Multi-Domain Cookies
Multi-domain cookies are an illusion. But you can have
workable simulations.
You are probably aware of the fact that cookies set by one
domain can't be read by another. It's a privacy feature,
and a good one.
When a domain sets a cookie on your browser, only that
domain can read it. And the webmaster of the domain is
relatively certain that no other domain can read the cookie
set on the visitor's browser. ("Relatively certain" because
some browser's security features seem to be compromised
with alarming frequency.)
If you have more than one domain, the domain restriction
can seem to be a barrier to what might be considered
legitimate and ethical use of cookies. Two examples of
such uses might be:
-
Having a popup display only once even when the
visitor surfs more than one domain.
-
Tracking page to page and domain to domain browser
movement for statistical analysis.
The basic technique is the same for both of the above. But
the implementation differs.
Basically, you decide which of your domains will be the
"cookie handler domain." Cookies set and read for the above
purposes are done by a script on your cookie handler domain.
For the popup functionality like the above example, the
script on the cookie handler domain is called with
JavaScript code. The script checks for the presence of a
specific cookie. If not found, the script sends a popup
launching JavaScript function to the browser. If found, the
script sends an empty function, instead.
For the tracking functionality, the script on the cookie
handler domain is called with an image tag. The script
updates the tracking database, reading and setting cookies
as required, and then sends an image to the browser.
Note that this technique uses what is called "third-party
cookies." That is, the cookies are read and set on a domain
other than what's in the browser's address bar.
The system is not infallible.
Some browsers can be set to ignore third party cookies.
Others, such IE6, can be set to require a privacy policy
of a specific format before accepting third party cookies.
The publisher of IE6 provides a guide for web site owners
to make an acceptable privacy policy. It is "How to Deploy
P3P Privacy Policies on Your Web Site" at
/a/12/pl.pl?129artie6p3p
If you're interested, an IE6 Privacy Feature FAQ is at
/a/12/pl.pl?129artie6faq
The Popup Functionality.
First: (i) Obtain the script from
/a/12/pl.pl?129artpuf
(ii) Make the modifications per instructions and upload it
into the cgi-bin of your cookie handler domain.
Next: (i) Change the URL in the following JavaScript to
the URL of the script you uploaded in the above paragraph.
(ii) Put the JavaScript on your web pages in the HEAD area.
<script language="JavaScript"
src="http://domain.com/cgi-bin/monitor.cgi">
</script>
If your web pages' BODY tags have onLoad or onUnload
attributes, those are still required. But your page doesn't
need the JavaScript popup function because the script
provides it as appropriate.
Last: Upload your web pages.
The above can be modified to do other tasks that require
inserting code or text into a web page, the nature of which
change depending on the presence or values of cookies.
The Tracking Functionality.
First: (i) Obtain the script from
/a/12/pl.pl?129arttrack
(ii) Make the modifications per instructions and upload it
into the cgi-bin of your cookie handler domain.
Next: (i) Change the URL in the following image tag to
the URL of the script you uploaded in the above paragraph.
(ii) When you modified the above script, you specified the
file name of an image. Here, modify the height and width
attributes as appropriate for that image. (iii) Put the
image tag into your web pages where you want the image to
appear.
<img src="http://domain.com/cgi-bin/monitor.cgi"
height="2" width="3" border="0">
Because many browsers are set to cache images, you may wish
to append a question mark and a unique string of characters
to the URL. So long as each page with this URL has a
different string of characters following the URL, most
browsers will load the image the first time the page is
loaded. Example:
<img src="http://domain.com/cgi-bin/monitor.cgi?skdefi332d"
height="2" width="3" border="0">
Last: Upload your web pages.
The above can be modified to do other server-related tasks,
the nature of which change depending on the presence or
values of cookies.
Now, dear reader, your several domains can function as if
you were actually using multi-domain cookies.
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Will Bontrager
©2002 Bontrager Connection, LLC
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