Email Tracking - GMail

Question

I am creating my own email tracking system for email marketing tracking. I have been able to determine each persons email client they are using by using the http referrer but for some reason GMAIL does not send a HTTP_REFERRER at all!

So I am trying to find another way of identifying when gmail requests a transparent image from my server. I get the following headers print_r($_SERVER);:

DOCUMENT_ROOT  =  /usr/local/apache/htdocs

GATEWAY_INTERFACE  =  CGI/1.1

HTTP_ACCEPT  =  */*

HTTP_ACCEPT_CHARSET  =  ISO-8859-1,utf-8;q=0.7,*;q=0.3

HTTP_ACCEPT_ENCODING  =  gzip,deflate,sdch

HTTP_ACCEPT_LANGUAGE  =  en-GB,en-US;q=0.8,en;q=0.6

HTTP_CONNECTION  =  keep-alive

HTTP_COOKIE  =  __utmz=156230011.1290976484.1.1.utmcsr=(direct)|utmccn=(direct)|utmcmd=(none); __utma=156230011.422791272.1290976484.1293034866.1293050468.7

HTTP_HOST  =  xx.xxx.xx.xxx

HTTP_USER_AGENT  =  Mozilla/5.0 (Windows; U; Windows NT 6.1; en-US) AppleWebKit/534.10 (KHTML, like Gecko) Chrome/8.0.552.237 Safari/534.10

PATH  =  /bin:/usr/bin

QUERY_STRING  =  i=MTA=

REDIRECT_STATUS  =  200

REMOTE_ADDR  =  xx.xxx.xx.xxx

REMOTE_PORT  =  61296

REQUEST_METHOD  =  GET

Is there anything of use in that list? Or is there something else I can do to actually get the http referrer, if not how are other ESPs managing to find whether gmail was used to view an email?

Btw, I appreciate it if we can hold back on whether this is ethical or not as many ESPs do this already, I just don't want to pay for their service and I want to do it internally.

Thanks all for any implementation advice.

Update

Just thought I would update this question and make it clearer in light of the bounty.

I would like to find out when a user opens my email when sent to a GMail inbox. Assume, I have the usual transparent image tracking and the user does not block images.

I would like to do this with the single request and the header details I get when the transparent image is requested.

Answer 1

Are your images requested with HTTP or HTTPS?

If so, that's the problem.

HTTPS->HTTP referrals do not leak a Referer Header (HTTP_REFERER).

If you embed a HTTP hosted image in an email that is requested from an HTTPS page, it won't send a referrer. (HTTP pages requesting HTTPS, however, do send a referer).The solution is to embed the image as HTTPS. I've tested it, and sure enough, secure HTTPS images do indeed send the Referrer.

One way Gmail could block the referrer information on loaded images by default is if they used a referrer policy, which is supported on most modern browsers. (As of 2011, they did not implement such a policy.)

See the below screenshot of an embedded image that is generated dynamically with the HTTP REFERER of the request: