Uploads Administration

deutsche Übersetzung 21.06.2006 (teilweise, bitte vervollständigen)

PmWiki enthält ein Script upload.php, welches dem Benutzer erlaubt, Dateien mit Hilfe eine Webbrowsers auf den Wiki-Server hochzuladen?. Hochgeladene Dateien (auch attachments genannt) können leicht mittels markup auf den Wikiseiten angesprochen werden. Diese Seite beschreibt, wie das Upload feature installiert und konifuriert wird.

Some notes about security

PmWiki takes a somewhat paranoid stance when it comes to the uploads feature. Thus, the default settings for uploads tend to try to restrict the feature as much as possible:

• The upload function is disabled by default
• Even if you enable it, the function is password locked by default
• Even if you remove the password, you're restricted to uploading files with certain names and extensions
• The characters that may appear in upload filenames are (default) alphanumerics, hyphen, underscore, dot, and space.
• The maximum upload size is small (50K by default)

This way the potential damage is limited until/unless the wiki administrator explicitly relaxes the restrictions.

Keep in mind that letting users (anonymously!) upload files to your web server does entail some amount of risk. The upload.php script has been designed to reduce the hazards, but wiki administrators should be aware that the potential for vulnerabilities exist, and that misconfiguration of the upload utility could lead to unwanted consequences.

By default, authorized users are able to overwrite files that have already been uploaded, without the possibility of restoring the previous version of the file. If you want to disallow users from being able to overwrite files that have already been uploaded, add the following line to config.php:

Alternatively, an administrator can keep older versions of uploads.

An administrator can also configure PmWiki so the password mechanism controls access to uploaded files.

Grundinstallation

Das upload.php Script wird automatisch in stdconfig.php einbezogen, wenn die $EnableUpload Variable in config.php auf wahr (true) gesetzt ist. Zusätzlich können in config.php die $UploadDir und $UploadUrlFmt Variablen gesetzt werden, um das lokale Verzeichnis, in dem die hochgeladenen Dateien gespeichert werden sollen, und die URL die für den Zugriff benutzt werden können, anzugeben. Per default sind $UploadDir und $UploadUrlFmt so eingestellt, das Uploads in einem Verzeichnis uploads/ innerhalb des aktuellen Verzeichnisses gespeichert werden (gewöhnlich das Verzeichnis, das pmwiki.php enthält). Zusätzlich sollte in config.php ein voreingestelltes Uploadpasswort eingestellt werden (siehe PasswordsAdmin).

Demnach sieht eine Grundeinstellung in config.php für Uploads wie folgt aus:

<?php if (!defined('PmWiki')) exit();
##  Enable uploads and set a site-wide default upload password.
$EnableUpload = 1;
$DefaultPasswords['upload'] = crypt('secret');

Important: do NOT create the uploads directory yet! See the next paragraph.

You may also need to explicitly set which filesystem directory will hold uploads and provide a URL that corresponds to that directory like:

$UploadDir = "/home/john/public_html/uploads";
$UploadUrlFmt = "http://www.john.com/~john/uploads";

Uploads can be configured site-wide, by-group, or by-page by changing $UploadPrefixFmt. This determines whether all uploads go in one directory for the site, an individual directory for each group, or an individual directory for each page. The default is to organize upload by group.

For site-wide uploads, use

To organize uploads by page, use either of these:

$UploadPrefixFmt = '/$FullName';
$UploadPrefixFmt = '/$Group/$Name';

The upload directory

For the upload feature to work properly, the directory given by $UploadDir must be writable by the web server process, and it usually must be in a location that is accessible to the web somewhere (e.g., in a subdirectory of public_html). Executing PmWiki with uploads enabled will prompt you with the set of steps required to create the uploads directory on your server (it differs from one server to the next). Note that you are likely to be required to explicitly create writable group- or page-specific subdirectories as well!

Uploading a file

Once the upload feature is enabled, users can access the upload form by adding "?action=upload" to the end of a normal PmWiki URL. The user will be prompted for an upload password similar to the way other pages ask for passwords (see Passwords and PasswordsAdmin for information about setting passwords on pages, groups, and the entire site).

Another way to access the upload form to insert the markup "Attach:filename.ext" into an existing page, where filename.ext is the name of a new file to be uploaded. When the page is displayed, a '?-link' will be added to the end of the markup to take the author to the upload page. (See Uploads for syntax variations.)

By default, PmWiki will organize the uploaded files into separate subdirectories for each group. This can be changed by modifying the $UploadPrefixFmt variable. See Cookbook:UploadGroups for details.

Versioning Uploaded Files

PmWiki does not manage versioning of uploaded files by default. However, by setting $EnableUploadVersions=1; an administrator can have older versions of uploads preserved in the uploads directory along with the most recent version.

Restricting uploaded files for groups and pages

Uploads can be enabled only for specific groups or pages by using a per group customization?. Simply set $EnableUpload=1; for those groups or pages where uploading is to be enabled; alternately, set $EnableUpload=1; in the config.php file and then set $EnableUpload=0; in the per-group or per-page customization files where uploads are to be disabled.

Restricting uploaded files type and size

The upload script performs a number of verifications on an uploaded file before storing it in the upload directory. The basic verifications are described below.

filenames

the name for the uploaded file can contain only letters, digits, underscores, hyphens, spaces, and periods, and the name must begin and end with a letter or digit.

file extension

only files with approved extensions such as ".gif", ".jpeg", ".doc", etc. are allowed to be uploaded to the web server. This is vitally important for server security, since the web server might attempt to execute or specially process files with extensions like ".php", ".cgi", etc.

file size

By default all uploads are limited to 50K bytes, as specified by the $UploadMaxSize variable. Thus, to limit all uploads to 100K, simply specify a new value for $UploadMaxSize in config.php:

However, maximum file sizes can also be specified for each type of file uploaded. Thus, an administrator can restrict ".gif" and ".jpeg" files to 20K, ".doc" files to 200K, and all others to the size given by $UploadMaxSize. The $UploadExtSize array is used to determine which file extensions are valid and the maximum upload size (in bytes) for each file type. For example:

Setting an entry to zero disables file uploads of that type altogether:

Adding new file types to permitted uploads

To add a new extension to the list of allowed upload types, add a line like the following to a local customization file:

where ext is the extension to be added, and content-type is the content-type (MIME type) to be used for files with that extension. For example, to add the 'dxf' extension with a Content-Type of 'image/x-dxf', place the line

Each entry in $UploadExts needs to be the extension and the mime-type associated with that extension, thus:

$UploadExts = array(
  'gif' => 'image/gif',
  'jpeg' => 'image/jpeg',
  'jpg' => 'image/jpeg',
  'png' => 'image/png',
  'xxx' => 'yyyy/zzz'
)

For the types that PmWiki already knows about it's not necessary to repeat them here (the upload.php script adds PmWiki's defaults to whatever the administrator supplies).

Other file size limits

There are other factors involved that affect upload file sizes. In Apache 2.0, there is a LimitRequestBody directive that controls the maximum size of anything that is posted (including file uploads). Apache has this defaulted to unlimited size. However, some Linux distributions (e.g., Red Hat Linux) limit postings to 512K so this may need to be changed or increased. (Normally these settings are in an httpd.conf configuration file or in a file in /etc/httpd/conf.d.)

Problem noted on Red Hat 8.0/9.0 with Apache 2.0.x, the error "Requested content-length of 670955 is larger than the configured limit of 524288" was occurring under Apache and a "Page not found" would appear in the browser. Trying the above settings made no change with PHP, but on Red Hat 8.0/9.0 there is an additional PHP config file, /etc/httpd/conf.d/php.conf, and increasing the number on the line "LimitRequestBody 524288" solves the issue.

PHP itself has two limits on file uploads (usually located in /etc/php.ini). The first is the upload_max_filesize parameter, which is set to 2M by default. The second is post_max_size, which is set to 6M by default.

With the variables in place--PmWiki's maximum file size, Apache's request-size limits, and the PHP file size parameters, the maximum uploaded file size will be the smallest of the three variables.

Password protecting uploaded files

Setting a read password for pages (and groups) will prevent an attached file from being seen or accessed through the page, but to prevent direct access to the file location (the uploads/ directory) one can do the following:

• In local/config.php set $EnableDirectDownload=0;
• Deny public access to the uploads/ directory through moving it out of the html/ or public_html/ directory tree, or through a .htaccess file.

See Cookbook:SecureAttachments.

Other notes

• If uploads doesn't seem to work, make sure that your PHP installation allows uploads. The php.ini file (usually /etc/php.ini or /usr/local/lib/php.ini) should have

Note that if you change this value, httpd must generally be restarted. Another way to check if uploads are allowed by the server is to set $EnableDiag to 1 in config.php, and set ?action=phpinfo on a URL. The "file_uploads" variable must have a value of 1 (if it says "no value", that means it's off).

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