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contents/writing/about/cookieless-user-tracking/index.md

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+---
+title: Cookieless User Tracking
+author: Brett Langdon
+date: 2013-11-30
+template: article.jade
+---
+
+A look into various methods of online user tracking without cookies.
+
+---
+
+Over the past few months, in my free time, I have been researching various
+methods for cookieless user tracking. I have a previous article that talks
+on how to write a
+<a href="/writing/about/third-party-tracking-pixels/" target="_blank">tracking server</a>
+which uses cookies to follow people between requests. However, recently
+browsers are beginning to disallow third party cookies by default which means
+developers have to come up with other ways of tracking users.
+
+
+## Browser Fingerprinting
+
+You can use client side javascript to generate a
+<a href="/writing/about/browser-fingerprinting/" target="_blank">browser fingerprint</a>,
+or, a unique identifier for a specific users browser (since that is what cookies
+are actually tracking). Once you have the browser's fingerprint you can then
+send that id along with any other requests you make.
+
+```javascript
+var user_id = generateBrowserFingerprint();
+document.write(
+    '<script type="text/javascript" src="/track/user/"' + user_id + '></ sc' + 'ript>'
+);
+```
+
+
+## Local Storage
+
+Newer browsers come equipped with a feature called
+<a href="http://diveintohtml5.info/storage.html" target="_blank">local storage</a>
+, which is used as a simple key-value store accessible through javascript.
+So instead of relying on cookies as your persistent storage, you can store the
+user id in local storage instead.
+
+```javascript
+var user_id = localStorage.getItem("user_id");
+if(user_id == null){
+    user_id = generateNewId();
+    localStorage.setItem("user_id", user_id);
+}
+document.write(
+    '<script type="text/javascript" src="/track/user/"' + user_id + '></ sc' + 'ript>'
+);
+```
+
+This can also be combined with a browser fingerprinting library for generating
+the new id.
+
+
+## ETag Header
+
+There is a feature of HTTP requests called an
+<a href="http://en.wikipedia.org/wiki/HTTP_ETag" target="_blank">ETag Header</a>
+which can be exploited for the sake of user tracking. The way an ETag works is
+simply when a request is made the server will respond with an ETag header with
+a given value (usually it is an id for the requested document, or maybe a hash
+of it), whenever the bowser then makes another request for that document it will
+send an _If-None-Match_ header with the value of _ETag_ provided by the server
+last time. The server can then make a decision as to whether or not new content
+needs to be served based on the id/hash provided by the browser.
+
+As you may have figured out, instead we can assign a unique user id as the ETag
+header for a response, then when the browser makes a request for that page again
+it will send us the user id.
+
+This is useful, except for the fact that we can only provide a single id per
+user per endpoint. For example, if I use the urls `/track/user` and
+`/collect/data` there is no way for me to get the browser to send the same
+_If-None-Match_ header for both urls.
+
+### Example Server
+
+```python
+from uuid import uuid4
+from wsgiref.simple_server import make_server
+
+
+def tracking_server(environ, start_response):
+    user_id = environ.get("HTTP_IF_NONE_MATCH")
+    if not user_id:
+        user_id = uuid4().hex
+
+    start_response("200 Ok", [
+        ("ETag", user_id),
+    ])
+    return [user_id]
+
+
+if __name__ == "__main__":
+    try:
+        httpd = make_server("", 8000, tracking_server)
+        print "Tracking Server Listening on Port 8000..."
+        httpd.serve_forever()
+    except KeyboardInterrupt:
+        print "Exiting..."
+```
+
+
+## Redirect Caching
+
+Redirect caching works in a similar matter to using ETag headers,
+you end up relying on browser caches to store your user ids.
+
+
+## Ever Cookie
+
+A project worth noting is Samy Kamkar's
+<a href="http://samy.pl/evercookie/" target="_blank">Evercookie</a>
+which uses standard cookies, flash objects, silverlight isolated storage,
+web history, etags, web cache, local storage, global storage... and more.
+
+
+## Other Methods
+
+I am sure there are other methods out there, these are just the few that I
+decided to focus on. If anyone has any other methods or ideas please leave a comment.