Yes, as a matter of fact I can explain that. In the core DNN code the www is removed when checking the Alias so that you do not need to have both entries. In other words it is assumed that they are the same portal.
This discussion has led me to think that it may be beneficial to include this capability in PageBlaster. So I'll be adding that feature soon.
If you want to do it now, you could place the code in a skin or the default.aspx page.
For SEO, the redirect should be a "301 Moved Permanently". Here is the code you would need. (the word s c r i p t has spaces to make it post here in the forums).
< s c r i p t runat="server">
Private Sub Page_Init(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Init
If Not Request.Url.Host.ToLower().StartsWith("www") Then
Response.Status = "301 Moved Permanently"
Response.AddHeader("Location", Request.Url.Scheme & "://" & "www." & Request.Url.Host & Request.Url.PathAndQuery)
Response.End
End If
End Sub
< /s c r i p t >
I recommend putting it in the skin for the home page so it only works on the specific portal.
If you put this in the default.aspx page, then this will only work if you are not using other host names as the Portal Alias for different portals.
If you are using other portals with different host names then you could also make the code above be more specific.