How to configure IIS7 when using mixed mode authentication with asp.net

Let's start with server roles configuration (this is under server manager, roles, IIS)

You're gonna want to make sure that the windows auth and anonymous auth sections are enabled/installed, and also the forms auth (which presumably you already have). After those are installed/configured, you'll need to define the following stuff:

In your Web.Config you're going to want to have the following sections defined:

<configuration>
  <system.web>
  <authentication mode="Forms">
      <forms cookieless="UseDeviceProfile" defaultUrl="~/Default.aspx" enableCrossAppRedirects="true" loginUrl="~/WindowsLogin.aspx" name=".ASPXAUTH" path="/" protection="All" requireSSL="false" slidingExpiration="true" timeout="10080"/>
    </authentication>
    <authorization>
        <deny users="?"/>
    </authorization>
  </system.web>
  <location path="Login.aspx">
      <system.web>
          <authorization>
              <allow users="?"/>
          </authorization>
      </system.web>
      <system.webServer>
          <security>
              <authentication>
                  <anonymousAuthentication enabled="true"/>
                  <windowsAuthentication enabled="false"/>
              </authentication>
          </security>
      </system.webServer>
  </location>
  <location path="WindowsLogin.aspx">
      <system.web>
          <authorization>
              <deny users="?"/>
              <allow users="*"/>
          </authorization>
      </system.web>
      <system.webServer>
          <security>
              <authentication>
                  <anonymousAuthentication enabled="false"/>
                  <windowsAuthentication enabled="true"/>
              </authentication>
          </security>
      </system.webServer>
  </location>
</configuration>

Then you'll need two files:

Login.aspx (this does forms auth)
WindowsLogin.aspx (this does Windows auth)

LOGIN does forms, right, so that's just bog standard ASP.NET forms auth It's WindowsLogin that does the magic (and here's that file)

using System;
using System.Web;
using System.Web.Security;
using App_Code.Biz;

public partial class WindowsLogin : System.Web.UI.Page {
    protected string UserIsInRoles = string.Empty;
    private static readonly BAL _mBAL = new BAL();
    protected void Page_Load(object sender, EventArgs e) {
        string redirectUrl = Request["returnurl"] ?? "~/default.aspx";
        string username = Request.ServerVariables["LOGON_USER"];
        try {
            if ( Roles.GetRolesForUser( username ).Length < 1 )
                Roles.AddUserToRole( username, Global.defaultRole );
            int status;
            _mBAL.aspnet_Membership_CreateUser( username, out status );
        } catch ( Exception ex ) {
            ErrHandler.WriteXML( ex );
        }

        /* Test to see if the user is in any roles */
        if ( Roles.GetRolesForUser( username ).Length < 1 ) {
            UserIsInRoles = "<br />" + username + "You are not in any rules. This must be your first visit to our site!<br /> Adding you to the " + Global.defaultRole + " role now!";

        } else {
            UserIsInRoles = "You are in the following roles: ";
            string[] roles = Roles.GetRolesForUser( username );
            foreach ( string role in roles )
                UserIsInRoles += role + ", ";
            UserIsInRoles = UserIsInRoles.Remove( UserIsInRoles.Length - 2 ) + "!";

            if ( Login( username, String.Join( ",", roles ) ) )
                Response.Redirect( redirectUrl );
        }

        //we shouldn't get here, so if we do, redirect back to a page they can use.
        if ( Page.IsPostBack ) {
            if ( Response.StatusCode == 401 )
                Response.Redirect( "~/Login.aspx" );

        }
    }

    private bool Login(string strUser, string strRole) {
        if ( strRole != null ) {
            FormsAuthenticationTicket ticket = new FormsAuthenticationTicket(
               1,                            // version
               strUser,                      // user name
               DateTime.Now,                 // create time
               DateTime.Now.AddYears(1),     // expire time
               false,                        // persistent
               strRole );                     // user data
            string strEncryptedTicket = FormsAuthentication.Encrypt( ticket );
            HttpCookie cookie = new HttpCookie( FormsAuthentication.FormsCookieName, strEncryptedTicket );
            Context.Response.Cookies.Add( cookie );
            return true;
        }
        return false;
    }
}

After all this, you might get a config error for section locked at a parent level. Lock is either by default (overrideModeDefault="Deny") or set explicitly by a location tag ... and if so, then the fastest way to fix that is to open C:\Windows\System32\inetsrv\config\applicationHost.config and edit the following block:

<configSections>
  <sectionGroup name="system.webServer">
    <sectionGroup name="security">
      <sectionGroup name="authentication">
        <section name="anonymousAuthentication" overrideModeDefault="Allow">
        <section name="windowsAuthentication" overrideModeDefault="Allow">
      </sectionGroup>
    </sectionGroup>
  </sectionGroup>
</configSections>

Also see the chat log: https://chat.stackoverflow.com/rooms/5/conversation/configuring-iis7-and-mixed-mode-authentication-in-asp-net