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40 <h1 class="SECT1"><a name="FILTER-FILE" id="FILTER-FILE">9. Filter
43 <p>On-the-fly text substitutions need to be defined in a <span class=
44 "QUOTE">"filter file"</span>. Once defined, they can then be invoked as
45 an <span class="QUOTE">"action"</span>.</p>
47 <p><span class="APPLICATION">Privoxy</span> supports three different
48 pcrs-based filter actions: <tt class="LITERAL"><a href=
49 "actions-file.html#FILTER">filter</a></tt> to rewrite the content that is
50 send to the client, <tt class="LITERAL"><a href=
51 "actions-file.html#CLIENT-HEADER-FILTER">client-header-filter</a></tt> to
52 rewrite headers that are send by the client, and <tt class=
54 "actions-file.html#SERVER-HEADER-FILTER">server-header-filter</a></tt> to
55 rewrite headers that are send by the server.</p>
57 <p><span class="APPLICATION">Privoxy</span> also supports two tagger
58 actions: <tt class="LITERAL"><a href=
59 "actions-file.html#CLIENT-HEADER-TAGGER">client-header-tagger</a></tt>
60 and <tt class="LITERAL"><a href=
61 "actions-file.html#SERVER-HEADER-TAGGER">server-header-tagger</a></tt>.
62 Taggers and filters use the same syntax in the filter files, the
63 difference is that taggers don't modify the text they are filtering, but
64 use a rewritten version of the filtered text as tag. The tags can then be
65 used to change the applying actions through sections with <a href=
66 "actions-file.html#TAG-PATTERN">tag-patterns</a>.</p>
68 <p>Finally <span class="APPLICATION">Privoxy</span> supports the
69 <tt class="LITERAL"><a href=
70 "actions-file.html#EXTERNAL-FILTER">external-filter</a></tt> action to
71 enable <tt class="LITERAL"><a href=
72 "filter-file.html#EXTERNAL-FILTER-SYNTAX">external filters</a></tt>
73 written in proper programming languages.</p>
75 <p>Multiple filter files can be defined through the <tt class=
76 "LITERAL"><a href="config.html#FILTERFILE">filterfile</a></tt> config
77 directive. The filters as supplied by the developers are located in
78 <tt class="FILENAME">default.filter</tt>. It is recommended that any
79 locally defined or modified filters go in a separately defined file such
80 as <tt class="FILENAME">user.filter</tt>.</p>
82 <p>Common tasks for content filters are to eliminate common annoyances in
83 HTML and JavaScript, such as pop-up windows, exit consoles, crippled
84 windows without navigation tools, the infamous <BLINK> tag etc, to
85 suppress images with certain width and height attributes (standard banner
86 sizes or web-bugs), or just to have fun.</p>
88 <p>Enabled content filters are applied to any content whose <span class=
89 "QUOTE">"Content Type"</span> header is recognised as a sign of
90 text-based content, with the exception of <tt class=
91 "LITERAL">text/plain</tt>. Use the <a href=
92 "actions-file.html#FORCE-TEXT-MODE">force-text-mode</a> action to also
93 filter other content.</p>
95 <p>Substitutions are made at the source level, so if you want to
96 <span class="QUOTE">"roll your own"</span> filters, you should first be
97 familiar with HTML syntax, and, of course, regular expressions.</p>
99 <p>Just like the <a href="actions-file.html">actions files</a>, the
100 filter file is organized in sections, which are called <span class=
101 "emphasis"><i class="EMPHASIS">filters</i></span> here. Each filter
102 consists of a heading line, that starts with one of the <span class=
103 "emphasis"><i class="EMPHASIS">keywords</i></span> <tt class=
104 "LITERAL">FILTER:</tt>, <tt class="LITERAL">CLIENT-HEADER-FILTER:</tt> or
105 <tt class="LITERAL">SERVER-HEADER-FILTER:</tt> followed by the filter's
106 <span class="emphasis"><i class="EMPHASIS">name</i></span>, and a short
107 (one line) <span class="emphasis"><i class=
108 "EMPHASIS">description</i></span> of what it does. Below that line come
109 the <span class="emphasis"><i class="EMPHASIS">jobs</i></span>, i.e.
110 lines that define the actual text substitutions. By convention, the name
111 of a filter should describe what the filter <span class=
112 "emphasis"><i class="EMPHASIS">eliminates</i></span>. The comment is used
113 in the <a href="http://config.privoxy.org/" target="_top">web-based user
116 <p>Once a filter called <tt class="REPLACEABLE"><i>name</i></tt> has been
117 defined in the filter file, it can be invoked by using an action of the
118 form +<tt class="LITERAL"><a href=
119 "actions-file.html#FILTER">filter</a>{<tt class=
120 "REPLACEABLE"><i>name</i></tt>}</tt> in any <a href=
121 "actions-file.html">actions file</a>.</p>
123 <p>Filter definitions start with a header line that contains the filter
124 type, the filter name and the filter description. A content filter header
125 line for a filter called <span class="QUOTE">"foo"</span> could look like
128 <table border="0" bgcolor="#E0E0E0" width="100%">
132 FILTER: foo Replace all "foo" with "bar"
138 <p>Below that line, and up to the next header line, come the jobs that
139 define what text replacements the filter executes. They are specified in
140 a syntax that imitates <a href="http://www.perl.org/" target=
141 "_top">Perl</a>'s <tt class="LITERAL">s///</tt> operator. If you are
142 familiar with Perl, you will find this to be quite intuitive, and may
143 want to look at the PCRS documentation for the subtle differences to Perl
146 <p>Most notably, the non-standard option letter <tt class=
147 "LITERAL">U</tt> is supported, which turns the default to ungreedy
148 matching (add <tt class="LITERAL">?</tt> to quantifiers to turn them
151 <p>The non-standard option letter <tt class="LITERAL">D</tt> (dynamic)
152 allows to use the variables $host, $origin (the IP address the request
153 came from), $path, $url and $listen-address (the address on which Privoxy
154 accepted the client request. Example: 127.0.0.1:8118). They will be
155 replaced with the value they refer to before the filter is executed.</p>
157 <p>Note that '$' is a bad choice for a delimiter in a dynamic filter as
158 you might end up with unintended variables if you use a variable name
159 directly after the delimiter. Variables will be resolved without escaping
160 anything, therefore you also have to be careful not to chose delimiters
161 that appear in the replacement text. For example '<' should be save,
162 while '?' will sooner or later cause conflicts with $url.</p>
164 <p>The non-standard option letter <tt class="LITERAL">T</tt> (trivial)
165 prevents parsing for backreferences in the substitute. Use it if you want
166 to include text like '$&' in your substitute without quoting.</p>
168 <p>If you are new to <a href=
169 "http://en.wikipedia.org/wiki/Regular_expressions" target=
170 "_top"><span class="QUOTE">"Regular Expressions"</span></a>, you might
171 want to take a look at the <a href="appendix.html#REGEX">Appendix on
172 regular expressions</a>, and see the <a href=
173 "http://perldoc.perl.org/perlre.html" target="_top">Perl manual</a> for
174 <a href="http://perldoc.perl.org/perlop.html" target="_top">the
175 <tt class="LITERAL">s///</tt> operator's syntax</a> and <a href=
176 "http://perldoc.perl.org/perlre.html" target="_top">Perl-style regular
177 expressions</a> in general. The below examples might also help to get you
181 <h2 class="SECT2"><a name="FILTER-FILE-TUT" id="FILTER-FILE-TUT">9.1.
182 Filter File Tutorial</a></h2>
184 <p>Now, let's complete our <span class="QUOTE">"foo"</span> content
185 filter. We have already defined the heading, but the jobs are still
186 missing. Since all it does is to replace <span class=
187 "QUOTE">"foo"</span> with <span class="QUOTE">"bar"</span>, there is
188 only one (trivial) job needed:</p>
190 <table border="0" bgcolor="#E0E0E0" width="100%">
200 <p>But wait! Didn't the comment say that <span class=
201 "emphasis"><i class="EMPHASIS">all</i></span> occurrences of
202 <span class="QUOTE">"foo"</span> should be replaced? Our current job
203 will only take care of the first <span class="QUOTE">"foo"</span> on
204 each page. For global substitution, we'll need to add the <tt class=
205 "LITERAL">g</tt> option:</p>
207 <table border="0" bgcolor="#E0E0E0" width="100%">
217 <p>Our complete filter now looks like this:</p>
219 <table border="0" bgcolor="#E0E0E0" width="100%">
223 FILTER: foo Replace all "foo" with "bar"
230 <p>Let's look at some real filters for more interesting examples. Here
231 you see a filter that protects against some common annoyances that
232 arise from JavaScript abuse. Let's look at its jobs one after the
235 <table border="0" bgcolor="#E0E0E0" width="100%">
239 FILTER: js-annoyances Get rid of particularly annoying JavaScript abuse
241 # Get rid of JavaScript referrer tracking. Test page: http://www.randomoddness.com/untitled.htm
243 s|(<script.*)document\.referrer(.*</script>)|$1"Not Your Business!"$2|Usg
249 <p>Following the header line and a comment, you see the job. Note that
250 it uses <tt class="LITERAL">|</tt> as the delimiter instead of
251 <tt class="LITERAL">/</tt>, because the pattern contains a forward
252 slash, which would otherwise have to be escaped by a backslash
253 (<tt class="LITERAL">\</tt>).</p>
255 <p>Now, let's examine the pattern: it starts with the text <tt class=
256 "LITERAL"><script.*</tt> enclosed in parentheses. Since the dot
257 matches any character, and <tt class="LITERAL">*</tt> means:
258 <span class="QUOTE">"Match an arbitrary number of the element left of
259 myself"</span>, this matches <span class="QUOTE">"<script"</span>,
260 followed by <span class="emphasis"><i class="EMPHASIS">any</i></span>
261 text, i.e. it matches the whole page, from the start of the first
262 <script> tag.</p>
264 <p>That's more than we want, but the pattern continues: <tt class=
265 "LITERAL">document\.referrer</tt> matches only the exact string
266 <span class="QUOTE">"document.referrer"</span>. The dot needed to be
267 <span class="emphasis"><i class="EMPHASIS">escaped</i></span>, i.e.
268 preceded by a backslash, to take away its special meaning as a joker,
269 and make it just a regular dot. So far, the meaning is: Match from the
270 start of the first <script> tag in a the page, up to, and
271 including, the text <span class="QUOTE">"document.referrer"</span>, if
272 <span class="emphasis"><i class="EMPHASIS">both</i></span> are present
273 in the page (and appear in that order).</p>
275 <p>But there's still more pattern to go. The next element, again
276 enclosed in parentheses, is <tt class="LITERAL">.*</script></tt>.
277 You already know what <tt class="LITERAL">.*</tt> means, so the whole
278 pattern translates to: Match from the start of the first <script>
279 tag in a page to the end of the last <script> tag, provided that
280 the text <span class="QUOTE">"document.referrer"</span> appears
281 somewhere in between.</p>
283 <p>This is still not the whole story, since we have ignored the options
284 and the parentheses: The portions of the page matched by sub-patterns
285 that are enclosed in parentheses, will be remembered and be available
286 through the variables <tt class="LITERAL">$1, $2, ...</tt> in the
287 substitute. The <tt class="LITERAL">U</tt> option switches to ungreedy
288 matching, which means that the first <tt class="LITERAL">.*</tt> in the
289 pattern will only <span class="QUOTE">"eat up"</span> all text in
290 between <span class="QUOTE">"<script"</span> and the <span class=
291 "emphasis"><i class="EMPHASIS">first</i></span> occurrence of
292 <span class="QUOTE">"document.referrer"</span>, and that the second
293 <tt class="LITERAL">.*</tt> will only span the text up to the
294 <span class="emphasis"><i class="EMPHASIS">first</i></span>
295 <span class="QUOTE">"</script>"</span> tag. Furthermore, the
296 <tt class="LITERAL">s</tt> option says that the match may span multiple
297 lines in the page, and the <tt class="LITERAL">g</tt> option again
298 means that the substitution is global.</p>
300 <p>So, to summarize, the pattern means: Match all scripts that contain
301 the text <span class="QUOTE">"document.referrer"</span>. Remember the
302 parts of the script from (and including) the start tag up to (and
303 excluding) the string <span class="QUOTE">"document.referrer"</span> as
304 <tt class="LITERAL">$1</tt>, and the part following that string, up to
305 and including the closing tag, as <tt class="LITERAL">$2</tt>.</p>
307 <p>Now the pattern is deciphered, but wasn't this about substituting
308 things? So lets look at the substitute: <tt class="LITERAL">$1"Not Your
309 Business!"$2</tt> is easy to read: The text remembered as <tt class=
310 "LITERAL">$1</tt>, followed by <tt class="LITERAL">"Not Your
311 Business!"</tt> (<span class="emphasis"><i class=
312 "EMPHASIS">including</i></span> the quotation marks!), followed by the
313 text remembered as <tt class="LITERAL">$2</tt>. This produces an exact
314 copy of the original string, with the middle part (the <span class=
315 "QUOTE">"document.referrer"</span>) replaced by <tt class=
316 "LITERAL">"Not Your Business!"</tt>.</p>
318 <p>The whole job now reads: Replace <span class=
319 "QUOTE">"document.referrer"</span> by <tt class="LITERAL">"Not Your
320 Business!"</tt> wherever it appears inside a <script> tag. Note
321 that this job won't break JavaScript syntax, since both the original
322 and the replacement are syntactically valid string objects. The script
323 just won't have access to the referrer information anymore.</p>
325 <p>We'll show you two other jobs from the JavaScript taming department,
326 but this time only point out the constructs of special interest:</p>
328 <table border="0" bgcolor="#E0E0E0" width="100%">
332 # The status bar is for displaying link targets, not pointless blahblah
334 s/window\.status\s*=\s*(['"]).*?\1/dUmMy=1/ig
340 <p><tt class="LITERAL">\s</tt> stands for whitespace characters (space,
341 tab, newline, carriage return, form feed), so that <tt class=
342 "LITERAL">\s*</tt> means: <span class="QUOTE">"zero or more
343 whitespace"</span>. The <tt class="LITERAL">?</tt> in <tt class=
344 "LITERAL">.*?</tt> makes this matching of arbitrary text ungreedy.
345 (Note that the <tt class="LITERAL">U</tt> option is not set). The
346 <tt class="LITERAL">['"]</tt> construct means: <span class="QUOTE">"a
347 single <span class="emphasis"><i class="EMPHASIS">or</i></span> a
348 double quote"</span>. Finally, <tt class="LITERAL">\1</tt> is a
349 back-reference to the first parenthesis just like <tt class=
350 "LITERAL">$1</tt> above, with the difference that in the <span class=
351 "emphasis"><i class="EMPHASIS">pattern</i></span>, a backslash
352 indicates a back-reference, whereas in the <span class=
353 "emphasis"><i class="EMPHASIS">substitute</i></span>, it's the
356 <p>So what does this job do? It replaces assignments of single- or
357 double-quoted strings to the <span class="QUOTE">"window.status"</span>
358 object with a dummy assignment (using a variable name that is hopefully
359 odd enough not to conflict with real variables in scripts). Thus, it
360 catches many cases where e.g. pointless descriptions are displayed in
361 the status bar instead of the link target when you move your mouse over
364 <table border="0" bgcolor="#E0E0E0" width="100%">
368 # Kill OnUnload popups. Yummy. Test: http://www.zdnet.com/zdsubs/yahoo/tree/yfs.html
370 s/(<body [^>]*)onunload(.*>)/$1never$2/iU
376 <p>Including the <a href=
377 "http://www.w3.org/TR/2000/REC-DOM-Level-2-Events-20001113/events.html#Events-eventgroupings-htmlevents"
378 target="_top">OnUnload event binding</a> in the HTML DOM was a
379 <span class="emphasis"><i class="EMPHASIS">CRIME</i></span>. When I
380 close a browser window, I want it to close and die. Basta. This job
381 replaces the <span class="QUOTE">"onunload"</span> attribute in
382 <span class="QUOTE">"<body>"</span> tags with the dummy word
383 <tt class="LITERAL">never</tt>. Note that the <tt class=
384 "LITERAL">i</tt> option makes the pattern matching case-insensitive.
385 Also note that ungreedy matching alone doesn't always guarantee a
386 minimal match: In the first parenthesis, we had to use <tt class=
387 "LITERAL">[^>]*</tt> instead of <tt class="LITERAL">.*</tt> to
388 prevent the match from exceeding the <body> tag if it doesn't
389 contain <span class="QUOTE">"OnUnload"</span>, but the page's content
392 <p>The last example is from the fun department:</p>
394 <table border="0" bgcolor="#E0E0E0" width="100%">
398 FILTER: fun Fun text replacements
400 # Spice the daily news:
402 s/microsoft(?!\.com)/MicroSuck/ig
408 <p>Note the <tt class="LITERAL">(?!\.com)</tt> part (a so-called
409 negative lookahead) in the job's pattern, which means: Don't match, if
410 the string <span class="QUOTE">".com"</span> appears directly following
411 <span class="QUOTE">"microsoft"</span> in the page. This prevents links
412 to microsoft.com from being trashed, while still replacing the word
415 <table border="0" bgcolor="#E0E0E0" width="100%">
419 # Buzzword Bingo (example for extended regex syntax)
421 s* industry[ -]leading \
423 | customer[ -]focused \
425 | award[ -]winning # Comments are OK, too! \
426 | high[ -]performance \
427 | solutions[ -]based \
431 *<font color="red"><b>BINGO!</b></font> \
438 <p>The <tt class="LITERAL">x</tt> option in this job turns on extended
439 syntax, and allows for e.g. the liberal use of (non-interpreted!)
440 whitespace for nicer formatting.</p>
442 <p>You get the idea?</p>
446 <h2 class="SECT2"><a name="PREDEFINED-FILTERS" id=
447 "PREDEFINED-FILTERS">9.2. The Pre-defined Filters</a></h2>
449 <p>The distribution <tt class="FILENAME">default.filter</tt> file
450 contains a selection of pre-defined filters for your convenience:</p>
452 <div class="VARIABLELIST">
454 <dt><span class="emphasis"><i class=
455 "EMPHASIS">js-annoyances</i></span></dt>
458 <p>The purpose of this filter is to get rid of particularly
459 annoying JavaScript abuse. To that end, it</p>
463 <p>replaces JavaScript references to the browser's referrer
464 information with the string "Not Your Business!". This
465 compliments the <tt class="LITERAL"><a href=
466 "actions-file.html#HIDE-REFERRER">hide-referrer</a></tt>
467 action on the content level.</p>
471 <p>removes the bindings to the DOM's <a href=
472 "http://www.w3.org/TR/2000/REC-DOM-Level-2-Events-20001113/events.html#Events-eventgroupings-htmlevents"
473 target="_top">unload event</a> which we feel has no right to
474 exist and is responsible for most <span class="QUOTE">"exit
475 consoles"</span>, i.e. nasty windows that pop up when you
476 close another one.</p>
480 <p>removes code that causes new windows to be opened with
481 undesired properties, such as being full-screen,
482 non-resizeable, without location, status or menu bar etc.</p>
486 <p>Use with caution. This is an aggressive filter, and can break
487 sites that rely heavily on JavaScript.</p>
490 <dt><span class="emphasis"><i class=
491 "EMPHASIS">js-events</i></span></dt>
494 <p>This is a very radical measure. It removes virtually all
495 JavaScript event bindings, which means that scripts can not react
496 to user actions such as mouse movements or clicks, window
497 resizing etc, anymore. Use with caution!</p>
499 <p>We <span class="emphasis"><i class="EMPHASIS">strongly
500 discourage</i></span> using this filter as a default since it
501 breaks many legitimate scripts. It is meant for use only on
502 extra-nasty sites (should you really need to go there).</p>
505 <dt><span class="emphasis"><i class=
506 "EMPHASIS">html-annoyances</i></span></dt>
509 <p>This filter will undo many common instances of HTML based
512 <p>The <tt class="LITERAL">BLINK</tt> and <tt class=
513 "LITERAL">MARQUEE</tt> tags are neutralized (yeah baby!), and
514 browser windows will be created as resizeable (as of course they
515 should be!), and will have location, scroll and menu bars -- even
516 if specified otherwise.</p>
519 <dt><span class="emphasis"><i class=
520 "EMPHASIS">content-cookies</i></span></dt>
523 <p>Most cookies are set in the HTTP dialog, where they can be
524 intercepted by the <tt class="LITERAL"><a href=
525 "actions-file.html#CRUNCH-INCOMING-COOKIES">crunch-incoming-cookies</a></tt>
526 and <tt class="LITERAL"><a href=
527 "actions-file.html#CRUNCH-OUTGOING-COOKIES">crunch-outgoing-cookies</a></tt>
528 actions. But web sites increasingly make use of HTML meta tags
529 and JavaScript to sneak cookies to the browser on the content
532 <p>This filter disables most HTML and JavaScript code that reads
533 or sets cookies. It cannot detect all clever uses of these types
534 of code, so it should not be relied on as an absolute fix. Use it
535 wherever you would also use the cookie crunch actions.</p>
538 <dt><span class="emphasis"><i class=
539 "EMPHASIS">refresh-tags</i></span></dt>
542 <p>Disable any refresh tags if the interval is greater than nine
543 seconds (so that redirections done via refresh tags are not
544 destroyed). This is useful for dial-on-demand setups, or for
545 those who find this HTML feature annoying.</p>
548 <dt><span class="emphasis"><i class=
549 "EMPHASIS">unsolicited-popups</i></span></dt>
552 <p>This filter attempts to prevent only <span class=
553 "QUOTE">"unsolicited"</span> pop-up windows from opening, yet
554 still allow pop-up windows that the user has explicitly chosen to
555 open. It was added in version 3.0.1, as an improvement over
556 earlier such filters.</p>
558 <p>Technical note: The filter works by redefining the window.open
559 JavaScript function to a dummy function, <tt class=
560 "LITERAL">PrivoxyWindowOpen()</tt>, during the loading and
561 rendering phase of each HTML page access, and restoring the
562 function afterward.</p>
564 <p>This is recommended only for browsers that cannot perform this
565 function reliably themselves. And be aware that some sites
566 require such windows in order to function normally. Use with
570 <dt><span class="emphasis"><i class=
571 "EMPHASIS">all-popups</i></span></dt>
574 <p>Attempt to prevent <span class="emphasis"><i class=
575 "EMPHASIS">all</i></span> pop-up windows from opening. Note this
576 should be used with even more discretion than the above, since it
577 is more likely to break some sites that require pop-ups for
578 normal usage. Use with caution.</p>
581 <dt><span class="emphasis"><i class=
582 "EMPHASIS">img-reorder</i></span></dt>
585 <p>This is a helper filter that has no value if used alone. It
586 makes the <tt class="LITERAL">banners-by-size</tt> and <tt class=
587 "LITERAL">banners-by-link</tt> (see below) filters more effective
588 and should be enabled together with them.</p>
591 <dt><span class="emphasis"><i class=
592 "EMPHASIS">banners-by-size</i></span></dt>
595 <p>This filter removes image tags purely based on what size they
596 are. Fortunately for us, many ads and banner images tend to
597 conform to certain standardized sizes, which makes this filter
598 quite effective for ad stripping purposes.</p>
600 <p>Occasionally this filter will cause false positives on images
601 that are not ads, but just happen to be of one of the standard
604 <p>Recommended only for those who require extreme ad blocking.
605 The default block rules should catch 95+% of all ads <span class=
606 "emphasis"><i class="EMPHASIS">without</i></span> this filter
610 <dt><span class="emphasis"><i class=
611 "EMPHASIS">banners-by-link</i></span></dt>
614 <p>This is an experimental filter that attempts to kill any
615 banners if their URLs seem to point to known or suspected click
616 trackers. It is currently not of much value and is not
617 recommended for use by default.</p>
620 <dt><span class="emphasis"><i class=
621 "EMPHASIS">webbugs</i></span></dt>
624 <p>Webbugs are small, invisible images (technically 1X1 GIF
625 images), that are used to track users across websites, and
626 collect information on them. As an HTML page is loaded by the
627 browser, an embedded image tag causes the browser to contact a
628 third-party site, disclosing the tracking information through the
629 requested URL and/or cookies for that third-party domain, without
630 the user ever becoming aware of the interaction with the
631 third-party site. HTML-ized spam also uses a similar technique to
632 verify email addresses.</p>
634 <p>This filter removes the HTML code that loads such <span class=
635 "QUOTE">"webbugs"</span>.</p>
638 <dt><span class="emphasis"><i class=
639 "EMPHASIS">tiny-textforms</i></span></dt>
642 <p>A rather special-purpose filter that can be used to enlarge
643 textareas (those multi-line text boxes in web forms) and turn off
644 hard word wrap in them. It was written for the sourceforge.net
645 tracker system where such boxes are a nuisance, but it can be
646 handy on other sites, too.</p>
648 <p>It is not recommended to use this filter as a default.</p>
651 <dt><span class="emphasis"><i class=
652 "EMPHASIS">jumping-windows</i></span></dt>
655 <p>Many consider windows that move, or resize themselves to be
656 abusive. This filter neutralizes the related JavaScript code.
657 Note that some sites might not display or behave as intended when
658 using this filter. Use with caution.</p>
661 <dt><span class="emphasis"><i class=
662 "EMPHASIS">frameset-borders</i></span></dt>
665 <p>Some web designers seem to assume that everyone in the world
666 will view their web sites using the same browser brand and
667 version, screen resolution etc, because only that assumption
668 could explain why they'd use static frame sizes, yet prevent
669 their frames from being resized by the user, should they be too
670 small to show their whole content.</p>
672 <p>This filter removes the related HTML code. It should only be
673 applied to sites which need it.</p>
676 <dt><span class="emphasis"><i class=
677 "EMPHASIS">demoronizer</i></span></dt>
680 <p>Many Microsoft products that generate HTML use non-standard
681 extensions (read: violations) of the ISO 8859-1 aka Latin-1
682 character set. This can cause those HTML documents to display
683 with errors on standard-compliant platforms.</p>
685 <p>This filter translates the MS-only characters into Latin-1
686 equivalents. It is not necessary when using MS products, and will
687 cause corruption of all documents that use 8-bit character sets
688 other than Latin-1. It's mostly worthwhile for Europeans on
689 non-MS platforms, if weird garbage characters sometimes appear on
690 some pages, or user agents that don't correct for this on the
694 <dt><span class="emphasis"><i class=
695 "EMPHASIS">shockwave-flash</i></span></dt>
698 <p>A filter for shockwave haters. As the name suggests, this
699 filter strips code out of web pages that is used to embed
700 shockwave flash objects.</p>
703 <dt><span class="emphasis"><i class=
704 "EMPHASIS">quicktime-kioskmode</i></span></dt>
707 <p>Change HTML code that embeds Quicktime objects so that
708 kioskmode, which prevents saving, is disabled.</p>
711 <dt><span class="emphasis"><i class="EMPHASIS">fun</i></span></dt>
714 <p>Text replacements for subversive browsing fun. Make fun of
715 your favorite Monopolist or play buzzword bingo.</p>
718 <dt><span class="emphasis"><i class=
719 "EMPHASIS">crude-parental</i></span></dt>
722 <p>A demonstration-only filter that shows how <span class=
723 "APPLICATION">Privoxy</span> can be used to delete web content on
727 <dt><span class="emphasis"><i class=
728 "EMPHASIS">ie-exploits</i></span></dt>
731 <p>An experimental collection of text replacements to disable
732 malicious HTML and JavaScript code that exploits known security
733 holes in Internet Explorer.</p>
735 <p>Presently, it only protects against Nimda and a cross-site
736 scripting bug, and would need active maintenance to provide more
737 substantial protection.</p>
740 <dt><span class="emphasis"><i class=
741 "EMPHASIS">site-specifics</i></span></dt>
744 <p>Some web sites have very specific problems, the cure for which
745 doesn't apply anywhere else, or could even cause damage on other
748 <p>This is a collection of such site-specific cures which should
749 only be applied to the sites they were intended for, which is
750 what the supplied <tt class="FILENAME">default.action</tt> file
751 does. Users shouldn't need to change anything regarding this
755 <dt><span class="emphasis"><i class=
756 "EMPHASIS">google</i></span></dt>
759 <p>A CSS based block for Google text ads. Also removes a width
760 limitation and the toolbar advertisement.</p>
763 <dt><span class="emphasis"><i class=
764 "EMPHASIS">yahoo</i></span></dt>
767 <p>Another CSS based block, this time for Yahoo text ads. And
768 removes a width limitation as well.</p>
771 <dt><span class="emphasis"><i class="EMPHASIS">msn</i></span></dt>
774 <p>Another CSS based block, this time for MSN text ads. And
775 removes tracking URLs, as well as a width limitation.</p>
778 <dt><span class="emphasis"><i class=
779 "EMPHASIS">blogspot</i></span></dt>
782 <p>Cleans up some Blogspot blogs. Read the fine print before
785 <p>This filter also intentionally removes some navigation stuff
786 and sets the page width to 100%. As a result, some rounded
787 <span class="QUOTE">"corners"</span> would appear to early or not
788 at all and as fixing this would require a browser that
789 understands background-size (CSS3), they are removed instead.</p>
792 <dt><span class="emphasis"><i class=
793 "EMPHASIS">xml-to-html</i></span></dt>
796 <p>Server-header filter to change the Content-Type from xml to
800 <dt><span class="emphasis"><i class=
801 "EMPHASIS">html-to-xml</i></span></dt>
804 <p>Server-header filter to change the Content-Type from html to
808 <dt><span class="emphasis"><i class=
809 "EMPHASIS">no-ping</i></span></dt>
812 <p>Removes the non-standard <tt class="LITERAL">ping</tt>
813 attribute from anchor and area HTML tags.</p>
816 <dt><span class="emphasis"><i class=
817 "EMPHASIS">hide-tor-exit-notation</i></span></dt>
820 <p>Client-header filter to remove the <b class="COMMAND">Tor</b>
821 exit node notation found in Host and Referer headers.</p>
823 <p>If <span class="APPLICATION">Privoxy</span> and <b class=
824 "COMMAND">Tor</b> are chained and <span class=
825 "APPLICATION">Privoxy</span> is configured to use socks4a, one
827 "QUOTE">"http://www.example.org.foobar.exit/"</span> to access
828 the host <span class="QUOTE">"www.example.org"</span> through the
829 <b class="COMMAND">Tor</b> exit node <span class=
830 "QUOTE">"foobar"</span>.</p>
832 <p>As the HTTP client isn't aware of this notation, it treats the
833 whole string <span class=
834 "QUOTE">"www.example.org.foobar.exit"</span> as host and uses it
835 for the <span class="QUOTE">"Host"</span> and <span class=
836 "QUOTE">"Referer"</span> headers. From the server's point of view
837 the resulting headers are invalid and can cause problems.</p>
839 <p>An invalid <span class="QUOTE">"Referer"</span> header can
840 trigger <span class="QUOTE">"hot-linking"</span> protections, an
841 invalid <span class="QUOTE">"Host"</span> header will make it
842 impossible for the server to find the right vhost (several
843 domains hosted on the same IP address).</p>
845 <p>This client-header filter removes the <span class=
846 "QUOTE">"foo.exit"</span> part in those headers to prevent the
847 mentioned problems. Note that it only modifies the HTTP headers,
848 it doesn't make it impossible for the server to detect your
849 <b class="COMMAND">Tor</b> exit node based on the IP address the
850 request is coming from.</p>
857 <h2 class="SECT2"><a name="EXTERNAL-FILTER-SYNTAX" id=
858 "EXTERNAL-FILTER-SYNTAX">9.3. External filter syntax</a></h2>
860 <p>External filters are scripts or programs that can modify the content
861 in case common <tt class="LITERAL"><a href=
862 "actions-file.html#FILTER">filters</a></tt> aren't powerful enough.</p>
864 <p>External filters can be written in any language the platform
865 <span class="APPLICATION">Privoxy</span> runs on supports.</p>
867 <p>They are controlled with the <tt class="LITERAL"><a href=
868 "actions-file.html#EXTERNAL-FILTER">external-filter</a></tt> action and
869 have to be defined in the <tt class="LITERAL"><a href=
870 "config.html#FILTERFILE">filterfile</a></tt> first.</p>
872 <p>The header looks like any other filter, but instead of pcrs jobs,
873 external filters contain a single job which can be a program or a shell
874 script (which may call other scripts or programs).</p>
876 <p>External filters read the content from STDIN and write the rewritten
877 content to STDOUT. The environment variables PRIVOXY_URL, PRIVOXY_PATH,
878 PRIVOXY_HOST, PRIVOXY_ORIGIN, PRIVOXY_LISTEN_ADDRESS can be used to get
879 some details about the client request.</p>
881 <p><span class="APPLICATION">Privoxy</span> will temporary store the
882 content to filter in the <tt class="LITERAL"><a href=
883 "config.html#TEMPORARY-DIRECTORY">temporary-directory</a></tt>.</p>
885 <table border="0" bgcolor="#E0E0E0" width="100%">
889 EXTERNAL-FILTER: cat Pointless example filter that doesn't actually modify the content
892 # Incorrect reimplementation of the filter above in POSIX shell.
894 # Note that it's a single job that spans multiple lines, the line
895 # breaks are not passed to the shell, thus the semicolons are required.
897 # If the script isn't trivial, it is recommended to put it into an external file.
899 # In general, writing external filters entirely in POSIX shell is not
900 # considered a good idea.
901 EXTERNAL-FILTER: cat2 Pointless example filter that despite its name may actually modify the content
907 EXTERNAL-FILTER: rotate-image Rotate an image by 180 degree. Test filter with limited value.
908 /usr/local/bin/convert - -rotate 180 -
910 EXTERNAL-FILTER: citation-needed Adds a "[citation needed]" tag to an image. The coordinates may need adjustment.
911 /usr/local/bin/convert - -pointsize 16 -fill white -annotate +17+418 "[citation needed]" -
917 <div class="WARNING">
918 <table class="WARNING" border="1" width="100%">
920 <td align="center"><b>Warning</b></td>
925 <p>Currently external filters are executed with <span class=
926 "APPLICATION">Privoxy</span>'s privileges! Only use external
927 filters you understand and trust.</p>
933 <p>External filters are experimental and the syntax may change in the
938 <div class="NAVFOOTER">
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