Windows Authentication Using Form Authentication

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Background

Last month I worked on a small assignment to authenticate windows account (Domain or Local) using form authentication. The purpose of this task was to facilitate our application users to login with any valid windows account (instead of automatic authentication of windows logged in user).

As it was an interesting task, I decided to share my experience with you.

Requirement

The application should authenticate windows user using form authentication so that the currently logged in user shouldn't be bound to logged-in in the application only with his windows account. He should be able to log-in with any valid windows account.

Solution

We need to do the following steps to get the desired functionality:

1. Configure Authorization and Authentication settings in web.config
2. A login page and execute logic to authenticate provided credential of windows user
3. If provided credentials are authenticated in step 2, then generate an authentication token so that user should be able to navigate into the authorized pages of your application.

1. Configure Authorization and Authentication Settings in web.config

We need to use Form authentication. The user will enter his windows credential in the form and we will validate provided windows credential using custom logic in step 2.

<authentication mode="Forms">
	<forms loginUrl="login.aspx" name=".ASPXFORMSAUTH"></forms>
</authentication>

To restrict anonymous access, you need to make the following authorization settings in web.config:

<authorization>
	<deny users="?"/>
</authorization>

2. Create a Login Page and Execute Logic to Authenticate Provided Credential of Windows User

We need to create a login page (e.g. login.aspx) to get username and password information from user and then validate them. We can have different options to validate windows credentials, the way I chose is LogonUser() method of a win32 API called Advapi32.dll.

The LogonUser function attempts to log a user on to the local computer. This method takes username, password and other information as input and returns a Boolean value to indicate that either user is logged or not. If it returns true, it means the provided username and password are correct. To use this method in our class, we need to include the following namespace:

using System.Runtime.InteropServices;

And the add method declaration with DLLImport attribute (as this is a method of Win32 DLL which is an unmanaged DLL).

[DllImport("ADVAPI32.dll", EntryPoint = 
	"LogonUserW", SetLastError = true, CharSet = CharSet.Auto)]
public static extern bool LogonUser(string lpszUsername, string lpszDomain, 
	string lpszPassword, int dwLogonType, int dwLogonProvider, ref IntPtr phToken);

According to the MSDN documentation:

lpszUsername [in]

A pointer to a null-terminated string that specifies the name of the user. This is the name of the user account to log on to. If you use the user principal name (UPN) format, User@DNSDomainName, the lpszDomain parameter must be NULL.

lpszDomain [in, optional] 

A pointer to a null-terminated string that specifies the name of the domain or server whose account database contains the lpszUsername account. If this parameter is NULL, the user name must be specified in UPN format. If this parameter is ".", the function validates the account by using only the local account database.

lpszPassword [in] 

A pointer to a null-terminated string that specifies the plaintext password for the user account specified by lpszUsername. When you have finished using the password, clear the password from memory by calling the SecureZeroMemory function. For more information about protecting passwords, see Handling Passwords.

dwLogonType [in] 

The type of logon operation to perform.

dwLogonProvider [in] 

Specifies the logon provider.

phToken [out] 

A pointer to a handle variable that receives a handle to a token that represents the specified user.

3. Generate an Authentication Token, If Provided Credentials are Authenticated in step 2

If provided credentials are authenticated by LogonUser() method, then we need to generate an authentication token so that users should be able to navigate into the authorized pages of the application.

FormsAuthentication.RedirectFromLoginPage()

Or:

FormsAuthentication.SetAuthCookie()

can be used for this purpose.

Here is login button’s Click handler code for authentication and generating authentication token. The comments will help you to understand the code.

protected void btnLogin_Click(object sender, EventArgs e)
    {
        string domainName = GetDomainName(txtUserName.Text); // Extract domain name 
			// form provided DomainUsername e.g Domainname\Username
        string userName = GetUsername(txtUserName.Text);  // Extract user name 
			// from provided DomainUsername e.g Domainname\Username
        IntPtr token = IntPtr.Zero;

        //userName, domainName and Password parameters are very obvious.
        //dwLogonType (3rd parameter): 
        //    I used LOGON32_LOGON_INTERACTIVE, This logon type 
        //    is intended for users who will be interactively using the computer, 
        //    such as a user being logged on by a terminal server, remote shell, 
        //    or similar process. 
        //    This logon type has the additional expense of caching 
        //    logon information for disconnected operations.
        //    For more details about this parameter please 
        //    see http://msdn.microsoft.com/en-us/library/aa378184(VS.85).aspx
        //dwLogonProvider (4th parameter) :
        //    I used LOGON32_PROVIDER_DEFAUL, This provider use the standard 
        //    logon provider for the system. 
        //    The default security provider is negotiate, unless you pass 
        //    NULL for the domain name and the user name is not in UPN format. 
        //    In this case, the default provider is NTLM. For more details 
        //    about this parameter please see 
        //    http://msdn.microsoft.com/en-us/library/aa378184(VS.85).aspx
        //phToken (5th parameter):
        //    A pointer to a handle variable that receives a handle to 
        //    a token that represents the specified user. 
        //    We can use this handler for impersonation purpose. 
        bool result = LogonUser(userName, domainName, 
				txtPassword.Text, 2, 0, ref token);
        if (result)
        {
            //If Successfully authenticated

            //When an unauthenticated user try to visit any page of your 
            //application that is only allowed to view by authenticated users 
            //then ASP.NET automatically redirect that user to login form 
            //and add ReturnUrl query string parameter that contain the URL of 
            //a page that user want to visit, So that we can redirect the 
            //user to that page after authenticated. 
            //FormsAuthentication.RedirectFromLoginPage() method not only 
            //redirect the user to that page but also generate an authentication 
            //token for that user.
            if (string.IsNullOrEmpty(Request.QueryString["ReturnUrl"]))
            {
                FormsAuthentication.RedirectFromLoginPage(txtUserName.Text, false);
            }
            //If ReturnUrl query string parameter is not present, 
            //then we need to generate authentication token and redirect the user 
            //to any page ( according to your application need). 
            //FormsAuthentication.SetAuthCookie() method will 
            //generate Authentication token 
            else
            {
                FormsAuthentication.SetAuthCookie(txtUserName.Text, false);
                Response.Redirect("default.aspx");
            }
        }
        else
        {
            //If not authenticated then display an error message
            Response.Write("Invalid username or password.");
        }
    }

Let's Put It All Together

Login.aspx

<%@ Page Language="C#" AutoEventWireup="true" CodeFile="Login.aspx.cs" 
	Inherits="Login" %>

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" 
	"http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">

<html xmlns="http://www.w3.org/1999/xhtml">
<head runat="server">
    <title>Windows Authentication Using Form Authentication</title>
    <style type="text/css">
        .style1
        {
            width: 100%;
        }
    </style>
</head>
<body>
    <form id="form1" runat="server">
    <div>
    
        <table class="style1">
            <tr>
                <td>
                    <asp:Label ID="lblUserName" runat="server" Text="User Name:">
		 </asp:Label>
                </td>
                <td>
                    <asp:TextBox ID="txtUserName" runat="server"></asp:TextBox>
                </td>
            </tr>
            <tr>
                <td>
                    <asp:Label ID="lblPassword" runat="server" Text="Password:">
		  </asp:Label>
                </td>
                <td>
                    <asp:TextBox ID="txtPassword" runat="server" TextMode="Password" >
		  </asp:TextBox>
                </td>
            </tr>
            <tr>
                <td>
                     </td>
                <td>
                    <asp:Button ID="btnLogin" runat="server" onclick="btnLogin_Click" 
                        Text="Login" />
                </td>
            </tr>
        </table>
    
    </div>
    <p>
         </p>
    </form>
</body>
</html>

Login.aspx.cs

using System;
using System.Collections;
using System.Configuration;
using System.Data;
using System.Linq;
using System.Web;
using System.Web.Security;
using System.Web.UI;
using System.Web.UI.HtmlControls;
using System.Web.UI.WebControls;
using System.Web.UI.WebControls.WebParts;
using System.Runtime.InteropServices;

public partial class Login : System.Web.UI.Page
{
    [DllImport("ADVAPI32.dll", EntryPoint = 
	"LogonUserW", SetLastError = true, CharSet = CharSet.Auto)]
    public static extern bool LogonUser(string lpszUsername, 
	string lpszDomain, string lpszPassword, int dwLogonType, 
	int dwLogonProvider, ref IntPtr phToken);

    /// <summary>
    /// Parses the string to pull the domain name out.
    /// </summary>
    /// <param name="usernameDomain">The string to parse that must 
    /// contain the domain in either the domain\username or UPN format 
    /// username@domain</param>
    /// <returns>The domain name or "" if not domain is found.</returns>
    public static string GetDomainName(string usernameDomain)
    {
        if (string.IsNullOrEmpty(usernameDomain))
        {
            throw (new ArgumentException("Argument can't be null.", "usernameDomain"));
        }
        if (usernameDomain.Contains("\\"))
        {
            int index = usernameDomain.IndexOf("\\");
            return usernameDomain.Substring(0, index);
        }
        else if (usernameDomain.Contains("@"))
        {
            int index = usernameDomain.IndexOf("@");
            return usernameDomain.Substring(index + 1);
        }
        else
        {
            return "";
        }
    }

    /// <summary>
    /// Parses the string to pull the user name out.
    /// </summary>
    /// <param name="usernameDomain">The string to parse that must 
    /// contain the username in either the domain\username or UPN format 
    /// username@domain</param>
    /// <returns>The username or the string if no domain is found.</returns>
    public static string GetUsername(string usernameDomain)
    {
        if (string.IsNullOrEmpty(usernameDomain))
        {
            throw (new ArgumentException("Argument can't be null.", "usernameDomain"));
        }
        if (usernameDomain.Contains("\\"))
        {
            int index = usernameDomain.IndexOf("\\");
            return usernameDomain.Substring(index + 1);
        }
        else if (usernameDomain.Contains("@"))
        {
            int index = usernameDomain.IndexOf("@");
            return usernameDomain.Substring(0, index);
        }
        else
        {
            return usernameDomain;
        }
    }  

    protected void btnLogin_Click(object sender, EventArgs e)
    {
        string domainName = GetDomainName(txtUserName.Text); // Extract domain name 
			//form provide DomainUsername e.g Domainname\Username
        string userName = GetUsername(txtUserName.Text);  // Extract user name 
			//from provided DomainUsername e.g Domainname\Username
        IntPtr token = IntPtr.Zero;

        //userName, domainName and Password parameters are very obvious.
        //dwLogonType (3rd parameter): 
        //    I used LOGON32_LOGON_INTERACTIVE, This logon type is 
        //    intended for users who will be interactively using the computer, 
        //    such as a user being logged on by a terminal server, remote shell, 
        //    or similar process. 
        //    This logon type has the additional expense of caching 
        //    logon information for disconnected operations.
        //    For more details about this parameter please see 
        //    http://msdn.microsoft.com/en-us/library/aa378184(VS.85).aspx
        //dwLogonProvider (4th parameter) :
        //    I used LOGON32_PROVIDER_DEFAUL, This provider use the standard 
        //    logon provider for the system. 
        //    The default security provider is negotiate, unless you pass 
        //    NULL for the domain name and the user name is not in UPN format. 
        //    In this case, the default provider is NTLM. For more details 
        //    about this parameter please see 
        //    http://msdn.microsoft.com/en-us/library/aa378184(VS.85).aspx
        //phToken (5th parameter):
        //    A pointer to a handle variable that receives a handle to a 
        //    token that represents the specified user. We can use this handler 
        //    for impersonation purpose. 
        bool result = LogonUser(userName, domainName, txtPassword.Text, 2, 0, ref token);
        if (result)
        {
            //If Successfully authenticated

            //When an unauthenticated user try to visit any page of your 
            //application that is only allowed to view by authenticated users then,
            //ASP.NET automatically redirect the user to login form and add 
            //ReturnUrl query string parameter that contain the URL of a page that 
            //user want to visit, So that we can redirect the user to that page after 
            //authenticated. FormsAuthentication.RedirectFromLoginPage() method 
            //not only redirect the user to that page but also generate an 
            //authentication token for that user.
            if (string.IsNullOrEmpty(Request.QueryString["ReturnUrl"]))
            {
                FormsAuthentication.RedirectFromLoginPage(txtUserName.Text, false);
            }
            //If ReturnUrl query string parameter is not present, 
            //then we need to generate authentication token and redirect 
            //the user to any page ( according to your application need). 
            //FormsAuthentication.SetAuthCookie() 
            //method will generate Authentication token 
            else
            {
                FormsAuthentication.SetAuthCookie(txtUserName.Text, false);
                Response.Redirect("default.aspx");
            }
        }
        else
        {
            //If not authenticated then display an error message
            Response.Write("Invalid username or password.");
        }
    }
}

And We Are DONE!!!

We are done with authentication of windows account using form authentication.

Feedback

You feedback will be very helpful for me. You can send me an email at akhhttar@gmail.com. Thanks!