Making user login persistant with ASP .Net Membership

Question

I have a website that is built in ASP.NET 3.5 & SQL Server 2005, using the sql membership provider, and presumably forms authentication.

Since security needs on my site are very low, I would like to authenticate once, and then keep the log in persisted indefinitely (without giving the user the choice).

What is happening is that the user logs in, stays logged in for the session, and then the next time they arrive, they are logged out.

How can I get the log-in persisted?

Here's the technical details, I have experimented with many variations of durations.

 Try
            If Membership.ValidateUser(UserName.Text, Password.Text) Then
                Security.UserManager.AuthenticateUser(UserName.Text)

                If FormsAuthentication.GetRedirectUrl(UserName.Text, False) = "/default.aspx" Then
                    Try
                        'Custom Logic'
                    Catch Ex As Exception
                        'Custom Error handling'
                    End Try
                Else
                    FormsAuthentication.RedirectFromLoginPage(UserName.Text, True)
                End If
            End If
        Catch ex As Exception
            RaiseEvent ExceptionThrown(New ApplicationException("An error occurred trying to log the user in after account creation.", ex))
        End Try

Public Shared Sub AuthenticateUser(ByVal Username As String)
    Dim Expiration As DateTime = DateTime.Now.AddMonths(3)

    Dim authTicket As FormsAuthenticationTicket = New FormsAuthenticationTicket(Username, True, Expiration.Subtract(Expiration).TotalMinutes)
    Dim EncryptedTicket As String = FormsAuthentication.Encrypt(authTicket)
    Dim AuthCookie As New HttpCookie(FormsAuthentication.FormsCookieName, EncryptedTicket)
    AuthCookie.Expires = Expiration
    HttpContext.Current.Response.Cookies.Add(AuthCookie)
End Sub

Web Config:

<membership defaultProvider="SqlProvider" userIsOnlineTimeWindow="15">
   <providers>
      <clear />
      <add
         name="SqlProvider"
         type="System.Web.Security.SqlMembershipProvider"
         connectionStringName="GlobalConnString"
         applicationName="/"
         enablePasswordRetrieval="false"
         enablePasswordReset="true"
         requiresQuestionAndAnswer="false"
         requiresUniqueEmail="true"
         passwordFormat="Hashed"
         minRequiredPasswordLength="5"
         minRequiredNonalphanumericCharacters="0"
      />
   </providers>
</membership>


<authentication mode="Forms">
    <forms timeout="1439200" name="SITENAME" loginUrl="/login.aspx" />
</authentication>

Edit: Here is the cookie information that is getting stored by the client:

Name    ASP.NET_SessionId
Value   r4dz1555f1pdne45n1zrlkmg
Host    SITEADDRESS.com
Path    /
Secure  No
Expires At End Of Session

Name    .ASPXAUTH
Value   648767AC72A60DBA49650A361A2FA446BA992F792055EF5B488CADC95DF495315C1C577F1C8E67E67BD937A7AB6CC5DAED85D8D64E4ED7867FC0FC395F48FED7FB631033CE441DE85223E8B3EBAE616C
Host    www.SITEADDRESS.com
Path    /
Secure  No
Expires Tue, 09 Jun 2009 17:51:31 GMT

Name    ASP.NET_SessionId
Value   gn5pcymhfsnua455yp45wpej
Host    www.SITEADDRESS.com
Path    /
Secure  No
Expires At End Of Session

Name    SITENAME
Value   9610E8515F3DBC088DAC286E1F44311A20CB2BBB57C97F906F49BC878A6C6AC0B9011777402AEA130DCDC521EF4FBB3393DB310083F72EB502AE971183306C24F07F696B3695C67DD73166F1653DF52B
Host    www.SITEADDRESS.com
Path    /
Secure  No
Expires Tue, 20 Dec 2011 06:14:10 GMT

Answer 1

I finally figured out the last piece of the puzzle. When the app pool of my server was being recycled (configured by the hosting provider), the viewstate encryption key was being auto-re-generated. This meant that even though the cookies were valid & non expired (pre-return visit), when the user returned the encyrption had changed, and the cookie was no longer valid.

The solution was to manually specify a static validation key. The following link can be used to generate the neccessary web.config tag for this.

http://www.aspnetresources.com/tools/keycreator.aspx

UPDATE:

Here's a more configurable site to generate Machine Key

Source Tree - Generage attribute

I realize that this might have a minor security impact, I guess theoritically it's safer to have a changing key in case your key gets brute forced and compromises any data you might be storing in the view state, but you probably shouldn't be storing sensitive information in the viewstate as it's not inherently safe anyway.

Example:

<configuration>
  <system.web>
    <machineKey
        validationKey="97CEB2D3DEBF853649EAB851F56F08BA328423F935C97244CF5300925B6FF7D2C43084C73CBAF19D5193755EF7F87A3FFC714675F9197C822BAEEC97E853B91E"
        decryptionKey="A72F69A4650348E3AA249A8179516D67C8553B3D4BD165B9"
        validation="SHA1" />
  </system.web>
</configuration>