Management.SqlServices.GetSqlConnection(String server, String user, String password, Boolean trusted, String connectionString)  
> ClientConnectionId:00000000-0000-0000-0000-000000000000  
> Error Number:-1,State:0,Class:20
>
> at System.Web.Management.SqlServices.GetSqlConnection(String server, String user, String password, Boolean trusted, String connectionString)  
> at System.Web.Management.SqlServices.SetupApplicationServices(String server, String user, String password, Boolean trusted, String connectionString, String database, String dbFileName, SqlFeatures features, Boolean install)  
> at System.Web.Management.SqlServices.Install(String database, String dbFileName, String connectionString)  
> at System.Web.DataAccess.SqlConnectionHelper.CreateMdfFile(String fullFileName, String dataDir, String connectionString)
   at System.Web.DataAccess.SqlConnectionHelper.CreateMdfFile(String fullFileName, String dataDir, String connectionString)
   at System.Web.DataAccess.SqlConnectionHelper.EnsureDBFile(String connectionString)
   at System.Web.DataAccess.SqlConnectionHelper.GetConnection(String connectionString, Boolean revertImpersonation)
   at System.Web.Security.SqlMembershipProvider.GetPasswordWithFormat(String username, Boolean updateLastLoginActivityDate, Int32& status, String& password, Int32& passwordFormat, String& passwordSalt, Int32& failedPasswordAttemptCount, Int32& failedPasswordAnswerAttemptCount, Boolean& isApproved, DateTime& lastLoginDate, DateTime& lastActivityDate)
   at System.Web.Security.SqlMembershipProvider.CheckPassword(String username, String password, Boolean updateLastLoginActivityDate, Boolean failIfNotApproved, String& salt, Int32& passwordFormat)
   at System.Web.Security.SqlMembershipProvider.ValidateUser(String username, String password)
   at System.Web.UI.WebControls.Login.AuthenticateUsingMembershipProvider(AuthenticateEventArgs e)
   at System.Web.UI.WebControls.Login.OnAuthenticate(AuthenticateEventArgs e)
   at System.Web.UI.WebControls.Login.AttemptLogin()
   at System.Web.UI.WebControls.Login.OnBubbleEvent(Object source, EventArgs e)
   at System.Web.UI.Control.RaiseBubbleEvent(Object source, EventArgs args)
   at System.Web.UI.WebControls.Button.OnCommand(CommandEventArgs e)
   at System.Web.UI.WebControls.Button.RaisePostBackEvent(String eventArgument)
   at System.Web.UI.WebControls.Button.System.Web.UI.IPostBackEventHandler.RaisePostBackEvent(String eventArgument)
   at System.Web.UI.Page.RaisePostBackEvent(IPostBackEventHandler sourceControl, String eventArgument)
   at System.Web.UI.Page.RaisePostBackEvent(NameValueCollection postData)
   at System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint)
   at System.Web.UI.Page.ProcessRequest(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint)
   at System.Web.UI.Page.ProcessRequest()
   at System.Web.UI.Page.ProcessRequestWithNoAssert(HttpContext context)
   at System.Web.UI.Page.ProcessRequest(HttpContext context)
   at ASP.logon_aspx.ProcessRequest(HttpContext context) in c:\Windows\Microsoft.NET\Framework\v4.0.30319\Temporary ASP.NET Files\presentationroot\c140bf05\aac314b1\App_Web_oqgoiock.2.cs:line 0
   at System.Web.HttpApplication.CallHandlerExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute()
   at System.Web.HttpApplication.ExecuteStepImpl(IExecutionStep step)
   at System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously)
```

We are using SQL Server to store session state with a custom partition resolver.

There could be several issues causing your error:

1) Make sure your server name is correct, e.g., no typo on the name.

2) Make sure your instance name is correct and there is actually such an instance on your target machine. [Update: Some application converts \ to . If you are not sure about your application, please try both ServerInstance and Server\Instance in your connection string]

3) Make sure the server machine is reachable, e.g, DNS can be resolve correctly, you are able to ping the server (not always true). Simply open Command Prompt and type "-ping yourservername" if it is a named instance or "-ping yourserverIP" if it is an unnamed instance.

4) Make sure SQL Browser service is running on the server.

5) If firewall is enabled on the server, you need to put sqlbrowser.exe and/or UDP port 1434 into exception.

I had the same issue with an application of mine and it was resolved by 4), enabling the SQL Browsing service on the server.

Reference and additional information: https://blogs.msdn.microsoft.com/sql_protocols/2007/05/13/sql-network-interfaces-error-26-error-locating-serverinstance-specified/