HTTP Headers for ZIP File Downloads

You know when you you’re working on a project and get stuck on something, so you scour the Web for solutions only to find that everyone else seems to be experiencing the exact same thing. Then, after many hours trying everything possible, you finally stumble onto something that seems to work. This time, the project was setting up a secure downloads area for Digging into WordPress. And when I finally discovered a solution, I told myself that it was definitely something I had to share here at Perishable Press.

Apparently, there is much to be desired when it comes to sending proper HTTP headers for file downloads. Different browsers (and not just IE) require different headers, and will error if not present exactly the way they expected. Confounding that equation is the fact that different file types also require specific headers. Then there are issues with sending an accurate (or should I say “acceptable”?) Content-Length headers when file compression is involved. Needless to say, finding a set of headers that works for all file types in all browsers is next to impossible. And I won’t even get into the issues involved with readfile() and large-download file-sizes.

Download Headers that actually work

After trying hundreds of different headers and combinations, I hit upon a set that works great for ZIP downloads (and other file types as well) in all tested browsers. Here’s what they look like using PHP:

<?php // HTTP Headers for ZIP File Downloads

// set example variables
$filename = "";
$filepath = "/var/www/domain/httpdocs/download/path/";

// http headers for zip downloads
header("Pragma: public");
header("Expires: 0");
header("Cache-Control: must-revalidate, post-check=0, pre-check=0");
header("Cache-Control: public");
header("Content-Description: File Transfer");
header("Content-type: application/octet-stream");
header("Content-Disposition: attachment; filename=\"".$filename."\"");
header("Content-Transfer-Encoding: binary");
header("Content-Length: ".filesize($filepath.$filename));

This PHP script is known to work under the following conditions:

  • Operating System: Linux
  • Server: Apache/2.2.3 (CentOS)
  • MYSQL Version: 5.0.77-log
  • PHP Version: 5.2.6
  • PHP Safe Mode: Off
  • PHP Allow URL fopen: On
  • PHP Memory Limit: 256M
  • PHP Max Upload Size: 2M
  • PHP Max Post Size: 8M
  • PHP Max Script Execute Time: 30s

With this code, the downloads work in the following tested browsers:

  • Firefox 3.0, 3.5 (Mac & PC)
  • Opera 8, 9, 10 (Mac & PC)
  • Internet Explorer 7, 8
  • Chrome 7.0.517
  • Camino 2
  • Safari 5 (PC)
  • Safari 3 (Mac)

The downloads work for the following types of files (including small and large file types):

  • .zip
  • .txt
  • .pdf
  • .jpg

Obviously, I didn’t test every file type in every browser, but the positive results from those listed here suggest a much wider range of files and browsers that will work. For the file sizes, I tested small files only a few bytes in length, and also large files up to around 20MB or so. Also would like to give a shout to the Live HTTP Headers extension for Firefox. It proved indispensable throughout the troubleshooting/testing/pulling-my-hair-out process.

As always, if you can contribute to the content of this post with further information about sending proper HTTP Headers for file downloads, you may just save a life ;)