CustomError mode in web.config file and Use
CustomErrors Element : "Provides information about custom error messages for an ASP.NET application. The customErrors element can be defined at any level in the application file hierarchy".
The ASP.NET framework provides built-in settings to control how to respond when an application error occurs. This functionality is part of the Web.Config customErrors section.
Configuration Options for Web.Config <customErrors>
Like most web.config settings, customErrors can be configured within the Machine.config, root web.config or your application web.config file. Usually, it is set per application.
CustomErrors supports the following modes: The Error mode attribute determines whether or not an ASP.NET error message is displayed. By default, the mode value is set to "RemoteOnly".
1) Off Mode : When the error attribute is set to "Off", ASP.NET uses its default error page for both local and remote users in case of an error.
2) On Mode : In case of "On" Mode, ASP.NET uses user-defined custom error page instead of its default error page for both local and remote users. If a custom error page is not specified, ASP.NET shows the error page describing how to enable remote viewing of errors.
3) RemoteOnly : ASP.NET error page is shown only to local users. Remote requests will first check the configuration settings for the custom error page or finally show an IIS error.
<configuration>
<system.web>
<customErrors defaultRedirect="ErrorPage.aspx" mode="RemoteOnly">
<error statusCode="500" redirect="ErrorPage.aspx"/>
<error statusCode="403" redirect="ErrorPage.aspx" />
<error statusCode="404" redirect="ErrorPage.aspx" />
</customErrors>
</system.web>
</configuration>