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= = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = configset listname attachment_filters <<TAG [VALUE LINES] TAG - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - Default Value : no default Data Type : attachment_filters Category : deliver Password Notes: Visible only with password. = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = EXAMPLE: configset listname attachment_filters << ENDTAG text/plain | allow text/html | clean /.+/ | discard ENDTAG The attachment_filters setting contains a table of rules, which describe how to process the different body parts of a posted message. A rule consists of a MIME type (or pattern matching a MIME type) and an action to perform when a message with this type or containing a part of this type is delivered to the subscribers of a mailing list. Each rule looks something like this: mime/type | action=argument Some common MIME types include text/plain, text/html, and image/jpeg. Possible actions are: allow - let the body part pass untouched. clean - remove portions of a HTML part that could pose a security hazard. discard - remove the part from the message and pass the rest. (This has no effect upon single-part messages.) format - convert the body part to plain text. A numerical argument will adjust the right margin of the text, which occurs at position 72 by default. Each message that is posted to a mailing list consists of one or more body parts. Each body part has a particular type, sometimes called a "MIME type," which gives some indication about the contents of the part. For example, a body part containing a WWW document might have a "text/html" MIME type. (MIME is an abbreviation for the name of the Internet standard for giving names to types) Most plain text messages consist of one body part whose type is "text/plain". When a message has more than one body part, the second and succeeding parts are usually called "attachments." The "attachment_filters" setting is somewhat misnamed, because the allow and format actions apply to every body part, including the first part. Each attachment filter occupies one line. It is possible to place comments before, between, and after the individual filters, to indicate to other administrators what each filter is intended to accomplish. Comments are lines that begin with a '#'. The first rule that matches a particular body part takes precedence. For example, in the following two filters: text/html | allow /^text/i | discard any "text/html" body part will not be changed; any other kind of text body part, for instance "text/plain" or "text/enriched," will be discarded. Each part of a message body is examined separately. If no rule matches, the part will not be altered. The type on the left-hand side of an access rule can be an exact MIME type, or it can be a pattern that potentially matches many different MIME types (see "help patterns" for an explanation of how to write patterns). For example, to discard all GIF images, use the following filter: image/gif | discard and to discard all images of any type, use this filter: /^image/i | discard Exact MIME type matches are case-insensitive; for instance, "image/jpeg" and "Image/JPEG" are considered identical. The allow action ---------------- The allow action causes a body part to be passed untouched. The clean action ---------------- The clean action will remove unwanted elements ("tags") and their attributes from text/html body parts, to reduce the likelihood that malicious code appears in the message. At present, the following elements and attributes will be removed by this action: attributes: background onblur onchange onclick ondblclick onfocus onkeydown onkeypress onkeyup onload onmousedown onmousemove onmouseout onmouseover onmouseup onreset onselect onunload elements: applet base embed form frame iframe ilayer img input layer link meta object option script select textarea These attributes and elements are not configurable. The discard action ------------------ The discard action causes body parts to be discarded entirely, from every message posted to a mailing list. For example, many messages have "multipart/alternative" bodies, with a "text/plain" part and a "text/html" part. Because "text/html" parts can contain a malicious computer virus, list owners may want to use the following attachment rule to remove them: text/html | discard The discard action only applies to messages with more than one body part. As a result, if someone posts a message that consists only of a "text/html" body part, the discard rule in the previous example will have no effect. In the following example, both filters will have an effect, even though they both match the "text/html" MIME type: text/html | discard text/html | format The first rule will cause text/html parts in multi-part messages to be removed. The second rule will cause text/html parts in single-part messages to have their HTML formatting removed. For each body part removed due to a discard action, an X-Content-Discarded header will be added to the message when it is delivered. The discard action will not remove affect multipart subparts directly. Instead, the filters will be applied to each body part that is not a multipart subpart. For example, the following rule: # By design, this rule has no effect. multipart/alternative | discard will not work because a multipart/alternative body part cannot be discarded. The discard action should be used with caution, because it is possible to discard every individual part of a multipart message. The format action ----------------- The format action alters message contents in two ways. It removes hypertext markup tags from "text/html" or "text/enriched" body parts, and converts them into "text/plain" body parts. It can also establish the position of the width of the resulting text; by default, the width of the text is 72 characters. For example, to turn all rich text body parts into plain text, use the following rule: text/enriched | format To turn all HTML body parts into text with an 80 character width, use this rule: text/html | format=80 For each body part changed due to a format action, an X-Content-Reformatted header will be added to the message when it is delivered. The reformatted text will be left-justified, regardless of its original placement. Effect upon the archives ------------------------ The attachment_filters setting also filters the copy of each new message that is stored in the list's archive. Otherwise, recipients of each message and recipients of MIME digests would receive different content. Avoiding the filters -------------------- If a moderated message is approved by the moderators with the "accept-intact" command, or if a message is posted with the "post-intact" command, none of the filters in this setting will be applied when the message is delivered. Example - Remove all attachments -------------------------------- The following filters will remove all attachments that are not plain text, and convert HTML and enriched-text single-part messages into plain text. text/plain | allow ALL | discard text/html | format text/enriched | format The second filter will discard all parts that are not plain text from multipart messages. The second filter will take precedence for all message parts that are not plain text. However, the "discard" action is ignored for single-part messages, so the third and fourth filters will affect single-part messages that contain HTML or enriched text. This example has two potential side effects. If a single-part message is posted that is not text/plain, text/html, or text/enriched, it will not be affected by the filters. Also, if a multipart message is posted that does not have a text/plain part, all of its parts will be discarded, leaving the message blank. Example - Remove all non-text attachments ----------------------------------------- The following filters will remove all attachments that are not plain text, HTML, or enriched-text, and convert HTML and enriched-text parts into plain text. text/plain | allow text/html | format text/enriched | format ALL | discard This example has two potential side effects. If a single-part message is posted that is not text/plain, text/html, or text/enriched, it will not be affected by the filters. Also, if a multipart message is posted that has more than one text part, it may continue to have more than one text part after the message has been filtered. The best way to cope with messages that might trigger one of these side effects is to use the attachment_rules setting to restrict access for messages that do not contain a (plain) text part. See "help configset_attachment_rules" for more details. Unimplemented features ---------------------- The "attachment_filters" setting does not alter portions of text messages that contain non-MIME attachments, such as uuencoded documents. See Also: help accept help admin_delivery help configset_access_rules help configset_admin_body help configset_attachment_rules help configset_taboo_body help post This is the "configset_attachment_filters" help document for Majordomo 2, version 0.1200401130. For a list of all help documents, send the following command: help topics in the body of a message to firstname.lastname@example.org.
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