Developer

Handle errors with custom error pages in ASP.NET

Providing detailed information with user-friendly error pages can help prevent support calls. ASP.NET offers a simple way to provide an alternative to the default error page.

I recently took over an ASP.NET application utilized by a large healthcare company that wanted to implement additional functionally and correct many existing problems. The company didn't have a system for gracefully handling errors; that is, any error resulted in the display of the default error page. With that in mind, I chose to make it a top priority to add more user-friendly error pages.

Error messages

By default, ASP.NET displays error messages that include the .NET error description along with a stack trace. For example, here is an error message:

Server Error in '/Test' Application.

SQL Server does not exist or access denied.
Description: An unhandled exception occurred during the execution of the
 current web request. Please review the stack trace for more information about
the error and where it originated in the code.

Exception Details: System.Data.SqlClient.SqlException: SQL Server does not
exist or access denied.

Source Error:

Line 100:            }    catch(Exception ex)    {
Line 101:                ExceptionManager.Publish(ex);
Line 102:                throw ex;
Line 103:            }
Line 104:        }

Source File: c:\cvone\web application\secure\status.aspx.cs    Line: 102

Stack Trace:

[SqlException: SQL Server does not exist or access denied.]
Test.Application.Page_Load(Object sender, EventArgs e) in c:\test\web
application\test.aspx.cs:102
System.Web.UI.Control.OnLoad(EventArgs e) +67
System.Web.UI.Control.LoadRecursive() +35
System.Web.UI.Page.ProcessRequestMain() +725

Version Information: Microsoft .NET Framework Version:1.1.4322.573; ASP.NET
 Version:1.1.4322.573

There's a lot of information in this error message. First, the exception is thrown when attempting to access SQL Server. The message includes a code excerpt, along with the local path (on the server) to the file. The stack trace signals where the specific error occurred, and the last line tells you the .NET Framework version utilized.

This information is great if you're a developer because it tells you why and where the error occurred. On the other hand, this information may be confusing to the average user. They will not understand the displayed text, and it may result in more support calls or even in them possibly abandoning the site. Also, hackers may unscrupulously use the information that's in the error details.

One particularly vulnerable area is the section of the error page that displays the lines of code. If this includes names and passwords (although I don't recommend it) in code that accesses a database, a person may easily gain access to other resources, such as a database server. Also, the line that details the file location (not the Internet Information Services virtual directory) on the server provides valuable information if the server is compromised.

Create your own

ASP.NET offers a simple way to provide an alternative to the default error page. You may create a custom error page and tell Internet Information Services to display it instead of the default. The web.config file with the customErrors element contains the setting:

<customErrors mode="On" defaultRedirect="errors/ErrorPage.aspx">
</customErrors>

This tells the system to display the custom error page (errors/ErrorPage.aspx) instead of the default error page. Since errors may occur at any place within the application, the error page should be available to all users (security access of public). I usually create a public errors directory and place the appropriate resources within it.

The defaultRedirect attribute contains the path to the error page; in addition, the mode attribute tells the system if and how error handling is enabled. Here are the possible values of the attribute:

  • On: All errors utilize the custom error page.
  • Off: Custom error pages aren't used.
  • RemoteOnly: The custom error pages are used for clients other than the local machine. This is good for debugging a production application. It allows you to sit at the server and view errors while clients receive the custom error pages.

A custom error page may be a simple HTML file or an ASPX file--it depends upon your requirements. For example, you may use the following HTML as a custom error page:

<html>
<head>Custom Error Page</head>
<body>
<h1>An error occurred. Please try again later.</h1>
</body><html>

Handling specific errors

The customErrors section of the web.config file allows you to handle individual errors differently. Errors are distinguished by their HTTP error codes.

Let's look at a few of the more common errors:

  • 400--Bad request--The request could not be understood by the server.
  • 401--Unauthorized--The request requires authentication.
  • 404--Not found--The server has not found anything matching the request.
  • 500--Internal server error--The server encountered an unexpected condition that prevented it from fulfilling the request.

A complete HTTP status code listing is available online. You may expand the customErrors section of the web.config file to handle specific error codes. You can achieve the expansion by adding error elements under the customErrors element. The following sample shows how you may accomplish this:

<customErrors mode="RemoteOnly" defaultRedirect="errors/ErrorPage.aspx">
<error statusCode="400" redirect="errors/ErrorPage400.aspx" />
<error statusCode="401" redirect="errors/ErrorPage401.aspx" />
<error statusCode="403" redirect="errors/ErrorPage403.aspx" />
<error statusCode="404" redirect="errors/ErrorPage404.aspx" />
<error statusCode="408" redirect="errors/ErrorPage408.aspx" />
<error statusCode="500" redirect="errors/ErrorPage500.aspx" />
<error statusCode="503" redirect="errors/ErrorPage503.aspx" />
</customErrors>

This sample handles each specific error with the corresponding page. The statusCode attribute of the error element designates the error, and the redirect attribute defines the page to use. This allows you to tailor the error message based upon the error encountered. The default error page handles any error that isn't specified.

Custom error pages may be developed/used on individual applications, but Internet Information Services allows error pages to be designated server-wide via its properties.

Improving user experience

We all design applications with the goal of no errors, but they do occur. Providing detailed information with user-friendly error pages can help prevent support calls and reduce the amount of information that less-than-faithful users may use.

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About

Tony Patton has worn many hats over his 15+ years in the IT industry while witnessing many technologies come and go. He currently focuses on .NET and Web Development while trying to grasp the many facets of supporting such technologies in a productio...

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