The Filter File

Privoxy 3.0.8 User Manual9. The Filter File9. Filter Files

All text substitutions that can be invoked through the +> On-the-fly text substitutions need + to be defined in a "filter file". Once defined, they + can then be invoked as an "action".

Privoxy supports three different filter actions: filter action - must first be defined in the filter file, which is typically - called default.filter and which can be - selected through the to + rewrite the content that is send to the client, + client-header-filter + to rewrite headers that are send by the client, and + server-header-filter + to rewrite headers that are send by the server.

Privoxy also supports two tagger actions: + client-header-tagger + and + server-header-tagger. + Taggers and filters use the same syntax in the filter files, the difference + is that taggers don't modify the text they are filtering, but use a rewritten + version of the filtered text as tag. The tags can then be used to change the + applying actions through sections with tag-patterns.

Multiple filter files can be defined through the filterfile config - option.

config directive. The filters + as supplied by the developers are located in + default.filter. It is recommended that any locally + defined or modified filters go in a separately defined file such as + user.filter. + Typical reasons for doing such substitutions are to eliminate - common annoyances in HTML and JavaScript, such as pop-up windows, +> Common tasks for content filters are to eliminate common annoyances in + HTML and JavaScript, such as pop-up windows, exit consoles, crippled windows without navigation tools, the infamous <BLINK> tag etc, to suppress images with certain width and height attributes (standard banner sizes or web-bugs), - or just to have fun. The possibilities are endless. Filtering works on any text-based document type, including plain - text, HTML, JavaScript, CSS etc. (all Enabled content filters are applied to any content whose + "Content Type" header is recognised as a sign + of text-based content, with the exception of text/* - MIME types). Substitutions are made at the source level, so if - you want to text/plain. + Use the force-text-mode action + to also filter other content. Substitutions are made at the source level, so if you want to "roll your own" filters, you should be - familiar with HTML syntax."roll + your own" filters, you should first be familiar with HTML syntax, + and, of course, regular expressions. Just like the actions files, the - filter file is organized in sections, which are called filters - here. Each filter consists of a heading line, that starts with the - keywordkeywords FILTER:, followed by - the filter's , + CLIENT-HEADER-FILTER: or SERVER-HEADER-FILTER: + followed by the filter's name, and a short (one line) - description of what it does. Below that line - come the jobs, i.e. lines that define the actual text substitutions. By convention, the name of a filter - should describe what the filter eliminates. The comment is used in the actions file.

A filter header line for a filter called Filter definitions start with a header line that contains the filter + type, the filter name and the filter description. + A content filter header line for a filter called "foo" could look @@ -211,31 +316,35 @@ CLASS="LITERAL" >s/// operator. If you are familiar with Perl, you will find this to be quite intuitive, and may want to look at the - PCRS man page - for the subtle differences to Perl behaviour. Most notably, the non-standard - option letter U is supported, which turns the default - to ungreedy matching.

is supported, + which turns the default to ungreedy matching.
If you are new to regular expressions, you might want to take a look at +> If you are new to + "Regular + Expressions", you might want to take a look at the Appendix on regular expressions, and see the Perl manual for the s/// operator's syntax and Perl-style regular expressions
9.1. Filter File Tutorial
Now, let's complete our "foo" filter. We have already defined +> content filter. We have already defined the heading, but the jobs are still missing. Since all it does is to replace

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But wait! Didn't the comment say that But wait! Didn't the comment say that all occurrences of "<script", followed by , followed by any text, i.e. it matches the whole page, from the start of the first <script> tag.

"document.referrer". The dot needed to - be escaped, i.e. preceded by a backslash, to take away its special meaning as a joker, and make it just a regular dot. So far, the meaning is: Match from the start of the first <script> tag in a the page, up to, and including, the text "document.referrer", if , if both are present in the page (and appear in that order).

"<script" and the and the first occurrence of .* will - only span the text up to the first "</script>""Not Your Business!" ( (including the quotation marks!), followed by the text remembered as # The status bar is for displaying link targets, not pointless blahblah # -s/window\.status\s*=\s*['"].*?['"]/dUmMy=1/ig construct means: "a single - or a double quote".

. Finally, \1 is + a back-reference to the first parenthesis just like $1 above, + with the difference that in the pattern, a backslash indicates + a back-reference, whereas in the substitute, it's the dollar.So what does this job do? It replaces assignments of single- or double-quoted strings to the # Kill OnUnload popups. Yummy. Test: http://www.zdnet.com/zdsubs/yahoo/tree/yfs.html # -s/(<body .*)onunload(.*>)/$1never$2/iUOnUnload event binding in the HTML DOM was a in the HTML DOM was a CRIME. When I close a browser window, I want it to close and die. Basta. This job replaces the i option makes the pattern matching - case-insensitive.
[^>]* + instead of .* to prevent the match from exceeding the + <body> tag if it doesn't contain "OnUnload", but the page's + content does.
The last example is from the fun department:
"microsoft" - in the page. This prevents links to microsoft.com from being messed, while + in the page. This prevents links to microsoft.com from being trashed, while still replacing the word everywhere else.
x option in this job turns on extended syntax, and allows for - e.g. the liberal use of (non-interpreted!) whitespace for nicer formatting.
You get the idea?
9.2. The Pre-defined Filters
The distribution default.filter file contains a selection of +pre-defined filters for your convenience:
js-annoyances
The purpose of this filter is to get rid of particularly annoying JavaScript abuse. + To that end, it +
replaces JavaScript references to the browser's referrer information + with the string "Not Your Business!". This compliments the hide-referrer action on the content level. +
removes the bindings to the DOM's + unload + event which we feel has no right to exist and is responsible for most "exit consoles", i.e. + nasty windows that pop up when you close another one. +
removes code that causes new windows to be opened with undesired properties, such as being + full-screen, non-resizeable, without location, status or menu bar etc. +
+
Use with caution. This is an aggressive filter, and can break sites that + rely heavily on JavaScript. +
js-events
This is a very radical measure. It removes virtually all JavaScript event bindings, which + means that scripts can not react to user actions such as mouse movements or clicks, window + resizing etc, anymore. Use with caution! +
We strongly discourage using this filter as a default since it breaks + many legitimate scripts. It is meant for use only on extra-nasty sites (should you really + need to go there). +
html-annoyances
This filter will undo many common instances of HTML based abuse. +
The BLINK and MARQUEE tags + are neutralized (yeah baby!), and browser windows will be created as + resizeable (as of course they should be!), and will have location, + scroll and menu bars -- even if specified otherwise. +
content-cookies
Most cookies are set in the HTTP dialog, where they can be intercepted + by the + crunch-incoming-cookies + and crunch-outgoing-cookies + actions. But web sites increasingly make use of HTML meta tags and JavaScript + to sneak cookies to the browser on the content level. +
This filter disables most HTML and JavaScript code that reads or sets + cookies. It cannot detect all clever uses of these types of code, so it + should not be relied on as an absolute fix. Use it wherever you would also + use the cookie crunch actions. +
refresh tags
Disable any refresh tags if the interval is greater than nine seconds (so + that redirections done via refresh tags are not destroyed). This is useful + for dial-on-demand setups, or for those who find this HTML feature + annoying. +
unsolicited-popups
This filter attempts to prevent only "unsolicited" pop-up + windows from opening, yet still allow pop-up windows that the user + has explicitly chosen to open. It was added in version 3.0.1, + as an improvement over earlier such filters. +
Technical note: The filter works by redefining the window.open JavaScript + function to a dummy function, PrivoxyWindowOpen(), + during the loading and rendering phase of each HTML page access, and + restoring the function afterward. +
This is recommended only for browsers that cannot perform this function + reliably themselves. And be aware that some sites require such windows + in order to function normally. Use with caution. +
all-popups
Attempt to prevent all pop-up windows from opening. + Note this should be used with even more discretion than the above, since + it is more likely to break some sites that require pop-ups for normal + usage. Use with caution. +
img-reorder
This is a helper filter that has no value if used alone. It makes the + banners-by-size and banners-by-link + (see below) filters more effective and should be enabled together with them. +
banners-by-size
This filter removes image tags purely based on what size they are. Fortunately + for us, many ads and banner images tend to conform to certain standardized + sizes, which makes this filter quite effective for ad stripping purposes. +
Occasionally this filter will cause false positives on images that are not ads, + but just happen to be of one of the standard banner sizes. +
Recommended only for those who require extreme ad blocking. The default + block rules should catch 95+% of all ads without this filter enabled. +
banners-by-link
This is an experimental filter that attempts to kill any banners if + their URLs seem to point to known or suspected click trackers. It is currently + not of much value and is not recommended for use by default. +
webbugs
Webbugs are small, invisible images (technically 1X1 GIF images), that + are used to track users across websites, and collect information on them. + As an HTML page is loaded by the browser, an embedded image tag causes the + browser to contact a third-party site, disclosing the tracking information + through the requested URL and/or cookies for that third-party domain, without + the user ever becoming aware of the interaction with the third-party site. + HTML-ized spam also uses a similar technique to verify email addresses. +
This filter removes the HTML code that loads such "webbugs". +
tiny-textforms
A rather special-purpose filter that can be used to enlarge textareas (those + multi-line text boxes in web forms) and turn off hard word wrap in them. + It was written for the sourceforge.net tracker system where such boxes are + a nuisance, but it can be handy on other sites, too. +
It is not recommended to use this filter as a default. +
jumping-windows
Many consider windows that move, or resize themselves to be abusive. This filter + neutralizes the related JavaScript code. Note that some sites might not display + or behave as intended when using this filter. Use with caution. +
frameset-borders
Some web designers seem to assume that everyone in the world will view their + web sites using the same browser brand and version, screen resolution etc, + because only that assumption could explain why they'd use static frame sizes, + yet prevent their frames from being resized by the user, should they be too + small to show their whole content. +
This filter removes the related HTML code. It should only be applied to sites + which need it. +
demoronizer
Many Microsoft products that generate HTML use non-standard extensions (read: + violations) of the ISO 8859-1 aka Latin-1 character set. This can cause those + HTML documents to display with errors on standard-compliant platforms. +
This filter translates the MS-only characters into Latin-1 equivalents. + It is not necessary when using MS products, and will cause corruption of + all documents that use 8-bit character sets other than Latin-1. It's mostly + worthwhile for Europeans on non-MS platforms, if weird garbage characters + sometimes appear on some pages, or user agents that don't correct for this on + the fly. + +
shockwave-flash
A filter for shockwave haters. As the name suggests, this filter strips code + out of web pages that is used to embed shockwave flash objects. +

quicktime-kioskmode
Change HTML code that embeds Quicktime objects so that kioskmode, which + prevents saving, is disabled. +
fun
Text replacements for subversive browsing fun. Make fun of your favorite + Monopolist or play buzzword bingo. +
crude-parental
A demonstration-only filter that shows how Privoxy + can be used to delete web content on a keyword basis. +
ie-exploits
An experimental collection of text replacements to disable malicious HTML and JavaScript + code that exploits known security holes in Internet Explorer. +
Presently, it only protects against Nimda and a cross-site scripting bug, and + would need active maintenance to provide more substantial protection. +
site-specifics
Some web sites have very specific problems, the cure for which doesn't apply + anywhere else, or could even cause damage on other sites. +
This is a collection of such site-specific cures which should only be applied + to the sites they were intended for, which is what the supplied + default.action file does. Users shouldn't need to change + anything regarding this filter. +
google
A CSS based block for Google text ads. Also removes a width limitation + and the toolbar advertisement. +
yahoo
Another CSS based block, this time for Yahoo text ads. And removes + a width limitation as well. +
msn
Another CSS based block, this time for MSN text ads. And removes + tracking URLs, as well as a width limitation. +
blogspot
Cleans up some Blogspot blogs. Read the fine print before using this one! +
This filter also intentionally removes some navigation stuff and sets the + page width to 100%. As a result, some rounded "corners" would + appear to early or not at all and as fixing this would require a browser + that understands background-size (CSS3), they are removed instead. +
xml-to-html
Server-header filter to change the Content-Type from xml to html. +
html-to-xml
Server-header filter to change the Content-Type from html to xml. +
no-ping
Removes the non-standard ping attribute from + anchor and area HTML tags. +
hide-tor-exit-notation
Client-header filter to remove the Tor exit node notation + found in Host and Referer headers. +
If Privoxy and Tor are chained and Privoxy + is configured to use socks4a, one can use "http://www.example.org.foobar.exit/" + to access the host "www.example.org" through the + Tor exit node "foobar". +
As the HTTP client isn't aware of this notation, it treats the + whole string "www.example.org.foobar.exit" as host and uses it + for the "Host" and "Referer" headers. From the + server's point of view the resulting headers are invalid and can cause problems. +
An invalid "Referer" header can trigger "hot-linking" + protections, an invalid "Host" header will make it impossible for + the server to find the right vhost (several domains hosted on the same IP address). +
This client-header filter removes the "foo.exit" part in those headers + to prevent the mentioned problems. Note that it only modifies + the HTTP headers, it doesn't make it impossible for the server + to detect your Tor exit node based on the IP address + the request is coming from. +
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