NONE: Re: ONLINE-ADS>> Caching, IE and Guaranteed Impressions

Re: ONLINE-ADS>> Caching, IE and Guaranteed Impressions

Bob Stewart (
Tue, 08 Apr 1997 11:30:35 -0400

Michael Kunze wrote:

>the caching mechanism of IE 3.01 is really hard to deal with. IMHO, it
>is one of the most idiotic cache algorithms i ever came across. IE
>caches images on URL basis, never touching an image again if it has
>cached its URL. This is against all specs in HTTP 1.0 and 1.1. (BTW,
>Navigator does an conditional reguest, as it should be).

AOL, Prodigy, Compuserve and many other large providers have never
followed proper protocol when it comes to caching either. AOL can just be
atrocious. When I had to change the underlying IP # for one of my domains,
they were still sending people to the old site 3 months later! This is
something that isn't supposed to be cached for more than 2-3 days. I guess
that's why they call it "Almost OnLine." ;-)

Michael's right, adding "?RandomText" doesn't defeat IE's caching. What you
need to do is have the source of the image be a cgi script that loads the

Say you reference it like:

<img src="/ads/nph-load.cgi">

This is the perl code for nph-load.cgi:

#! /usr/bin/perl

($sec, $min, $hour, $mday, $mon, $year, $wday, $yday, $isdst)=gmtime(time +

# List of rotating image files (in same directory as script):
_at_adgifs = ("one.gif", "two.gif", "three.gif", "four.gif");

# This randomizes the loading based on the second:
$adbanner = $adgifs[int($sec/(60/($#adgifs+1)))];

# This screams "DON'T CACHE THIS!", but generally cgi scripts aren't cached.
print "HTTP/1.0 200 OK\n";
print "Expires: 23-Sep-94 18:28:33 GMT\n";
print "Last-Modified: 23-Sep-94 18:28:33 GMT\n";
print "Pragma: no-cache\n";

# And this sends the gif file:
open(IMAGE, $adbanner);
print "Content-type: image/gif\n\n";
$n = 8192; # read buffer size - tune for your system
while ( ($nread = read(IMAGE,$buf,$n)) > 0 ) {
print $buf;
close (IMAGE);


That definitely defeats IE's cache. You can also use the script to record
stats, or cookies, etc.

Bob Stewart

The Virtual Mirror -

138 Montgomery St., #1M
Highland Park, NJ 08904


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