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15.9. Serving Images Securely

15.9.3. Discussion

The first line in the Solution sends the Content-type header to the browser, so the browser knows what type of object is coming and displays it accordingly. The second opens a file off a disk (or from a remote URL) for reading, reads it in, dumps it directly to the browser, and closes the file.

The typical way to serve up an image is to use an <img> tag and set the src attribute to point to a file on your web site. If you want to protect those images, you probably should use some form of password authentication. One method is HTTP Basic Authentication, which is covered in Recipe 8.10.

The typical way, however, may not always be the best. First, what happens if you want to restrict the files people can view, but you don't want to make things complex by using usernames and passwords? One option is to link only to the files; if users can't click on the link, they can't view the file. They might, however, bookmark old files, or they may also try and guess other filenames based on your naming scheme and manually enter the URL into the browser.

If your content is embargoed, you don't want people to be able to guess your naming scheme and view images. When information is embargoed, a select group of people, usually reporters, are given a preview release, so they can write stories about the topic or be ready to distribute it the moment the embargo is lifted. You can fix this by making sure only legal content is under the document root, but this requires a lot of file shuffling back and forth from directory to directory. Instead, you can keep all the files in one constant place, and deliver only files that pass a check inside your code.

For example, let's say you have a contract with a publishing corporation to redistribute one of their comics on your web site. However, they don't want you to create a virtual archive, so you agree to let your users view only the last two weeks worth of strips. For everything else, they'll need to go to the official site. Also, you may get comics in advance of their publication date, but you don't want to let people get a free preview; you want them to keep coming back to your site on a daily basis.

Here's the solution. Files arrive named by date, so it's easy to identify which files belong to which day. Now, to lock out strips outside the rolling 14-day window, use code like this:

// display a comic if it's less than 14 days old and not in the future

// calculate the current date
list($now_m,$now_d,$now_y) = explode(',',date('m,d,Y'));
$now = mktime(0,0,0,$now_m,$now_d,$now_y);

// two hour boundary on either side to account for dst
$min_ok = $now - 14*86400 - 7200; // 14 days ago
$max_ok = $now + 7200;            // today

// find the time stamp of the requested comic
$asked_for = mktime(0,0,0,$_REQUEST['mo'],$_REQUEST['dy'],$_REQUEST['yr']);

// compare the dates
if (($min_ok > $asked_for) || ($max_ok < $asked_for)) {
    echo 'You are not allowed to view the comic for that day.';
} else {
    header('Content-type: image/png');
    readfile("/www/comics/$_REQUEST['mo']$_REQUEST['dy'] $_REQUEST['yr'].png");
}

15.9.4. See Also

Recipe 18.6 for more on reading files.



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