UrlReferrer problem

bhavlice · March 19th, 2016, 09:31 PM

I'm using the following simple code in VB.NET:

Dim MyUrl As Uri = Request.UrlReferrer
Response.Write("Referrer URL Port: " & Server.HtmlEncode(MyUrl.Port.ToString()) & "<br>")
Response.Write("Referrer URL Protocol: " & Server.HtmlEncode(MyUrl.Scheme) & "<br>")

I am receiving the following error:

Object reference not set to an instance of an object.
Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code.

Exception Details: System.NullReferenceException: Object reference not set to an instance of an object.

Source Error:

Line 54:
Line 55: Dim MyUrl As Uri = Request.UrlReferrer
Line 56: Response.Write("Referrer URL Port: " & Server.HtmlEncode(MyUrl.Port.ToString()) & "<br>")
Line 57: Response.Write("Referrer URL Protocol: " & Server.HtmlEncode(MyUrl.Scheme) & "<br>")

This used to work but for some reason it no longer does and I don't know why.

Imar · March 20th, 2016, 05:39 AM

Hi there,

The referrer is not guaranteed to be there. It won't be there when you browse to a page directly or when you have local software removing it fore example. Before you write out the port and other info, check if Request.UrlReferrer is not Nothing.

Hope this helps,

Imar

bhavlice · March 21st, 2016, 12:29 PM

Thanks for the response. Yes in deed, you are right, the Referrer port is nothing. However, it is definitely coming from another page with a URL reference. so I'm still not at all clear why it does not show up in the Request. Any ideas why that would be?

Imar · March 21st, 2016, 03:33 PM

Like I said, it could be stripped away by security software. Also, when you're using SSL you won't see the referrer.

In general, don't count on the referrer being there; always assume it could be null / Nothing.

Imar

Imar · March 21st, 2016, 03:34 PM

Or are you saying the URL is there but not the pot? Could it be the default port?

Imar

bhavlice · March 24th, 2016, 10:00 AM

Thanks very much for all your help. I'm not sure I can count on any referrer being there which is not good. This function used to provide me with the ability to return to the page that sent the functional page after the processing is completed. This is particularly useful for Mobile sites to eliminate backing up to previous pages on a close. I guess with all the recent security concerns this request function is no longer a viable option. What a shame. Does anyone know of an alternative way to return to the referrer?

Imar · March 25th, 2016, 07:29 AM

I don't think there's an alternative if you don't control the previous page / site. If the browser doesn't send the referrer, or it gets stripped from the request by software, you'll never see it. If you do control the previous site you could send it as a request parameter.

Cheers,

Imar

bhavlice · March 25th, 2016, 11:43 AM

Once again, thank you so much for responding to my question. Your input has been invaluable to me. I've been at this for more years than I care to remember, but I'm still learning.

I do normally control the entire site where the requests are coming from which is why I was surprised that the referrer stopped working. How can I send it as a request parameter? I thought it was included automatically. However, if I use: Request.ServerVariables("ALL_HTTP"), no UrlReferrer shows up.

Imar · March 28th, 2016, 09:29 AM

>> How can I send it as a request parameter? I thought it was included automatically.

It will be, when it's there. But if the browser doesn't send it, it won't be there.

I was thinking of something like thsi:

<a href="http://www.secondsite.com?From=http://www.firstsite.com/somepage.aspx">....

Imar

bhavlice · March 28th, 2016, 03:45 PM

It is definitely not there. Is it possible that the latest IE version 11 has something to do with blocking the UrlReferrer? It seems that the last time this worked for me was on an earlier version but unfortunately, IE is, as you know, the most commonly used.

Is there some way you can send me the sample code you referenced?