Privoxy 3.0.32 User Manual
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# 14. Appendix

## 14.1. Regular Expressions

Privoxy uses Perl-style "regular expressions" in its actions files and filter file, through the PCRE and PCRS libraries.

If you are reading this, you probably don't understand what "regular expressions" are, or what they can do. So this will be a very brief introduction only. A full explanation would require a book ;-)

Regular expressions provide a language to describe patterns that can be run against strings of characters (letter, numbers, etc), to see if they match the string or not. The patterns are themselves (sometimes complex) strings of literal characters, combined with wild-cards, and other special characters, called meta-characters. The "meta-characters" have special meanings and are used to build complex patterns to be matched against. Perl Compatible Regular Expressions are an especially convenient "dialect" of the regular expression language.

To make a simple analogy, we do something similar when we use wild-card characters when listing files with the dir command in DOS. `*.*` matches all filenames. The "special" character here is the asterisk which matches any and all characters. We can be more specific and use `?` to match just individual characters. So "dir file?.text" would match "file1.txt", "file2.txt", etc. We are pattern matching, using a similar technique to "regular expressions"!

Regular expressions do essentially the same thing, but are much, much more powerful. There are many more "special characters" and ways of building complex patterns however. Let's look at a few of the common ones, and then some examples:

. - Matches any single character, e.g. "a", "A", "4", ":", or "@".

? - The preceding character or expression is matched ZERO or ONE times. Either/or.

+ - The preceding character or expression is matched ONE or MORE times.

* - The preceding character or expression is matched ZERO or MORE times.

\ - The "escape" character denotes that the following character should be taken literally. This is used where one of the special characters (e.g. ".") needs to be taken literally and not as a special meta-character. Example: "example\.com", makes sure the period is recognized only as a period (and not expanded to its meta-character meaning of any single character).

[ ] - Characters enclosed in brackets will be matched if any of the enclosed characters are encountered. For instance, "[0-9]" matches any numeric digit (zero through nine). As an example, we can combine this with "+" to match any digit one of more times: "[0-9]+".

( ) - parentheses are used to group a sub-expression, or multiple sub-expressions.

| - The "bar" character works like an "or" conditional statement. A match is successful if the sub-expression on either side of "|" matches. As an example: "/(this|that) example/" uses grouping and the bar character and would match either "this example" or "that example", and nothing else.

These are just some of the ones you are likely to use when matching URLs with Privoxy, and is a long way from a definitive list. This is enough to get us started with a few simple examples which may be more illuminating:

`/.*/banners/.*` - A simple example that uses the common combination of "." and "*" to denote any character, zero or more times. In other words, any string at all. So we start with a literal forward slash, then our regular expression pattern (".*") another literal forward slash, the string "banners", another forward slash, and lastly another ".*". We are building a directory path here. This will match any file with the path that has a directory named "banners" in it. The ".*" matches any characters, and this could conceivably be more forward slashes, so it might expand into a much longer looking path. For example, this could match: "/eye/hate/spammers/banners/annoy_me_please.gif", or just "/banners/annoying.html", or almost an infinite number of other possible combinations, just so it has "banners" in the path somewhere.

And now something a little more complex:

`/.*/adv((er)?ts?|ertis(ing|ements?))?/` - We have several literal forward slashes again ("/"), so we are building another expression that is a file path statement. We have another ".*", so we are matching against any conceivable sub-path, just so it matches our expression. The only true literal that must match our pattern is adv, together with the forward slashes. What comes after the "adv" string is the interesting part.

Remember the "?" means the preceding expression (either a literal character or anything grouped with "(...)" in this case) can exist or not, since this means either zero or one match. So "((er)?ts?|ertis(ing|ements?))" is optional, as are the individual sub-expressions: "(er)", "(ing|ements?)", and the "s". The "|" means "or". We have two of those. For instance, "(ing|ements?)", can expand to match either "ing" OR "ements?". What is being done here, is an attempt at matching as many variations of "advertisement", and similar, as possible. So this would expand to match just "adv", or "advert", or "adverts", or "advertising", or "advertisement", or "advertisements". You get the idea. But it would not match "advertizements" (with a "z"). We could fix that by changing our regular expression to: "/.*/adv((er)?ts?|erti(s|z)(ing|ements?))?/", which would then match either spelling.

`/.*/advert[0-9]+\.(gif|jpe?g)` - Again another path statement with forward slashes. Anything in the square brackets "[ ]" can be matched. This is using "0-9" as a shorthand expression to mean any digit one through nine. It is the same as saying "0123456789". So any digit matches. The "+" means one or more of the preceding expression must be included. The preceding expression here is what is in the square brackets -- in this case, any digit one through nine. Then, at the end, we have a grouping: "(gif|jpe?g)". This includes a "|", so this needs to match the expression on either side of that bar character also. A simple "gif" on one side, and the other side will in turn match either "jpeg" or "jpg", since the "?" means the letter "e" is optional and can be matched once or not at all. So we are building an expression here to match image GIF or JPEG type image file. It must include the literal string "advert", then one or more digits, and a "." (which is now a literal, and not a special character, since it is escaped with "\"), and lastly either "gif", or "jpeg", or "jpg". Some possible matches would include: "//advert1.jpg", "/nasty/ads/advert1234.gif", "/banners/from/hell/advert99.jpg". It would not match "advert1.gif" (no leading slash), or "/adverts232.jpg" (the expression does not include an "s"), or "/advert1.jsp" ("jsp" is not in the expression anywhere).

We are barely scratching the surface of regular expressions here so that you can understand the default Privoxy configuration files, and maybe use this knowledge to customize your own installation. There is much, much more that can be done with regular expressions. Now that you know enough to get started, you can learn more on your own :/

More reading on Perl Compatible Regular expressions: http://perldoc.perl.org/perlre.html

For information on regular expression based substitutions and their applications in filters, please see the filter file tutorial in this manual.

## 14.2. Privoxy's Internal Pages

Since Privoxy proxies each requested web page, it is easy for Privoxy to trap certain special URLs. In this way, we can talk directly to Privoxy, and see how it is configured, see how our rules are being applied, change these rules and other configuration options, and even turn Privoxy's filtering off, all with a web browser.

The URLs listed below are the special ones that allow direct access to Privoxy. Of course, Privoxy must be running to access these. If not, you will get a friendly error message. Internet access is not necessary either.

- Privoxy main page:

  > http://config.privoxy.org/

  There is a shortcut: http://p.p/ (But it doesn't provide a fall-back to a real page, in case the request is not sent through Privoxy)

- View and toggle client tags:

  > http://config.privoxy.org/client-tags

- Show information about the current configuration, including viewing and editing of actions files:

  > http://config.privoxy.org/show-status

- Show the browser's request headers:

  > http://config.privoxy.org/show-request

- Show which actions apply to a URL and why:

  > http://config.privoxy.org/show-url-info

- Toggle Privoxy on or off. This feature can be turned off/on in the main `config` file. When toggled "off", "Privoxy" continues to run, but only as a pass-through proxy, with no actions taking place:

  > http://config.privoxy.org/toggle

  Short cuts. Turn off, then on:

  > http://config.privoxy.org/toggle?set=disable

  > http://config.privoxy.org/toggle?set=enable

## 14.3. Chain of Events

Let's take a quick look at how some of Privoxy's core features are triggered, and the ensuing sequence of events when a web page is requested by your browser:

- First, your web browser requests a web page. The browser knows to send the request to Privoxy, which will in turn, relay the request to the remote web server after passing the following tests:

- Privoxy traps any request for its own internal CGI pages (e.g http://p.p/) and sends the CGI page back to the browser.

- Next, Privoxy checks to see if the URL matches any "+block" patterns. If so, the URL is then blocked, and the remote web server will not be contacted. "+handle-as-image" and "+handle-as-empty-document" are then checked, and if there is no match, an HTML "BLOCKED" page is sent back to the browser. Otherwise, if it does match, an image is returned for the former, and an empty text document for the latter. The type of image would depend on the setting of "+set-image-blocker" (blank, checkerboard pattern, or an HTTP redirect to an image elsewhere).

- Untrusted URLs are blocked. If URLs are being added to the `trust` file, then that is done.

- If the URL pattern matches the "+fast-redirects" action, it is then processed. Unwanted parts of the requested URL are stripped.

- Now the rest of the client browser's request headers are processed. If any of these match any of the relevant actions (e.g. "+hide-user-agent", etc.), headers are suppressed or forged as determined by these actions and their parameters.

- Now the web server starts sending its response back (i.e. typically a web page).

- First, the server headers are read and processed to determine, among other things, the MIME type (document type) and encoding. The headers are then filtered as determined by the "+crunch-incoming-cookies", "+session-cookies-only", and "+downgrade-http-version" actions.

- If any "+filter" action or "+deanimate-gifs" action applies (and the document type fits the action), the rest of the page is read into memory (up to a configurable limit). Then the filter rules (from `default.filter` and any other filter files) are processed against the buffered content. Filters are applied in the order they are specified in one of the filter files. Animated GIFs, if present, are reduced to either the first or last frame, depending on the action setting.The entire page, which is now filtered, is then sent by Privoxy back to your browser.

  If neither a "+filter" action or "+deanimate-gifs" matches, then Privoxy passes the raw data through to the client browser as it becomes available.

- As the browser receives the now (possibly filtered) page content, it reads and then requests any URLs that may be embedded within the page source, e.g. ad images, stylesheets, JavaScript, other HTML documents (e.g. frames), sounds, etc. For each of these objects, the browser issues a separate request (this is easily viewable in Privoxy's logs). And each such request is in turn processed just as above. Note that a complex web page will have many, many such embedded URLs. If these secondary requests are to a different server, then quite possibly a very differing set of actions is triggered.

NOTE: This is somewhat of a simplistic overview of what happens with each URL request. For the sake of brevity and simplicity, we have focused on Privoxy's core features only.

## 14.4. Troubleshooting: Anatomy of an Action

The way Privoxy applies actions and filters to any given URL can be complex, and not always so easy to understand what is happening. And sometimes we need to be able to see just what Privoxy is doing. Especially, if something Privoxy is doing is causing us a problem inadvertently. It can be a little daunting to look at the actions and filters files themselves, since they tend to be filled with regular expressions whose consequences are not always so obvious.

One quick test to see if Privoxy is causing a problem or not, is to disable it temporarily. This should be the first troubleshooting step (be sure to flush caches afterward!). Looking at the logs is a good idea too. (Note that both the toggle feature and logging are enabled via `config` file settings, and may need to be turned "on".)

Another easy troubleshooting step to try is if you have done any customization of your installation, revert back to the installed defaults and see if that helps. There are times the developers get complaints about one thing or another, and the problem is more related to a customized configuration issue.

Privoxy also provides the http://config.privoxy.org/show-url-info page that can show us very specifically how actions are being applied to any given URL. This is a big help for troubleshooting.

First, enter one URL (or partial URL) at the prompt, and then Privoxy will tell us how the current configuration will handle it. This will not help with filtering effects (i.e. the "+filter" action) from one of the filter files since this is handled very differently and not so easy to trap! It also will not tell you about any other URLs that may be embedded within the URL you are testing. For instance, images such as ads are expressed as URLs within the raw page source of HTML pages. So you will only get info for the actual URL that is pasted into the prompt area -- not any sub-URLs. If you want to know about embedded URLs like ads, you will have to dig those out of the HTML source. Use your browser's "View Page Source" option for this. Or right click on the ad, and grab the URL.

Let's try an example, google.com, and look at it one section at a time in a sample configuration (your real configuration may vary):

     Matches for http://www.google.com:

     In file: default.action [ View ] [ Edit ]

     {+change-x-forwarded-for{block}
     +deanimate-gifs {last}
     +fast-redirects {check-decoded-url}
     +filter {refresh-tags}
     +filter {img-reorder}
     +filter {banners-by-size}
     +filter {webbugs}
     +filter {jumping-windows}
     +filter {ie-exploits}
     +hide-from-header {block}
     +hide-referrer {forge}
     +session-cookies-only
     +set-image-blocker {pattern}
    /

     { -session-cookies-only }
     .google.com

     { -fast-redirects }
     .google.com

    In file: user.action [ View ] [ Edit ]
    (no matches in this file)

This is telling us how we have defined our "actions", and which ones match for our test case, "google.com". Displayed is all the actions that are available to us. Remember, the `+` sign denotes "on". `-` denotes "off". So some are "on" here, but many are "off". Each example we try may provide a slightly different end result, depending on our configuration directives.

The first listing is for our `default.action` file. The large, multi-line listing, is how the actions are set to match for all URLs, i.e. our default settings. If you look at your "actions" file, this would be the section just below the "aliases" section near the top. This will apply to all URLs as signified by the single forward slash at the end of the listing -- " / ".

But we have defined additional actions that would be exceptions to these general rules, and then we list specific URLs (or patterns) that these exceptions would apply to. Last match wins. Just below this then are two explicit matches for ".google.com". The first is negating our previous cookie setting, which was for "+session-cookies-only" (i.e. not persistent). So we will allow persistent cookies for google, at least that is how it is in this example. The second turns off any "+fast-redirects" action, allowing this to take place unmolested. Note that there is a leading dot here -- ".google.com". This will match any hosts and sub-domains, in the google.com domain also, such as "www.google.com" or "mail.google.com". But it would not match "www.google.de"! So, apparently, we have these two actions defined as exceptions to the general rules at the top somewhere in the lower part of our `default.action` file, and "google.com" is referenced somewhere in these latter sections.

Then, for our `user.action` file, we again have no hits. So there is nothing google-specific that we might have added to our own, local configuration. If there was, those actions would over-rule any actions from previously processed files, such as `default.action`. `user.action` typically has the last word. This is the best place to put hard and fast exceptions,

And finally we pull it all together in the bottom section and summarize how Privoxy is applying all its "actions" to "google.com":

     Final results:

     -add-header
     -block
     +change-x-forwarded-for{block}
     -client-header-filter{hide-tor-exit-notation}
     -content-type-overwrite
     -crunch-client-header
     -crunch-if-none-match
     -crunch-incoming-cookies
     -crunch-outgoing-cookies
     -crunch-server-header
     +deanimate-gifs {last}
     -downgrade-http-version
     -fast-redirects
     -filter {js-events}
     -filter {content-cookies}
     -filter {all-popups}
     -filter {banners-by-link}
     -filter {tiny-textforms}
     -filter {frameset-borders}
     -filter {demoronizer}
     -filter {shockwave-flash}
     -filter {quicktime-kioskmode}
     -filter {fun}
     -filter {crude-parental}
     -filter {site-specifics}
     -filter {js-annoyances}
     -filter {html-annoyances}
     +filter {refresh-tags}
     -filter {unsolicited-popups}
     +filter {img-reorder}
     +filter {banners-by-size}
     +filter {webbugs}
     +filter {jumping-windows}
     +filter {ie-exploits}
     -filter {google}
     -filter {yahoo}
     -filter {msn}
     -filter {blogspot}
     -filter {no-ping}
     -force-text-mode
     -handle-as-empty-document
     -handle-as-image
     -hide-accept-language
     -hide-content-disposition
     +hide-from-header {block}
     -hide-if-modified-since
     +hide-referrer {forge}
     -hide-user-agent
     -limit-connect
     -overwrite-last-modified
     -prevent-compression
     -redirect
     -server-header-filter{xml-to-html}
     -server-header-filter{html-to-xml}
     -session-cookies-only
     +set-image-blocker {pattern}

Notice the only difference here to the previous listing, is to "fast-redirects" and "session-cookies-only", which are activated specifically for this site in our configuration, and thus show in the "Final Results".

Now another example, "ad.doubleclick.net":

     { +block{Domains starts with "ad"} }
      ad*.

     { +block{Domain contains "ad"} }
      .ad.

     { +block{Doubleclick banner server} +handle-as-image }
      .[a-vx-z]*.doubleclick.net

We'll just show the interesting part here - the explicit matches. It is matched three different times. Two "+block{}" sections, and a "+block{} +handle-as-image", which is the expanded form of one of our aliases that had been defined as: "+block-as-image". ("Aliases" are defined in the first section of the actions file and typically used to combine more than one action.)

Any one of these would have done the trick and blocked this as an unwanted image. This is unnecessarily redundant since the last case effectively would also cover the first. No point in taking chances with these guys though ;-) Note that if you want an ad or obnoxious URL to be invisible, it should be defined as "ad.doubleclick.net" is done here -- as both a "+block{}" and an "+handle-as-image". The custom alias "`+block-as-image`" just simplifies the process and make it more readable.

One last example. Let's try "http://www.example.net/adsl/HOWTO/". This one is giving us problems. We are getting a blank page. Hmmm ...

     Matches for http://www.example.net/adsl/HOWTO/:

     In file: default.action [ View ] [ Edit ]

     {-add-header
      -block
      +change-x-forwarded-for{block}
      -client-header-filter{hide-tor-exit-notation}
      -content-type-overwrite
      -crunch-client-header
      -crunch-if-none-match
      -crunch-incoming-cookies
      -crunch-outgoing-cookies
      -crunch-server-header
      +deanimate-gifs
      -downgrade-http-version
      +fast-redirects {check-decoded-url}
      -filter {js-events}
      -filter {content-cookies}
      -filter {all-popups}
      -filter {banners-by-link}
      -filter {tiny-textforms}
      -filter {frameset-borders}
      -filter {demoronizer}
      -filter {shockwave-flash}
      -filter {quicktime-kioskmode}
      -filter {fun}
      -filter {crude-parental}
      -filter {site-specifics}
      -filter {js-annoyances}
      -filter {html-annoyances}
      +filter {refresh-tags}
      -filter {unsolicited-popups}
      +filter {img-reorder}
      +filter {banners-by-size}
      +filter {webbugs}
      +filter {jumping-windows}
      +filter {ie-exploits}
      -filter {google}
      -filter {yahoo}
      -filter {msn}
      -filter {blogspot}
      -filter {no-ping}
      -force-text-mode
      -handle-as-empty-document
      -handle-as-image
      -hide-accept-language
      -hide-content-disposition
      +hide-from-header{block}
      +hide-referer{forge}
      -hide-user-agent
      -overwrite-last-modified
      +prevent-compression
      -redirect
      -server-header-filter{xml-to-html}
      -server-header-filter{html-to-xml}
      +session-cookies-only
      +set-image-blocker{blank} }
       /

     { +block{Path contains "ads".} +handle-as-image }
      /ads

Ooops, the "/adsl/" is matching "/ads" in our configuration! But we did not want this at all! Now we see why we get the blank page. It is actually triggering two different actions here, and the effects are aggregated so that the URL is blocked, and Privoxy is told to treat the block as if it were an image. But this is, of course, all wrong. We could now add a new action below this (or better in our own `user.action` file) that explicitly un blocks ( "{-block}") paths with "adsl" in them (remember, last match in the configuration wins). There are various ways to handle such exceptions. Example:

     { -block }
      /adsl

Now the page displays ;-) Remember to flush your browser's caches when making these kinds of changes to your configuration to insure that you get a freshly delivered page! Or, try using `Shift+Reload`.

But now what about a situation where we get no explicit matches like we did with:

     { +block{Path starts with "ads".} +handle-as-image }
     /ads

That actually was very helpful and pointed us quickly to where the problem was. If you don't get this kind of match, then it means one of the default rules in the first section of `default.action` is causing the problem. This would require some guesswork, and maybe a little trial and error to isolate the offending rule. One likely cause would be one of the "+filter" actions. These tend to be harder to troubleshoot. Try adding the URL for the site to one of aliases that turn off "+filter":

     { shop }
     .quietpc.com
     .worldpay.com   # for quietpc.com
     .jungle.com
     .scan.co.uk
     .forbes.com

"`{ shop }`" is an "alias" that expands to "`{ -filter -session-cookies-only }`". Or you could do your own exception to negate filtering:

     { -filter }
     # Disable ALL filter actions for sites in this section
     .forbes.com
     developer.ibm.com
     localhost

This would turn off all filtering for these sites. This is best put in `user.action`, for local site exceptions. Note that when a simple domain pattern is used by itself (without the subsequent path portion), all sub-pages within that domain are included automatically in the scope of the action.

Images that are inexplicably being blocked, may well be hitting the "+filter{banners-by-size}" rule, which assumes that images of certain sizes are ad banners (works well most of the time since these tend to be standardized).

"`{ fragile }`" is an alias that disables most actions that are the most likely to cause trouble. This can be used as a last resort for problem sites.

     { fragile }
     # Handle with care: easy to break
     mail.google.
     mybank.example.com

Remember to flush caches! Note that the `mail.google` reference lacks the TLD portion (e.g. ".com"). This will effectively match any TLD with `google` in it, such as `mail.google.de.`, just as an example.

If this still does not work, you will have to go through the remaining actions one by one to find which one(s) is causing the problem.

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