Understanding ASP.NET Authentication

Authentication is the process of obtaining identification credentials such as name and password from a user and validating those credentials against some authority. If the credentials are valid, the entity that submitted the credentials is considered an authenticated identity.

Once an identity has been authenticated, the authorization process determines whether that identity has access to a given resource.

ASP.NET implements authentication through authentication providers, the code modules that contain the code necessary to authenticate the requestor's credentials. The topics in this section describe the authentication providers built into ASP.NET.

ASP.NET Forms Authentication

Forms authentication enables you to authenticate the user name and password of your users using a login form that you create. Unauthenticated requests are redirected to a login page, where the user provides credentials and submits the form. If the application authenticates the request, the system issues a ticket (authentication token) that contains a key for reestablishing the identity for subsequent requests. You can maintain the authentication token in a cookie or in the page URL.

To use forms authentication, you create a login page that collects credentials from the user and that includes code to authenticate the credentials. Typically you configure the application by using the authentication configuration element(<authentication>) in the web.config file to redirect requests to the login page when users try to access a protected resource, such as a page that requires authentication.

Forms Authentication Control Flow

The flow of control for ASP.NET forms authentication is shown in the following table.

 

Browser and HTTP operation	Server reply
Requests a protected resource from a server. The HTTP operation is: GET /default.aspx	If there is no authentication cookie, redirects the request to a logon page to collect credentials. Information about the originating page is placed in the query string using RETURNURL as the key. The server HTTP reply is: 302 Found Location: http://samples.microsoft.com/logon.aspx?RETURNURL=/default.aspx
Follows the redirection to the logon page. The HTTP operation is: GET /logon.aspx?RETURNURL=/default.aspx	Returns the logon page. For security, we recommend that you use Secure Sockets Layer (SSL) for the logon page to keep the user's credentials from being sent in clear text. The server HTTP reply is: 200 OK
After user enters credentials into the logon page, submits the page. The HTTP operation is: POST /logon.aspx?RETURNURL=/default.aspx	Validates user credentials and, if the credentials are authenticated, redirects the browser to the original URL specified in the QueryString as the RETURNURL variable. By default, the authentication ticket is issued as a cookie. Note You can specify that the authentication ticket be included in the URL instead of a cookie using the CookieMode property. The server HTTP reply is: 302 Found Location: /default.aspx
Follows the redirection and requests the original resource again. The HTTP operation is: GET /default.aspx	If the user is authenticated, grants access and grants the authentication cookie, which contains an authentication ticket. Future requests by the same browser session will be authenticated when the FormsAuthenticationModule module inspects the cookie. It is possible to create a persistent cookie that can be used for future sessions, but only until the cookie's expiration date. The server HTTP reply is: 200 OK Set-Cookie: ASPXTICKET=ABCDEFG12345;Path=/ Note that the cookie path is set to /. Because cookie names are case-sensitive, this prevents inconsistent case in URLs on the site. For example, if the path were set to /SavingsPlan and a link contained /savingsplan, the user would be forced to re-authenticate because the browser would not send the cookie.

The following figure and steps shows the sequence of events that occur during forms authentication.

1. The user requests the Default.aspx file from your application's virtual directory. IIS allows the request because anonymous access is enabled in the IIS metabase. ASP.NET confirms that the authorization element includes a <deny users="?" /> tag.
2. The server looks for an authentication cookie. If it fails to find the authentication cookie, the user is redirected to the configured logon page (Login.aspx), as specified by the LoginUrl attribute of the forms element. The user supplies and submits credentials through this form. Information about the originating page is placed in the query string using RETURNURL as the key. The server HTTP reply is as follows:

   302 Found Location: 
   http://localhost/FormsAuthTest/login.aspx?RETURNURL=%2fFormAuthTest%2fDefault.aspx
     

3. The browser requests the Login.aspx page and includes the RETURNURL parameter in the query string.
4. The server returns the logon page and the 200 OK HTTP status code.
5. The user enters credentials on the logon page and posts the page, including the RETURNURL parameter from the query string, back to the server.
6. The server validates user credentials against a store, such as a SQL Server database or an Active Directory user store. Code in the logon page creates a cookie that contains a forms authentication ticket that is set for the session.

   In ASP.NET, the validation of user credentials can be performed by the membership system. The Membership class provides the ValidateUser method for this purpose as shown here:

   if (Membership.ValidateUser(userName.Text, password.Text))
   {
       //Maintain the authentication token in a cookie
       if (Request.QueryString["ReturnUrl"] != null)
       {
           FormsAuthentication.RedirectFromLoginPage(userName.Text, false);
       }
       else
       {
           FormsAuthentication.SetAuthCookie(userName.Text, false);
       }
   }
   else
   {
       Response.Write("Invalid UserID and Password");
   }
   

7. For the authenticated user, the server redirects the browser to the original URL that was specified in the query string by the RETURNURL parameter. The server HTTP reply is as follows:

   302 Found Location: 
   http://localhost/TestSample/default.aspx 

8. Following the redirection, the browser requests the Default.aspx page again. This request includes the forms authentication cookie.
9. The FormsAuthenticationModule class detects the forms authentication cookie and authenticates the user. After successful authentication, the FormsAuthenticationModule class populates the current User property, which is exposed by the HttpContext object, with information about the authenticated user.
10. Since the server has verified the authentication cookie, it grants access and returns the Default.aspx page.