Identify robots in asp.net httphandler?

Question

I am using UrlRewriter that is registered as http module in web.config, I have some 301 redirects in place that does works when i request the page through web browser however when request comes from google robots then it return 404 error.

Does it means httpmodules doesn't get activated when request comes through crawlers? How do i make sure that my 301 redirect does get forced even when request comes from a crawler?

Imports System.Web

Imports System.IO

Public Class UrlRewriter Implements System.Web.IHttpModule

Public Sub Init(ByVal app As System.Web.HttpApplication) Implements System.Web.IHttpModule.Init
    AddHandler app.AuthorizeRequest, AddressOf AuthorizeRequest
End Sub

Public Sub Dispose() Implements System.Web.IHttpModule.Dispose
End Sub

Protected Sub AuthorizeRequest(ByVal sender As Object, ByVal e As EventArgs)

    Dim app As HttpApplication = CType(sender, HttpApplication)
    Dim host As Eteach.Entity.Host = Eteach.UI.Web.Host.Current(app.Context)

    Dim rewriteUrl As String = app.Context.Request.Path
    Dim reqPath As String = app.Context.Request.Path.ToLower
    Dim reqUrl As String = reqPath
    If app.Context.Request.QueryString.Count > 0 Then
        rewriteUrl &= "?" & app.Context.Request.QueryString.ToString
        reqUrl &= "?" & app.Context.Request.QueryString.ToString.ToLower
    End If

    If Not host Is Nothing AndAlso host.UrlRewrites.ContainsKey(reqUrl) Then
        Dim url As String = host.UrlRewrites(reqUrl)

        Dim qs As String = String.Empty
        If url.Contains("?") Then
            qs = url.Substring(url.IndexOf("?") + 1)
            url = url.Substring(0, url.IndexOf("?"))
        End If

        app.Context.Items("Page_RewriteUrl") = rewriteUrl
        app.Context.RewritePath(url, String.Empty, qs)
    ElseIf Not host Is Nothing AndAlso host.UrlRedirect301s.ContainsKey(reqUrl) Then
        app.Context.Response.Clear()
        app.Context.Response.StatusCode = 301
        app.Context.Response.Status = "301 Moved Permanently"
        app.Context.Response.AddHeader("Location", host.UrlRedirect301s(reqUrl))
        app.Context.Response.End()
    ElseIf Not host Is Nothing AndAlso host.UrlRedirects.ContainsKey(reqUrl) Then
        app.Context.Response.Clear()
        app.Context.Response.Redirect(host.UrlRedirects(reqUrl))
        app.Context.Response.End()
    ElseIf reqPath.EndsWith("/") AndAlso File.Exists(app.Context.Server.MapPath(reqPath & "default.aspx")) Then
        app.Context.Items("Page_RewriteUrl") = rewriteUrl
        If app.Context.Request.QueryString.Count > 0 Then
            app.Context.RewritePath(reqPath & "default.aspx", String.Empty, app.Context.Request.QueryString.ToString)
        Else
            app.Context.RewritePath(reqPath & "default.aspx", String.Empty, String.Empty)
        End If
    ElseIf reqPath.EndsWith("/default.aspx") Then
        Dim url As String = app.Context.Request.Path
        url = url.Substring(0, url.Length - 12)
        If app.Context.Request.QueryString.Count > 0 Then
            url &= "?" & app.Context.Request.QueryString.ToString
        End If

        app.Context.Response.Clear()
        app.Context.Response.StatusCode = 301
        app.Context.Response.Status = "301 Moved Permanently"
        app.Context.Response.AddHeader("Location", url)
        app.Context.Response.End()

    ElseIf reqPath.StartsWith("/cms/" & host.ID.ToString & "/") Then
        Dim url As String = app.Context.Request.Path
        url = url.Remove(0, 6 + host.ID.ToString.Length)
        url = url.Insert(0, "/DataFiles/CMS/" & host.ID.ToString & "/pages/")

        app.Context.Items("Page_RewriteUrl") = rewriteUrl
        If app.Context.Request.QueryString.Count > 0 Then
            app.Context.RewritePath(url, String.Empty, app.Context.Request.QueryString.ToString)
        Else
            app.Context.RewritePath(url, String.Empty, String.Empty)
        End If
    End If
End Sub

End Class