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8. Actions Files

The actions files are used to define what actions Privoxy takes for which URLs, and thus determines how ad images, cookies and various other aspects of HTTP content and transactions are handled, and on which sites (or even parts thereof). There are a number of such actions, with a wide range of functionality. Each action does something a little different. These actions give us a veritable arsenal of tools with which to exert our control, preferences and independence. Actions can be combined so that their effects are aggregated when applied against a given set of URLs.

There are three action files included with Privoxy with differing purposes:

The list of actions files to be used are defined in the main configuration file, and are processed in the order they are defined (e.g. default.action is typically processed before user.action). The content of these can all be viewed and edited from http://config.privoxy.org/show-status. The over-riding principle when applying actions, is that the last action that matches a given URL wins. The broadest, most general rules go first (defined in default.action), followed by any exceptions (typically also in default.action), which are then followed lastly by any local preferences (typically in user.action). Generally, user.action has the last word.

An actions file typically has multiple sections. If you want to use "aliases" in an actions file, you have to place the (optional) alias section at the top of that file. Then comes the default set of rules which will apply universally to all sites and pages (be very careful with using such a universal set in user.action or any other actions file after default.action, because it will override the result from consulting any previous file). And then below that, exceptions to the defined universal policies. You can regard user.action as an appendix to default.action, with the advantage that it is a separate file, which makes preserving your personal settings across Privoxy upgrades easier.

Actions can be used to block anything you want, including ads, banners, or just some obnoxious URL whose content you would rather not see. Cookies can be accepted or rejected, or accepted only during the current browser session (i.e. not written to disk), content can be modified, some JavaScripts tamed, user-tracking fooled, and much more. See below for a complete list of actions.

8.1. Finding the Right Mix

Note that some actions, like cookie suppression or script disabling, may render some sites unusable that rely on these techniques to work properly. Finding the right mix of actions is not always easy and certainly a matter of personal taste. And, things can always change, requiring refinements in the configuration. In general, it can be said that the more "aggressive" your default settings (in the top section of the actions file) are, the more exceptions for "trusted" sites you will have to make later. If, for example, you want to crunch all cookies per default, you'll have to make exceptions from that rule for sites that you regularly use and that require cookies for actually useful purposes, like maybe your bank, favorite shop, or newspaper.

We have tried to provide you with reasonable rules to start from in the distribution actions files. But there is no general rule of thumb on these things. There just are too many variables, and sites are constantly changing. Sooner or later you will want to change the rules (and read this chapter again :).

8.2. How to Edit

The easiest way to edit the actions files is with a browser by using our browser-based editor, which can be reached from http://config.privoxy.org/show-status. Note: the config file option enable-edit-actions must be enabled for this to work. The editor allows both fine-grained control over every single feature on a per-URL basis, and easy choosing from wholesale sets of defaults like "Cautious", "Medium" or "Advanced". Warning: the "Advanced" setting is more aggressive, and will be more likely to cause problems for some sites. Experienced users only!

If you prefer plain text editing to GUIs, you can of course also directly edit the the actions files with your favorite text editor. Look at default.action which is richly commented with many good examples.

8.3. How Actions are Applied to Requests

Actions files are divided into sections. There are special sections, like the "alias" sections which will be discussed later. For now let's concentrate on regular sections: They have a heading line (often split up to multiple lines for readability) which consist of a list of actions, separated by whitespace and enclosed in curly braces. Below that, there is a list of URL and tag patterns, each on a separate line.

To determine which actions apply to a request, the URL of the request is compared to all URL patterns in each "action file". Every time it matches, the list of applicable actions for the request is incrementally updated, using the heading of the section in which the pattern is located. The same is done again for tags and tag patterns later on.

If multiple applying sections set the same action differently, the last match wins. If not, the effects are aggregated. E.g. a URL might match a regular section with a heading line of { +handle-as-image }, then later another one with just { +block }, resulting in both actions to apply. And there may well be cases where you will want to combine actions together. Such a section then might look like:

  { +handle-as-image  +block{Banner ads.} }
  # Block these as if they were images. Send no block page.
   banners.example.com
   media.example.com/.*banners
   .example.com/images/ads/

You can trace this process for URL patterns and any given URL by visiting http://config.privoxy.org/show-url-info.

Examples and more detail on this is provided in the Appendix, Troubleshooting: Anatomy of an Action section.

8.4. Patterns

As mentioned, Privoxy uses "patterns" to determine what actions might apply to which sites and pages your browser attempts to access. These "patterns" use wild card type pattern matching to achieve a high degree of flexibility. This allows one expression to be expanded and potentially match against many similar patterns.

Generally, an URL pattern has the form <domain><port>/<path>, where the <domain>, the <port> and the <path> are optional. (This is why the special / pattern matches all URLs). Note that the protocol portion of the URL pattern (e.g. http://) should not be included in the pattern. This is assumed already!

The pattern matching syntax is different for the domain and path parts of the URL. The domain part uses a simple globbing type matching technique, while the path part uses more flexible "Regular Expressions" (POSIX 1003.2).

The port part of a pattern is a decimal port number preceded by a colon (:). If the domain part contains a numerical IPv6 address, it has to be put into angle brackets (<, >).

www.example.com/

is a domain-only pattern and will match any request to www.example.com, regardless of which document on that server is requested. So ALL pages in this domain would be covered by the scope of this action. Note that a simple example.com is different and would NOT match.

www.example.com

means exactly the same. For domain-only patterns, the trailing / may be omitted.

www.example.com/index.html

matches all the documents on www.example.com whose name starts with /index.html.

www.example.com/index.html$

matches only the single document /index.html on www.example.com.

/index.html$

matches the document /index.html, regardless of the domain, i.e. on any web server anywhere.

/

Matches any URL because there's no requirement for either the domain or the path to match anything.

:8000/

Matches any URL pointing to TCP port 8000.

<2001:db8::1>/

Matches any URL with the host address 2001:db8::1. (Note that the real URL uses plain brackets, not angle brackets.)

index.html

matches nothing, since it would be interpreted as a domain name and there is no top-level domain called .html. So its a mistake.

8.4.1. The Domain Pattern

The matching of the domain part offers some flexible options: if the domain starts or ends with a dot, it becomes unanchored at that end. For example:

.example.com

matches any domain with first-level domain com and second-level domain example. For example www.example.com, example.com and foo.bar.baz.example.com. Note that it wouldn't match if the second-level domain was another-example.

www.

matches any domain that STARTS with www. (It also matches the domain www but most of the time that doesn't matter.)

.example.

matches any domain that CONTAINS .example.. And, by the way, also included would be any files or documents that exist within that domain since no path limitations are specified. (Correctly speaking: It matches any FQDN that contains example as a domain.) This might be www.example.com, news.example.de, or www.example.net/cgi/testing.pl for instance. All these cases are matched.

Additionally, there are wild-cards that you can use in the domain names themselves. These work similarly to shell globbing type wild-cards: "*" represents zero or more arbitrary characters (this is equivalent to the "Regular Expression" based syntax of ".*"), "?" represents any single character (this is equivalent to the regular expression syntax of a simple "."), and you can define "character classes" in square brackets which is similar to the same regular expression technique. All of this can be freely mixed:

ad*.example.com

matches "adserver.example.com", "ads.example.com", etc but not "sfads.example.com"

*ad*.example.com

matches all of the above, and then some.

.?pix.com

matches www.ipix.com, pictures.epix.com, a.b.c.d.e.upix.com etc.

www[1-9a-ez].example.c*

matches www1.example.com, www4.example.cc, wwwd.example.cy, wwwz.example.com etc., but not wwww.example.com.

While flexible, this is not the sophistication of full regular expression based syntax.

8.4.2. The Path Pattern

Privoxy uses "modern" POSIX 1003.2 "Regular Expressions" for matching the path portion (after the slash), and is thus more flexible.

There is an Appendix with a brief quick-start into regular expressions, you also might want to have a look at your operating system's documentation on regular expressions (try man re_format).

Note that the path pattern is automatically left-anchored at the "/", i.e. it matches as if it would start with a "^" (regular expression speak for the beginning of a line).

Please also note that matching in the path is CASE INSENSITIVE by default, but you can switch to case sensitive at any point in the pattern by using the "(?-i)" switch: www.example.com/(?-i)PaTtErN.* will match only documents whose path starts with PaTtErN in exactly this capitalization.

.example.com/.*

Is equivalent to just ".example.com", since any documents within that domain are matched with or without the ".*" regular expression. This is redundant

.example.com/.*/index.html$

Will match any page in the domain of "example.com" that is named "index.html", and that is part of some path. For example, it matches "www.example.com/testing/index.html" but NOT "www.example.com/index.html" because the regular expression called for at least two "/'s", thus the path requirement. It also would match "www.example.com/testing/index_html", because of the special meta-character ".".

.example.com/(.*/)?index\.html$

This regular expression is conditional so it will match any page named "index.html" regardless of path which in this case can have one or more "/'s". And this one must contain exactly ".html" (but does not have to end with that!).

.example.com/(.*/)(ads|banners?|junk)

This regular expression will match any path of "example.com" that contains any of the words "ads", "banner", "banners" (because of the "?") or "junk". The path does not have to end in these words, just contain them.

.example.com/(.*/)(ads|banners?|junk)/.*\.(jpe?g|gif|png)$

This is very much the same as above, except now it must end in either ".jpg", ".jpeg", ".gif" or ".png". So this one is limited to common image formats.

There are many, many good examples to be found in default.action, and more tutorials below in Appendix on regular expressions.

8.4.3. The Tag Pattern

Tag patterns are used to change the applying actions based on the request's tags. Tags can be created with either the client-header-tagger or the server-header-tagger action.

Tag patterns have to start with "TAG:", so Privoxy can tell them apart from URL patterns. Everything after the colon including white space, is interpreted as a regular expression with path pattern syntax, except that tag patterns aren't left-anchored automatically (Privoxy doesn't silently add a "^", you have to do it yourself if you need it).

To match all requests that are tagged with "foo" your pattern line should be "TAG:^foo$", "TAG:foo" would work as well, but it would also match requests whose tags contain "foo" somewhere. "TAG: foo" wouldn't work as it requires white space.

Sections can contain URL and tag patterns at the same time, but tag patterns are checked after the URL patterns and thus always overrule them, even if they are located before the URL patterns.

Once a new tag is added, Privoxy checks right away if it's matched by one of the tag patterns and updates the action settings accordingly. As a result tags can be used to activate other tagger actions, as long as these other taggers look for headers that haven't already be parsed.

For example you could tag client requests which use the POST method, then use this tag to activate another tagger that adds a tag if cookies are sent, and then use a block action based on the cookie tag. This allows the outcome of one action, to be input into a subsequent action. However if you'd reverse the position of the described taggers, and activated the method tagger based on the cookie tagger, no method tags would be created. The method tagger would look for the request line, but at the time the cookie tag is created, the request line has already been parsed.

While this is a limitation you should be aware of, this kind of indirection is seldom needed anyway and even the example doesn't make too much sense.

8.5. Actions

All actions are disabled by default, until they are explicitly enabled somewhere in an actions file. Actions are turned on if preceded with a "+", and turned off if preceded with a "-". So a +action means "do that action", e.g. +block means "please block URLs that match the following patterns", and -block means "don't block URLs that match the following patterns, even if +block previously applied."

Again, actions are invoked by placing them on a line, enclosed in curly braces and separated by whitespace, like in {+some-action -some-other-action{some-parameter}}, followed by a list of URL patterns, one per line, to which they apply. Together, the actions line and the following pattern lines make up a section of the actions file.

Actions fall into three categories:

If nothing is specified in any actions file, no "actions" are taken. So in this case Privoxy would just be a normal, non-blocking, non-filtering proxy. You must specifically enable the privacy and blocking features you need (although the provided default actions files will give a good starting point).

Later defined action sections always over-ride earlier ones of the same type. So exceptions to any rules you make, should come in the latter part of the file (or in a file that is processed later when using multiple actions files such as user.action). For multi-valued actions, the actions are applied in the order they are specified. Actions files are processed in the order they are defined in config (the default installation has three actions files). It also quite possible for any given URL to match more than one "pattern" (because of wildcards and regular expressions), and thus to trigger more than one set of actions! Last match wins.

The list of valid Privoxy actions are:

8.5.1. add-header

Typical use:

Confuse log analysis, custom applications

Effect:

Sends a user defined HTTP header to the web server.

Type:

Multi-value.

Parameter:

Any string value is possible. Validity of the defined HTTP headers is not checked. It is recommended that you use the "X-" prefix for custom headers.

Notes:

This action may be specified multiple times, in order to define multiple headers. This is rarely needed for the typical user. If you don't know what "HTTP headers" are, you definitely don't need to worry about this one.

Headers added by this action are not modified by other actions.

Example usage:
+add-header{X-User-Tracking: sucks}

8.5.2. block

Typical use:

Block ads or other unwanted content

Effect:

Requests for URLs to which this action applies are blocked, i.e. the requests are trapped by Privoxy and the requested URL is never retrieved, but is answered locally with a substitute page or image, as determined by the handle-as-image, set-image-blocker, and handle-as-empty-document actions.

Type:

Parameterized.

Parameter:

A block reason that should be given to the user.

Notes:

Privoxy sends a special "BLOCKED" page for requests to blocked pages. This page contains the block reason given as parameter, a link to find out why the block action applies, and a click-through to the blocked content (the latter only if the force feature is available and enabled).

A very important exception occurs if both block and handle-as-image, apply to the same request: it will then be replaced by an image. If set-image-blocker (see below) also applies, the type of image will be determined by its parameter, if not, the standard checkerboard pattern is sent.

It is important to understand this process, in order to understand how Privoxy deals with ads and other unwanted content. Blocking is a core feature, and one upon which various other features depend.

The filter action can perform a very similar task, by "blocking" banner images and other content through rewriting the relevant URLs in the document's HTML source, so they don't get requested in the first place. Note that this is a totally different technique, and it's easy to confuse the two.

Example usage (section):
{+block{No nasty stuff for you.}}
# Block and replace with "blocked" page
 .nasty-stuff.example.com

{+block{Doubleclick banners.} +handle-as-image}
# Block and replace with image
 .ad.doubleclick.net
 .ads.r.us/banners/

{+block{Layered ads.} +handle-as-empty-document}
# Block and then ignore
 adserver.example.net/.*\.js$

8.5.3. change-x-forwarded-for

Typical use:

Improve privacy by not forwarding the source of the request in the HTTP headers.

Effect:

Deletes the "X-Forwarded-For:" HTTP header from the client request, or adds a new one.

Type:

Parameterized.

Parameter:
  • "block" to delete the header.

  • "add" to create the header (or append the client's IP address to an already existing one).

Notes:

It is safe and recommended to use block.

Forwarding the source address of the request may make sense in some multi-user setups but is also a privacy risk.

Example usage:
+change-x-forwarded-for{block}

8.5.4. client-header-filter

Typical use:

Rewrite or remove single client headers.

Effect:

All client headers to which this action applies are filtered on-the-fly through the specified regular expression based substitutions.

Type:

Parameterized.

Parameter:

The name of a client-header filter, as defined in one of the filter files.

Notes:

Client-header filters are applied to each header on its own, not to all at once. This makes it easier to diagnose problems, but on the downside you can't write filters that only change header x if header y's value is z. You can do that by using tags though.

Client-header filters are executed after the other header actions have finished and use their output as input.

If the request URI gets changed, Privoxy will detect that and use the new one. This can be used to rewrite the request destination behind the client's back, for example to specify a Tor exit relay for certain requests.

Please refer to the filter file chapter to learn which client-header filters are available by default, and how to create your own.

Example usage (section):
# Hide Tor exit notation in Host and Referer Headers
{+client-header-filter{hide-tor-exit-notation}}
/

8.5.5. client-header-tagger

Typical use:

Block requests based on their headers.

Effect:

Client headers to which this action applies are filtered on-the-fly through the specified regular expression based substitutions, the result is used as tag.

Type:

Parameterized.

Parameter:

The name of a client-header tagger, as defined in one of the filter files.

Notes:

Client-header taggers are applied to each header on its own, and as the header isn't modified, each tagger "sees" the original.

Client-header taggers are the first actions that are executed and their tags can be used to control every other action.

Example usage (section):
# Tag every request with the User-Agent header
{+client-header-tagger{user-agent}}
/

# Tagging itself doesn't change the action
# settings, sections with TAG patterns do:
#
# If it's a download agent, use a different forwarding proxy,
# show the real User-Agent and make sure resume works.
{+forward-override{forward-socks5 10.0.0.2:2222 .} \
 -hide-if-modified-since      \
 -overwrite-last-modified     \
 -hide-user-agent             \
 -filter                      \
 -deanimate-gifs              \
}
TAG:^User-Agent: NetBSD-ftp/
TAG:^User-Agent: Novell ZYPP Installer
TAG:^User-Agent: RPM APT-HTTP/
TAG:^User-Agent: fetch libfetch/
TAG:^User-Agent: Ubuntu APT-HTTP/
TAG:^User-Agent: MPlayer/

# Tag all requests with the Range header set
{+client-header-tagger{range-requests}}
/

# Disable filtering for the tagged requests.
#
# With filtering enabled Privoxy would remove the Range headers
# to be able to filter the whole response. The downside is that
# it prevents clients from resuming downloads or skipping over
# parts of multimedia files.
{-filter -deanimate-gifs}
TAG:^RANGE-REQUEST$

8.5.6. content-type-overwrite

Typical use:

Stop useless download menus from popping up, or change the browser's rendering mode

Effect:

Replaces the "Content-Type:" HTTP server header.

Type:

Parameterized.

Parameter:

Any string.

Notes:

The "Content-Type:" HTTP server header is used by the browser to decide what to do with the document. The value of this header can cause the browser to open a download menu instead of displaying the document by itself, even if the document's format is supported by the browser.

The declared content type can also affect which rendering mode the browser chooses. If XHTML is delivered as "text/html", many browsers treat it as yet another broken HTML document. If it is send as "application/xml", browsers with XHTML support will only display it, if the syntax is correct.

If you see a web site that proudly uses XHTML buttons, but sets "Content-Type: text/html", you can use Privoxy to overwrite it with "application/xml" and validate the web master's claim inside your XHTML-supporting browser. If the syntax is incorrect, the browser will complain loudly.

You can also go the opposite direction: if your browser prints error messages instead of rendering a document falsely declared as XHTML, you can overwrite the content type with "text/html" and have it rendered as broken HTML document.

By default content-type-overwrite only replaces "Content-Type:" headers that look like some kind of text. If you want to overwrite it unconditionally, you have to combine it with force-text-mode. This limitation exists for a reason, think twice before circumventing it.

Most of the time it's easier to replace this action with a custom server-header filter. It allows you to activate it for every document of a certain site and it will still only replace the content types you aimed at.

Of course you can apply content-type-overwrite to a whole site and then make URL based exceptions, but it's a lot more work to get the same precision.

Example usage (sections):
# Check if www.example.net/ really uses valid XHTML
{ +content-type-overwrite{application/xml} }
www.example.net/

# but leave the content type unmodified if the URL looks like a style sheet
{-content-type-overwrite}
www.example.net/.*\.css$
www.example.net/.*style

8.5.7. crunch-client-header

Typical use:

Remove a client header Privoxy has no dedicated action for.

Effect:

Deletes every header sent by the client that contains the string the user supplied as parameter.

Type:

Parameterized.

Parameter:

Any string.

Notes:

This action allows you to block client headers for which no dedicated Privoxy action exists. Privoxy will remove every client header that contains the string you supplied as parameter.

Regular expressions are not supported and you can't use this action to block different headers in the same request, unless they contain the same string.

crunch-client-header is only meant for quick tests. If you have to block several different headers, or only want to modify parts of them, you should use a client-header filter.

Warning

Don't block any header without understanding the consequences.

Example usage (section):
# Block the non-existent "Privacy-Violation:" client header
{ +crunch-client-header{Privacy-Violation:} }
/

8.5.8. crunch-if-none-match

Typical use:

Prevent yet another way to track the user's steps between sessions.

Effect:

Deletes the "If-None-Match:" HTTP client header.

Type:

Boolean.

Parameter:

N/A

Notes:

Removing the "If-None-Match:" HTTP client header is useful for filter testing, where you want to force a real reload instead of getting status code "304" which would cause the browser to use a cached copy of the page.

It is also useful to make sure the header isn't used as a cookie replacement (unlikely but possible).

Blocking the "If-None-Match:" header shouldn't cause any caching problems, as long as the "If-Modified-Since:" header isn't blocked or missing as well.

It is recommended to use this action together with hide-if-modified-since and overwrite-last-modified.

Example usage (section):
# Let the browser revalidate cached documents but don't
# allow the server to use the revalidation headers for user tracking.
{+hide-if-modified-since{-60} \
 +overwrite-last-modified{randomize} \
 +crunch-if-none-match}
/

8.5.9. crunch-incoming-cookies

Typical use:

Prevent the web server from setting HTTP cookies on your system

Effect:

Deletes any "Set-Cookie:" HTTP headers from server replies.

Type:

Boolean.

Parameter:

N/A

Notes:

This action is only concerned with incoming HTTP cookies. For outgoing HTTP cookies, use crunch-outgoing-cookies. Use both to disable HTTP cookies completely.

It makes no sense at all to use this action in conjunction with the session-cookies-only action, since it would prevent the session cookies from being set. See also filter-content-cookies.

Example usage:
+crunch-incoming-cookies

8.5.10. crunch-server-header

Typical use:

Remove a server header Privoxy has no dedicated action for.

Effect:

Deletes every header sent by the server that contains the string the user supplied as parameter.

Type:

Parameterized.

Parameter:

Any string.

Notes:

This action allows you to block server headers for which no dedicated Privoxy action exists. Privoxy will remove every server header that contains the string you supplied as parameter.

Regular expressions are not supported and you can't use this action to block different headers in the same request, unless they contain the same string.

crunch-server-header is only meant for quick tests. If you have to block several different headers, or only want to modify parts of them, you should use a custom server-header filter.

Warning

Don't block any header without understanding the consequences.

Example usage (section):
# Crunch server headers that try to prevent caching
{ +crunch-server-header{no-cache} }
/

8.5.11. crunch-outgoing-cookies

Typical use:

Prevent the web server from reading any HTTP cookies from your system

Effect:

Deletes any "Cookie:" HTTP headers from client requests.

Type:

Boolean.

Parameter:

N/A

Notes:

This action is only concerned with outgoing HTTP cookies. For incoming HTTP cookies, use crunch-incoming-cookies. Use both to disable HTTP cookies completely.

It makes no sense at all to use this action in conjunction with the session-cookies-only action, since it would prevent the session cookies from being read.

Example usage:
+crunch-outgoing-cookies

8.5.12. deanimate-gifs

Typical use:

Stop those annoying, distracting animated GIF images.

Effect:

De-animate GIF animations, i.e. reduce them to their first or last image.

Type:

Parameterized.

Parameter:

"last" or "first"

Notes:

This will also shrink the images considerably (in bytes, not pixels!). If the option "first" is given, the first frame of the animation is used as the replacement. If "last" is given, the last frame of the animation is used instead, which probably makes more sense for most banner animations, but also has the risk of not showing the entire last frame (if it is only a delta to an earlier frame).

You can safely use this action with patterns that will also match non-GIF objects, because no attempt will be made at anything that doesn't look like a GIF.

Example usage:
+deanimate-gifs{last}

8.5.13. downgrade-http-version

Typical use:

Work around (very rare) problems with HTTP/1.1

Effect:

Downgrades HTTP/1.1 client requests and server replies to HTTP/1.0.

Type:

Boolean.

Parameter:

N/A

Notes:

This is a left-over from the time when Privoxy didn't support important HTTP/1.1 features well. It is left here for the unlikely case that you experience HTTP/1.1-related problems with some server out there.

Note that enabling this action is only a workaround. It should not be enabled for sites that work without it. While it shouldn't break any pages, it has an (usually negative) performance impact.

If you come across a site where enabling this action helps, please report it, so the cause of the problem can be analyzed. If the problem turns out to be caused by a bug in Privoxy it should be fixed so the following release works without the work around.

Example usage (section):
{+downgrade-http-version}
problem-host.example.com

8.5.14. fast-redirects

Typical use:

Fool some click-tracking scripts and speed up indirect links.

Effect:

Detects redirection URLs and redirects the browser without contacting the redirection server first.

Type:

Parameterized.

Parameter:
  • "simple-check" to just search for the string "http://" to detect redirection URLs.

  • "check-decoded-url" to decode URLs (if necessary) before searching for redirection URLs.

Notes:

Many sites, like yahoo.com, don't just link to other sites. Instead, they will link to some script on their own servers, giving the destination as a parameter, which will then redirect you to the final target. URLs resulting from this scheme typically look like: "http://www.example.org/click-tracker.cgi?target=http%3a//www.example.net/".

Sometimes, there are even multiple consecutive redirects encoded in the URL. These redirections via scripts make your web browsing more traceable, since the server from which you follow such a link can see where you go to. Apart from that, valuable bandwidth and time is wasted, while your browser asks the server for one redirect after the other. Plus, it feeds the advertisers.

This feature is currently not very smart and is scheduled for improvement. If it is enabled by default, you will have to create some exceptions to this action. It can lead to failures in several ways:

Not every URLs with other URLs as parameters is evil. Some sites offer a real service that requires this information to work. For example a validation service needs to know, which document to validate. fast-redirects assumes that every URL parameter that looks like another URL is a redirection target, and will always redirect to the last one. Most of the time the assumption is correct, but if it isn't, the user gets redirected anyway.

Another failure occurs if the URL contains other parameters after the URL parameter. The URL: "http://www.example.org/?redirect=http%3a//www.example.net/&foo=bar". contains the redirection URL "http://www.example.net/", followed by another parameter. fast-redirects doesn't know that and will cause a redirect to "http://www.example.net/&foo=bar". Depending on the target server configuration, the parameter will be silently ignored or lead to a "page not found" error. You can prevent this problem by first using the redirect action to remove the last part of the URL, but it requires a little effort.

To detect a redirection URL, fast-redirects only looks for the string "http://", either in plain text (invalid but often used) or encoded as "http%3a//". Some sites use their own URL encoding scheme, encrypt the address of the target server or replace it with a database id. In theses cases fast-redirects is fooled and the request reaches the redirection server where it probably gets logged.

Example usage:
 { +fast-redirects{simple-check} }
   one.example.com

 { +fast-redirects{check-decoded-url} }
   another.example.com/testing

8.5.15. filter

Typical use:

Get rid of HTML and JavaScript annoyances, banner advertisements (by size), do fun text replacements, add personalized effects, etc.

Effect:

All instances of text-based type, most notably HTML and JavaScript, to which this action applies, can be filtered on-the-fly through the specified regular expression based substitutions. (Note: as of version 3.0.3 plain text documents are exempted from filtering, because web servers often use the text/plain MIME type for all files whose type they don't know.)

Type:

Parameterized.

Parameter:

The name of a content filter, as defined in the filter file. Filters can be defined in one or more files as defined by the filterfile option in the config file. default.filter is the collection of filters supplied by the developers. Locally defined filters should go in their own file, such as user.filter.

When used in its negative form, and without parameters, all filtering is completely disabled.

Notes:

For your convenience, there are a number of pre-defined filters available in the distribution filter file that you can use. See the examples below for a list.

Filtering requires buffering the page content, which may appear to slow down page rendering since nothing is displayed until all content has passed the filters. (The total time until the page is completely rendered doesn't change much, but it may be perceived as slower since the page is not incrementally displayed.) This effect will be more noticeable on slower connections.

"Rolling your own" filters requires a knowledge of "Regular Expressions" and "HTML". This is very powerful feature, and potentially very intrusive. Filters should be used with caution, and where an equivalent "action" is not available.

The amount of data that can be filtered is limited to the buffer-limit option in the main config file. The default is 4096 KB (4 Megs). Once this limit is exceeded, the buffered data, and all pending data, is passed through unfiltered.

Inappropriate MIME types, such as zipped files, are not filtered at all. (Again, only text-based types except plain text). Encrypted SSL data (from HTTPS servers) cannot be filtered either, since this would violate the integrity of the secure transaction. In some situations it might be necessary to protect certain text, like source code, from filtering by defining appropriate -filter exceptions.

Compressed content can't be filtered either, but if Privoxy is compiled with zlib support and a supported compression algorithm is used (gzip or deflate), Privoxy can first decompress the content and then filter it.

If you use a Privoxy version without zlib support, but want filtering to work on as much documents as possible, even those that would normally be sent compressed, you must use the prevent-compression action in conjunction with filter.

Content filtering can achieve some of the same effects as the block action, i.e. it can be used to block ads and banners. But the mechanism works quite differently. One effective use, is to block ad banners based on their size (see below), since many of these seem to be somewhat standardized.

Feedback with suggestions for new or improved filters is particularly welcome!

The below list has only the names and a one-line description of each predefined filter. There are more verbose explanations of what these filters do in the filter file chapter.

Example usage (with filters from the distribution default.filter file). See the Predefined Filters section for more explanation on each:

+filter{js-annoyances}       # Get rid of particularly annoying JavaScript abuse.

+filter{js-events}           # Kill JavaScript event bindings and timers (Radically destructive! Only for extra nasty sites).

+filter{html-annoyances}     # Get rid of particularly annoying HTML abuse.

+filter{content-cookies}     # Kill cookies that come in the HTML or JS content.

+filter{refresh-tags}        # Kill automatic refresh tags if refresh time is larger than 9 seconds.

+filter{unsolicited-popups}  # Disable only unsolicited pop-up windows.

+filter{all-popups}          # Kill all popups in JavaScript and HTML.

+filter{img-reorder}         # Reorder attributes in <img> tags to make the banners-by-* filters more effective.

+filter{banners-by-size}     # Kill banners by size.

+filter{banners-by-link}     # Kill banners by their links to known clicktrackers.

+filter{webbugs}             # Squish WebBugs (1x1 invisible GIFs used for user tracking).

+filter{tiny-textforms}      # Extend those tiny textareas up to 40x80 and kill the hard wrap.

+filter{jumping-windows}     # Prevent windows from resizing and moving themselves.

+filter{frameset-borders}    # Give frames a border and make them resizable.

+filter{iframes}             # Removes all detected iframes. Should only be enabled for individual sites.

+filter{demoronizer}         # Fix MS's non-standard use of standard charsets.

+filter{shockwave-flash}     # Kill embedded Shockwave Flash objects.

+filter{quicktime-kioskmode} # Make Quicktime movies saveable.

+filter{fun}                 # Text replacements for subversive browsing fun!

+filter{crude-parental}      # Crude parental filtering. Note that this filter doesn't work reliably.

+filter{ie-exploits}         # Disable some known Internet Explorer bug exploits.

+filter{site-specifics}      # Cure for site-specific problems. Don't apply generally!

+filter{no-ping}             # Removes non-standard ping attributes in <a> and <area> tags.

+filter{google}              # CSS-based block for Google text ads. Also removes a width limitation and the toolbar advertisement.

+filter{yahoo}               # CSS-based block for Yahoo text ads. Also removes a width limitation.

+filter{msn}                 # CSS-based block for MSN text ads. Also removes tracking URLs and a width limitation.

+filter{blogspot}            # Cleans up some Blogspot blogs. Read the fine print before using this.

8.5.16. force-text-mode

Typical use:

Force Privoxy to treat a document as if it was in some kind of text format.

Effect:

Declares a document as text, even if the "Content-Type:" isn't detected as such.

Type:

Boolean.

Parameter:

N/A

Notes:

As explained above, Privoxy tries to only filter files that are in some kind of text format. The same restrictions apply to content-type-overwrite. force-text-mode declares a document as text, without looking at the "Content-Type:" first.

Warning

Think twice before activating this action. Filtering binary data with regular expressions can cause file damage.

Example usage:
+force-text-mode

8.5.17. forward-override

Typical use:

Change the forwarding settings based on User-Agent or request origin

Effect:

Overrules the forward directives in the configuration file.

Type:

Multi-value.

Parameter:
  • "forward ." to use a direct connection without any additional proxies.

  • "forward 127.0.0.1:8123" to use the HTTP proxy listening at 127.0.0.1 port 8123.

  • "forward-socks4a 127.0.0.1:9050 ." to use the socks4a proxy listening at 127.0.0.1 port 9050. Replace "forward-socks4a" with "forward-socks4" to use a socks4 connection (with local DNS resolution) instead, use "forward-socks5" for socks5 connections (with remote DNS resolution).

  • "forward-socks4a 127.0.0.1:9050 proxy.example.org:8000" to use the socks4a proxy listening at 127.0.0.1 port 9050 to reach the HTTP proxy listening at proxy.example.org port 8000. Replace "forward-socks4a" with "forward-socks4" to use a socks4 connection (with local DNS resolution) instead, use "forward-socks5" for socks5 connections (with remote DNS resolution).

Notes:

This action takes parameters similar to the forward directives in the configuration file, but without the URL pattern. It can be used as replacement, but normally it's only used in cases where matching based on the request URL isn't sufficient.

Warning

Please read the description for the forward directives before using this action. Forwarding to the wrong people will reduce your privacy and increase the chances of man-in-the-middle attacks.

If the ports are missing or invalid, default values will be used. This might change in the future and you shouldn't rely on it. Otherwise incorrect syntax causes Privoxy to exit.

Use the show-url-info CGI page to verify that your forward settings do what you thought the do.

Example usage:
# Always use direct connections for requests previously tagged as
# "User-Agent: fetch libfetch/2.0" and make sure
# resuming downloads continues to work.
# This way you can continue to use Tor for your normal browsing,
# without overloading the Tor network with your FreeBSD ports updates
# or downloads of bigger files like ISOs.
# Note that HTTP headers are easy to fake and therefore their
# values are as (un)trustworthy as your clients and users.
{+forward-override{forward .} \
 -hide-if-modified-since      \
 -overwrite-last-modified     \
}
TAG:^User-Agent: fetch libfetch/2\.0$

8.5.18. handle-as-empty-document

Typical use:

Mark URLs that should be replaced by empty documents if they get blocked

Effect:

This action alone doesn't do anything noticeable. It just marks URLs. If the block action also applies, the presence or absence of this mark decides whether an HTML "BLOCKED" page, or an empty document will be sent to the client as a substitute for the blocked content. The empty document isn't literally empty, but actually contains a single space.

Type:

Boolean.

Parameter:

N/A

Notes:

Some browsers complain about syntax errors if JavaScript documents are blocked with Privoxy's default HTML page; this option can be used to silence them. And of course this action can also be used to eliminate the Privoxy BLOCKED message in frames.

The content type for the empty document can be specified with content-type-overwrite{}, but usually this isn't necessary.

Example usage:
# Block all documents on example.org that end with ".js",
# but send an empty document instead of the usual HTML message.
{+block{Blocked JavaScript} +handle-as-empty-document}
example.org/.*\.js$

8.5.19. handle-as-image

Typical use:

Mark URLs as belonging to images (so they'll be replaced by images if they do get blocked, rather than HTML pages)

Effect:

This action alone doesn't do anything noticeable. It just marks URLs as images. If the block action also applies, the presence or absence of this mark decides whether an HTML "blocked" page, or a replacement image (as determined by the set-image-blocker action) will be sent to the client as a substitute for the blocked content.

Type:

Boolean.

Parameter:

N/A

Notes:

The below generic example section is actually part of default.action. It marks all URLs with well-known image file name extensions as images and should be left intact.

Users will probably only want to use the handle-as-image action in conjunction with block, to block sources of banners, whose URLs don't reflect the file type, like in the second example section.

Note that you cannot treat HTML pages as images in most cases. For instance, (in-line) ad frames require an HTML page to be sent, or they won't display properly. Forcing handle-as-image in this situation will not replace the ad frame with an image, but lead to error messages.

Example usage (sections):
# Generic image extensions:
#
{+handle-as-image}
/.*\.(gif|jpg|jpeg|png|bmp|ico)$

# These don't look like images, but they're banners and should be
# blocked as images:
#
{+block{Nasty banners.} +handle-as-image}
nasty-banner-server.example.com/junk.cgi\?output=trash

8.5.20. hide-accept-language

Typical use:

Pretend to use different language settings.

Effect:

Deletes or replaces the "Accept-Language:" HTTP header in client requests.

Type:

Parameterized.

Parameter:

Keyword: "block", or any user defined value.

Notes:

Faking the browser's language settings can be useful to make a foreign User-Agent set with hide-user-agent more believable.

However some sites with content in different languages check the "Accept-Language:" to decide which one to take by default. Sometimes it isn't possible to later switch to another language without changing the "Accept-Language:" header first.

Therefore it's a good idea to either only change the "Accept-Language:" header to languages you understand, or to languages that aren't wide spread.

Before setting the "Accept-Language:" header to a rare language, you should consider that it helps to make your requests unique and thus easier to trace. If you don't plan to change this header frequently, you should stick to a common language.

Example usage (section):
# Pretend to use Canadian language settings.
{+hide-accept-language{en-ca} \
+hide-user-agent{Mozilla/5.0 (X11; U; OpenBSD i386; en-CA; rv:1.8.0.4) Gecko/20060628 Firefox/1.5.0.4} \
}
/

8.5.21. hide-content-disposition

Typical use:

Prevent download menus for content you prefer to view inside the browser.

Effect:

Deletes or replaces the "Content-Disposition:" HTTP header set by some servers.

Type:

Parameterized.

Parameter:

Keyword: "block", or any user defined value.

Notes:

Some servers set the "Content-Disposition:" HTTP header for documents they assume you want to save locally before viewing them. The "Content-Disposition:" header contains the file name the browser is supposed to use by default.

In most browsers that understand this header, it makes it impossible to just view the document, without downloading it first, even if it's just a simple text file or an image.

Removing the "Content-Disposition:" header helps to prevent this annoyance, but some browsers additionally check the "Content-Type:" header, before they decide if they can display a document without saving it first. In these cases, you have to change this header as well, before the browser stops displaying download menus.

It is also possible to change the server's file name suggestion to another one, but in most cases it isn't worth the time to set it up.

This action will probably be removed in the future, use server-header filters instead.

Example usage:
# Disarm the download link in Sourceforge's patch tracker
{ -filter \
 +content-type-overwrite{text/plain}\
 +hide-content-disposition{block} }
 .sourceforge.net/tracker/download\.php

8.5.22. hide-if-modified-since

Typical use:

Prevent yet another way to track the user's steps between sessions.

Effect:

Deletes the "If-Modified-Since:" HTTP client header or modifies its value.

Type:

Parameterized.

Parameter:

Keyword: "block", or a user defined value that specifies a range of hours.

Notes:

Removing this header is useful for filter testing, where you want to force a real reload instead of getting status code "304", which would cause the browser to use a cached copy of the page.

Instead of removing the header, hide-if-modified-since can also add or subtract a random amount of time to/from the header's value. You specify a range of minutes where the random factor should be chosen from and Privoxy does the rest. A negative value means subtracting, a positive value adding.

Randomizing the value of the "If-Modified-Since:" makes it less likely that the server can use the time as a cookie replacement, but you will run into caching problems if the random range is too high.

It is a good idea to only use a small negative value and let overwrite-last-modified handle the greater changes.

It is also recommended to use this action together with crunch-if-none-match, otherwise it's more or less pointless.

Example usage (section):
# Let the browser revalidate but make tracking based on the time less likely.
{+hide-if-modified-since{-60} \
 +overwrite-last-modified{randomize} \
 +crunch-if-none-match}
/

8.5.23. hide-from-header

Typical use:

Keep your (old and ill) browser from telling web servers your email address

Effect:

Deletes any existing "From:" HTTP header, or replaces it with the specified string.

Type:

Parameterized.

Parameter:

Keyword: "block", or any user defined value.

Notes:

The keyword "block" will completely remove the header (not to be confused with the block action).

Alternately, you can specify any value you prefer to be sent to the web server. If you do, it is a matter of fairness not to use any address that is actually used by a real person.

This action is rarely needed, as modern web browsers don't send "From:" headers anymore.

Example usage:
+hide-from-header{block}
or
+hide-from-header{spam-me-senseless@sittingduck.example.com}

8.5.24. hide-referrer

Typical use:

Conceal which link you followed to get to a particular site

Effect:

Deletes the "Referer:" (sic) HTTP header from the client request, or replaces it with a forged one.

Type:

Parameterized.

Parameter:
  • "conditional-block" to delete the header completely if the host has changed.

  • "conditional-forge" to forge the header if the host has changed.

  • "block" to delete the header unconditionally.

  • "forge" to pretend to be coming from the homepage of the server we are talking to.

  • Any other string to set a user defined referrer.

Notes:

conditional-block is the only parameter, that isn't easily detected in the server's log file. If it blocks the referrer, the request will look like the visitor used a bookmark or typed in the address directly.

Leaving the referrer unmodified for requests on the same host allows the server owner to see the visitor's "click path", but in most cases she could also get that information by comparing other parts of the log file: for example the User-Agent if it isn't a very common one, or the user's IP address if it doesn't change between different requests.

Always blocking the referrer, or using a custom one, can lead to failures on servers that check the referrer before they answer any requests, in an attempt to prevent their content from being embedded or linked to elsewhere.

Both conditional-block and forge will work with referrer checks, as long as content and valid referring page are on the same host. Most of the time that's the case.

hide-referer is an alternate spelling of hide-referrer and the two can be can be freely substituted with each other. ("referrer" is the correct English spelling, however the HTTP specification has a bug - it requires it to be spelled as "referer".)

Example usage:
+hide-referrer{forge}
or
+hide-referrer{http://www.yahoo.com/}

8.5.25. hide-user-agent

Typical use:

Try to conceal your type of browser and client operating system

Effect:

Replaces the value of the "User-Agent:" HTTP header in client requests with the specified value.

Type:

Parameterized.

Parameter:

Any user-defined string.

Notes:
Warning

This can lead to problems on web sites that depend on looking at this header in order to customize their content for different browsers (which, by the way, is NOT the right thing to do: good web sites work browser-independently).

Using this action in multi-user setups or wherever different types of browsers will access the same Privoxy is not recommended. In single-user, single-browser setups, you might use it to delete your OS version information from the headers, because it is an invitation to exploit known bugs for your OS. It is also occasionally useful to forge this in order to access sites that won't let you in otherwise (though there may be a good reason in some cases).

More information on known user-agent strings can be found at http://www.user-agents.org/ and http://en.wikipedia.org/wiki/User_agent.

Example usage:
+hide-user-agent{Netscape 6.1 (X11; I; Linux 2.4.18 i686)}

8.5.26. limit-connect

Typical use:

Prevent abuse of Privoxy as a TCP proxy relay or disable SSL for untrusted sites

Effect:

Specifies to which ports HTTP CONNECT requests are allowable.

Type:

Parameterized.

Parameter:

A comma-separated list of ports or port ranges (the latter using dashes, with the minimum defaulting to 0 and the maximum to 65K).

Notes:

By default, i.e. if no limit-connect action applies, Privoxy allows HTTP CONNECT requests to all ports. Use limit-connect if fine-grained control is desired for some or all destinations.

The CONNECT methods exists in HTTP to allow access to secure websites ("https://" URLs) through proxies. It works very simply: the proxy connects to the server on the specified port, and then short-circuits its connections to the client and to the remote server. This means CONNECT-enabled proxies can be used as TCP relays very easily.

Privoxy relays HTTPS traffic without seeing the decoded content. Websites can leverage this limitation to circumvent Privoxy's filters. By specifying an invalid port range you can disable HTTPS entirely.

Example usages:
+limit-connect{443}                   # Port 443 is OK.
+limit-connect{80,443}                # Ports 80 and 443 are OK.
+limit-connect{-3, 7, 20-100, 500-}   # Ports less than 3, 7, 20 to 100 and above 500 are OK.
+limit-connect{-}                     # All ports are OK
+limit-connect{,}                     # No HTTPS/SSL traffic is allowed

8.5.27. limit-cookie-lifetime

Typical use:

Limit the lifetime of HTTP cookies to a couple of minutes or hours.

Effect:

Overwrites the expires field in Set-Cookie server headers if it's above the specified limit.

Type:

Parameterized.

Parameter:

The lifetime limit in minutes, or 0.

Notes:

This action reduces the lifetime of HTTP cookies coming from the server to the specified number of minutes, starting from the time the cookie passes Privoxy.

Cookies with a lifetime below the limit are not modified. The lifetime of session cookies is set to the specified limit.

The effect of this action depends on the server.

In case of servers which refresh their cookies with each response (or at least frequently), the lifetime limit set by this action is updated as well. Thus, a session associated with the cookie continues to work with this action enabled, as long as a new request is made before the last limit set is reached.

However, some servers send their cookies once, with a lifetime of several years (the year 2037 is a popular choice), and do not refresh them until a certain event in the future, for example the user logging out. In this case this action may limit the absolute lifetime of the session, even if requests are made frequently.

If the parameter is "0", this action behaves like session-cookies-only.

Example usages:
+limit-cookie-lifetime{60}

8.5.28. prevent-compression

Typical use:

Ensure that servers send the content uncompressed, so it can be passed through filters.

Effect:

Removes the Accept-Encoding header which can be used to ask for compressed transfer.

Type:

Boolean.

Parameter:

N/A

Notes:

More and more websites send their content compressed by default, which is generally a good idea and saves bandwidth. But the filter and deanimate-gifs actions need access to the uncompressed data.

When compiled with zlib support (available since Privoxy 3.0.7), content that should be filtered is decompressed on-the-fly and you don't have to worry about this action. If you are using an older Privoxy version, or one that hasn't been compiled with zlib support, this action can be used to convince the server to send the content uncompressed.

Most text-based instances compress very well, the size is seldom decreased by less than 50%, for markup-heavy instances like news feeds saving more than 90% of the original size isn't unusual.

Not using compression will therefore slow down the transfer, and you should only enable this action if you really need it. As of Privoxy 3.0.7 it's disabled in all predefined action settings.

Note that some (rare) ill-configured sites don't handle requests for uncompressed documents correctly. Broken PHP applications tend to send an empty document body, some IIS versions only send the beginning of the content. If you enable prevent-compression per default, you might want to add exceptions for those sites. See the example for how to do that.

Example usage (sections):
# Selectively turn off compression, and enable a filter
#
{ +filter{tiny-textforms} +prevent-compression }
# Match only these sites
 .google.
 sourceforge.net
 sf.net

# Or instead, we could set a universal default:
#
{ +prevent-compression }
 / # Match all sites

# Then maybe make exceptions for broken sites:
#
{ -prevent-compression }
.compusa.com/

8.5.29. overwrite-last-modified

Typical use:

Prevent yet another way to track the user's steps between sessions.

Effect:

Deletes the "Last-Modified:" HTTP server header or modifies its value.

Type:

Parameterized.

Parameter:

One of the keywords: "block", "reset-to-request-time" and "randomize"

Notes:

Removing the "Last-Modified:" header is useful for filter testing, where you want to force a real reload instead of getting status code "304", which would cause the browser to reuse the old version of the page.

The "randomize" option overwrites the value of the "Last-Modified:" header with a randomly chosen time between the original value and the current time. In theory the server could send each document with a different "Last-Modified:" header to track visits without using cookies. "Randomize" makes it impossible and the browser can still revalidate cached documents.

"reset-to-request-time" overwrites the value of the "Last-Modified:" header with the current time. You could use this option together with hide-if-modified-since to further customize your random range.

The preferred parameter here is "randomize". It is safe to use, as long as the time settings are more or less correct. If the server sets the "Last-Modified:" header to the time of the request, the random range becomes zero and the value stays the same. Therefore you should later randomize it a second time with hided-if-modified-since, just to be sure.

It is also recommended to use this action together with crunch-if-none-match.

Example usage:
# Let the browser revalidate without being tracked across sessions
{ +hide-if-modified-since{-60} \
 +overwrite-last-modified{randomize} \
 +crunch-if-none-match}
/

8.5.30. redirect

Typical use:

Redirect requests to other sites.

Effect:

Convinces the browser that the requested document has been moved to another location and the browser should get it from there.

Type:

Parameterized

Parameter:

An absolute URL or a single pcrs command.

Notes:

Requests to which this action applies are answered with a HTTP redirect to URLs of your choosing. The new URL is either provided as parameter, or derived by applying a single pcrs command to the original URL.

The syntax for pcrs commands is documented in the filter file section.

This action will be ignored if you use it together with block. It can be combined with fast-redirects{check-decoded-url} to redirect to a decoded version of a rewritten URL.

Use this action carefully, make sure not to create redirection loops and be aware that using your own redirects might make it possible to fingerprint your requests.

In case of problems with your redirects, or simply to watch them working, enable debug 128.

Example usages:
# Replace example.com's style sheet with another one
{ +redirect{http://localhost/css-replacements/example.com.css} }
 example.com/stylesheet\.css

# Create a short, easy to remember nickname for a favorite site
# (relies on the browser accept and forward invalid URLs to Privoxy)
{ +redirect{http://www.privoxy.org/user-manual/actions-file.html} }
 a

# Always use the expanded view for Undeadly.org articles
# (Note the $ at the end of the URL pattern to make sure
# the request for the rewritten URL isn't redirected as well)
{+redirect{s@$@&mode=expanded@}}
undeadly.org/cgi\?action=article&sid=\d*$

# Redirect Google search requests to MSN
{+redirect{s@^http://[^/]*/search\?q=([^&]*).*@http://search.msn.com/results.aspx?q=$1@}}
.google.com/search

# Redirect MSN search requests to Yahoo
{+redirect{s@^http://[^/]*/results\.aspx\?q=([^&]*).*@http://search.yahoo.com/search?p=$1@}}
search.msn.com//results\.aspx\?q=

# Redirect remote requests for this manual
# to the local version delivered by Privoxy
{+redirect{s@^http://www@http://config@}}
www.privoxy.org/user-manual/

8.5.31. server-header-filter

Typical use:

Rewrite or remove single server headers.

Effect:

All server headers to which this action applies are filtered on-the-fly through the specified regular expression based substitutions.

Type:

Parameterized.

Parameter:

The name of a server-header filter, as defined in one of the filter files.

Notes:

Server-header filters are applied to each header on its own, not to all at once. This makes it easier to diagnose problems, but on the downside you can't write filters that only change header x if header y's value is z. You can do that by using tags though.

Server-header filters are executed after the other header actions have finished and use their output as input.

Please refer to the filter file chapter to learn which server-header filters are available by default, and how to create your own.

Example usage (section):
{+server-header-filter{html-to-xml}}
example.org/xml-instance-that-is-delivered-as-html

{+server-header-filter{xml-to-html}}
example.org/instance-that-is-delivered-as-xml-but-is-not

8.5.32. server-header-tagger

Typical use:

Enable or disable filters based on the Content-Type header.

Effect:

Server headers to which this action applies are filtered on-the-fly through the specified regular expression based substitutions, the result is used as tag.

Type:

Parameterized.

Parameter:

The name of a server-header tagger, as defined in one of the filter files.

Notes:

Server-header taggers are applied to each header on its own, and as the header isn't modified, each tagger "sees" the original.

Server-header taggers are executed before all other header actions that modify server headers. Their tags can be used to control all of the other server-header actions, the content filters and the crunch actions (redirect and block).

Obviously crunching based on tags created by server-header taggers doesn't prevent the request from showing up in the server's log file.

Example usage (section):
# Tag every request with the content type declared by the server
{+server-header-tagger{content-type}}
/

8.5.33. session-cookies-only

Typical use:

Allow only temporary "session" cookies (for the current browser session only).

Effect:

Deletes the "expires" field from "Set-Cookie:" server headers. Most browsers will not store such cookies permanently and forget them in between sessions.

Type:

Boolean.

Parameter:

N/A

Notes:

This is less strict than crunch-incoming-cookies / crunch-outgoing-cookies and allows you to browse websites that insist or rely on setting cookies, without compromising your privacy too badly.

Most browsers will not permanently store cookies that have been processed by session-cookies-only and will forget about them between sessions. This makes profiling cookies useless, but won't break sites which require cookies so that you can log in for transactions. This is generally turned on for all sites, and is the recommended setting.

It makes no sense at all to use session-cookies-only together with crunch-incoming-cookies or crunch-outgoing-cookies. If you do, cookies will be plainly killed.

Note that it is up to the browser how it handles such cookies without an "expires" field. If you use an exotic browser, you might want to try it out to be sure.

This setting also has no effect on cookies that may have been stored previously by the browser before starting Privoxy. These would have to be removed manually.

Privoxy also uses the content-cookies filter to block some types of cookies. Content cookies are not effected by session-cookies-only.

Example usage:
+session-cookies-only

8.5.34. set-image-blocker

Typical use:

Choose the replacement for blocked images

Effect:

This action alone doesn't do anything noticeable. If both block and handle-as-image also apply, i.e. if the request is to be blocked as an image, then the parameter of this action decides what will be sent as a replacement.

Type:

Parameterized.

Parameter:
  • "pattern" to send a built-in checkerboard pattern image. The image is visually decent, scales very well, and makes it obvious where banners were busted.

  • "blank" to send a built-in transparent image. This makes banners disappear completely, but makes it hard to detect where Privoxy has blocked images on a given page and complicates troubleshooting if Privoxy has blocked innocent images, like navigation icons.

  • "target-url" to send a redirect to target-url. You can redirect to any image anywhere, even in your local filesystem via "file:///" URL. (But note that not all browsers support redirecting to a local file system).

    A good application of redirects is to use special Privoxy-built-in URLs, which send the built-in images, as target-url. This has the same visual effect as specifying "blank" or "pattern" in the first place, but enables your browser to cache the replacement image, instead of requesting it over and over again.

Notes:

The URLs for the built-in images are "http://config.privoxy.org/send-banner?type=type", where type is either "blank" or "pattern".

There is a third (advanced) type, called "auto". It is NOT to be used in set-image-blocker, but meant for use from filters. Auto will select the type of image that would have applied to the referring page, had it been an image.

Example usage:

Built-in pattern:

+set-image-blocker{pattern}

Redirect to the BSD daemon:

+set-image-blocker{http://www.freebsd.org/gifs/dae_up3.gif}

Redirect to the built-in pattern for better caching:

+set-image-blocker{http://config.privoxy.org/send-banner?type=pattern}

8.5.35. Summary

Note that many of these actions have the potential to cause a page to misbehave, possibly even not to display at all. There are many ways a site designer may choose to design his site, and what HTTP header content, and other criteria, he may depend on. There is no way to have hard and fast rules for all sites. See the Appendix for a brief example on troubleshooting actions.

8.6. Aliases

Custom "actions", known to Privoxy as "aliases", can be defined by combining other actions. These can in turn be invoked just like the built-in actions. Currently, an alias name can contain any character except space, tab, "=", "{" and "}", but we strongly recommend that you only use "a" to "z", "0" to "9", "+", and "-". Alias names are not case sensitive, and are not required to start with a "+" or "-" sign, since they are merely textually expanded.

Aliases can be used throughout the actions file, but they must be defined in a special section at the top of the file! And there can only be one such section per actions file. Each actions file may have its own alias section, and the aliases defined in it are only visible within that file.

There are two main reasons to use aliases: One is to save typing for frequently used combinations of actions, the other one is a gain in flexibility: If you decide once how you want to handle shops by defining an alias called "shop", you can later change your policy on shops in one place, and your changes will take effect everywhere in the actions file where the "shop" alias is used. Calling aliases by their purpose also makes your actions files more readable.

Currently, there is one big drawback to using aliases, though: Privoxy's built-in web-based action file editor honors aliases when reading the actions files, but it expands them before writing. So the effects of your aliases are of course preserved, but the aliases themselves are lost when you edit sections that use aliases with it.

Now let's define some aliases...

 # Useful custom aliases we can use later.
 #
 # Note the (required!) section header line and that this section
 # must be at the top of the actions file!
 #
 {{alias}}

 # These aliases just save typing later:
 # (Note that some already use other aliases!)
 #
 +crunch-all-cookies = +crunch-incoming-cookies +crunch-outgoing-cookies
 -crunch-all-cookies = -crunch-incoming-cookies -crunch-outgoing-cookies
 +block-as-image      = +block{Blocked image.} +handle-as-image
 allow-all-cookies   = -crunch-all-cookies -session-cookies-only -filter{content-cookies}

 # These aliases define combinations of actions
 # that are useful for certain types of sites:
 #
 fragile     = -block -filter -crunch-all-cookies -fast-redirects -hide-referrer -prevent-compression

 shop        = -crunch-all-cookies -filter{all-popups}

 # Short names for other aliases, for really lazy people ;-)
 #
 c0 = +crunch-all-cookies
 c1 = -crunch-all-cookies

...and put them to use. These sections would appear in the lower part of an actions file and define exceptions to the default actions (as specified further up for the "/" pattern):

 # These sites are either very complex or very keen on
 # user data and require minimal interference to work:
 #
 {fragile}
 .office.microsoft.com
 .windowsupdate.microsoft.com
 # Gmail is really mail.google.com, not gmail.com
 mail.google.com

 # Shopping sites:
 # Allow cookies (for setting and retrieving your customer data)
 #
 {shop}
 .quietpc.com
 .worldpay.com   # for quietpc.com
 mybank.example.com

 # These shops require pop-ups:
 #
 {-filter{all-popups} -filter{unsolicited-popups}}
  .dabs.com
  .overclockers.co.uk

Aliases like "shop" and "fragile" are typically used for "problem" sites that require more than one action to be disabled in order to function properly.

8.7. Actions Files Tutorial

The above chapters have shown which actions files there are and how they are organized, how actions are specified and applied to URLs, how patterns work, and how to define and use aliases. Now, let's look at an example match-all.action, default.action and user.action file and see how all these pieces come together:

8.7.1. match-all.action

Remember all actions are disabled when matching starts, so we have to explicitly enable the ones we want.

While the match-all.action file only contains a single section, it is probably the most important one. It has only one pattern, "/", but this pattern matches all URLs. Therefore, the set of actions used in this "default" section will be applied to all requests as a start. It can be partly or wholly overridden by other actions files like default.action and user.action, but it will still be largely responsible for your overall browsing experience.

Again, at the start of matching, all actions are disabled, so there is no need to disable any actions here. (Remember: a "+" preceding the action name enables the action, a "-" disables!). Also note how this long line has been made more readable by splitting it into multiple lines with line continuation.

{ \
 +change-x-forwarded-for{block} \
 +hide-from-header{block} \
 +set-image-blocker{pattern} \
}
/ # Match all URLs

The default behavior is now set.

8.7.2. default.action

If you aren't a developer, there's no need for you to edit the default.action file. It is maintained by the Privoxy developers and if you disagree with some of the sections, you should overrule them in your user.action.

Understanding the default.action file can help you with your user.action, though.

The first section in this file is a special section for internal use that prevents older Privoxy versions from reading the file:

##########################################################################
# Settings -- Don't change! For internal Privoxy use ONLY.
##########################################################################
{{settings}}
for-privoxy-version=3.0.11

After that comes the (optional) alias section. We'll use the example section from the above chapter on aliases, that also explains why and how aliases are used:

##########################################################################
# Aliases
##########################################################################
{{alias}}

 # These aliases just save typing later:
 # (Note that some already use other aliases!)
 #
 +crunch-all-cookies = +crunch-incoming-cookies +crunch-outgoing-cookies
 -crunch-all-cookies = -crunch-incoming-cookies -crunch-outgoing-cookies
 +block-as-image      = +block{Blocked image.} +handle-as-image
 mercy-for-cookies   = -crunch-all-cookies -session-cookies-only -filter{content-cookies}

 # These aliases define combinations of actions
 # that are useful for certain types of sites:
 #
 fragile     = -block -filter -crunch-all-cookies -fast-redirects -hide-referrer
 shop        = -crunch-all-cookies -filter{all-popups}

The first of our specialized sections is concerned with "fragile" sites, i.e. sites that require minimum interference, because they are either very complex or very keen on tracking you (and have mechanisms in place that make them unusable for people who avoid being tracked). We will simply use our pre-defined fragile alias instead of stating the list of actions explicitly:

##########################################################################
# Exceptions for sites that'll break under the default action set:
##########################################################################

# "Fragile" Use a minimum set of actions for these sites (see alias above):
#
{ fragile }
.office.microsoft.com           # surprise, surprise!
.windowsupdate.microsoft.com
mail.google.com

Shopping sites are not as fragile, but they typically require cookies to log in, and pop-up windows for shopping carts or item details. Again, we'll use a pre-defined alias:

# Shopping sites:
#
{ shop }
.quietpc.com
.worldpay.com   # for quietpc.com
.jungle.com
.scan.co.uk

The fast-redirects action, which may have been enabled in match-all.action, breaks some sites. So disable it for popular sites where we know it misbehaves:

{ -fast-redirects }
login.yahoo.com
edit.*.yahoo.com
.google.com
.altavista.com/.*(like|url|link):http
.altavista.com/trans.*urltext=http
.nytimes.com

It is important that Privoxy knows which URLs belong to images, so that if they are to be blocked, a substitute image can be sent, rather than an HTML page. Contacting the remote site to find out is not an option, since it would destroy the loading time advantage of banner blocking, and it would feed the advertisers information about you. We can mark any URL as an image with the handle-as-image action, and marking all URLs that end in a known image file extension is a good start:

##########################################################################
# Images:
##########################################################################

# Define which file types will be treated as images, in case they get
# blocked further down this file:
#
{ +handle-as-image }
/.*\.(gif|jpe?g|png|bmp|ico)$

And then there are known banner sources. They often use scripts to generate the banners, so it won't be visible from the URL that the request is for an image. Hence we block them and mark them as images in one go, with the help of our +block-as-image alias defined above. (We could of course just as well use +block +handle-as-image here.) Remember that the type of the replacement image is chosen by the set-image-blocker action. Since all URLs have matched the default section with its +set-image-blocker{pattern} action before, it still applies and needn't be repeated:

# Known ad generators:
#
{ +block-as-image }
ar.atwola.com
.ad.doubleclick.net
.ad.*.doubleclick.net
.a.yimg.com/(?:(?!/i/).)*$
.a[0-9].yimg.com/(?:(?!/i/).)*$
bs*.gsanet.com
.qkimg.net

One of the most important jobs of Privoxy is to block banners. Many of these can be "blocked" by the filter{banners-by-size} action, which we enabled above, and which deletes the references to banner images from the pages while they are loaded, so the browser doesn't request them anymore, and hence they don't need to be blocked here. But this naturally doesn't catch all banners, and some people choose not to use filters, so we need a comprehensive list of patterns for banner URLs here, and apply the block action to them.

First comes many generic patterns, which do most of the work, by matching typical domain and path name components of banners. Then comes a list of individual patterns for specific sites, which is omitted here to keep the example short:

##########################################################################
# Block these fine banners:
##########################################################################
{ +block{Banner ads.} }

# Generic patterns:
#
ad*.
.*ads.
banner?.
count*.
/.*count(er)?\.(pl|cgi|exe|dll|asp|php[34]?)
/(?:.*/)?(publicite|werbung|rekla(ma|me|am)|annonse|maino(kset|nta|s)?)/

# Site-specific patterns (abbreviated):
#
.hitbox.com

It's quite remarkable how many advertisers actually call their banner servers ads.company.com, or call the directory in which the banners are stored simply "banners". So the above generic patterns are surprisingly effective.

But being very generic, they necessarily also catch URLs that we don't want to block. The pattern .*ads. e.g. catches "nasty-ads.nasty-corp.com" as intended, but also "downloads.sourcefroge.net" or "adsl.some-provider.net." So here come some well-known exceptions to the +block section above.

Note that these are exceptions to exceptions from the default! Consider the URL "downloads.sourcefroge.net": Initially, all actions are deactivated, so it wouldn't get blocked. Then comes the defaults section, which matches the URL, but just deactivates the block action once again. Then it matches .*ads., an exception to the general non-blocking policy, and suddenly +block applies. And now, it'll match .*loads., where -block applies, so (unless it matches again further down) it ends up with no block action applying.

##########################################################################
# Save some innocent victims of the above generic block patterns:
##########################################################################

# By domain:
#
{ -block }
adv[io]*.  # (for advogato.org and advice.*)
adsl.      # (has nothing to do with ads)
adobe.     # (has nothing to do with ads either)
ad[ud]*.   # (adult.* and add.*)
.edu       # (universities don't host banners (yet!))
.*loads.   # (downloads, uploads etc)

# By path:
#
/.*loads/

# Site-specific:
#
www.globalintersec.com/adv # (adv = advanced)
www.ugu.com/sui/ugu/adv

Filtering source code can have nasty side effects, so make an exception for our friends at sourceforge.net, and all paths with "cvs" in them. Note that -filter disables all filters in one fell swoop!

# Don't filter code!
#
{ -filter }
/(.*/)?cvs
bugzilla.
developer.
wiki.
.sourceforge.net

The actual default.action is of course much more comprehensive, but we hope this example made clear how it works.

8.7.3. user.action

So far we are painting with a broad brush by setting general policies, which would be a reasonable starting point for many people. Now, you might want to be more specific and have customized rules that are more suitable to your personal habits and preferences. These would be for narrowly defined situations like your ISP or your bank, and should be placed in user.action, which is parsed after all other actions files and hence has the last word, over-riding any previously defined actions. user.action is also a safe place for your personal settings, since default.action is actively maintained by the Privoxy developers and you'll probably want to install updated versions from time to time.

So let's look at a few examples of things that one might typically do in user.action:

# My user.action file. <fred@example.com>

As aliases are local to the actions file that they are defined in, you can't use the ones from default.action, unless you repeat them here:

# Aliases are local to the file they are defined in.
# (Re-)define aliases for this file:
#
{{alias}}
#
# These aliases just save typing later, and the alias names should
# be self explanatory.
#
+crunch-all-cookies = +crunch-incoming-cookies +crunch-outgoing-cookies
-crunch-all-cookies = -crunch-incoming-cookies -crunch-outgoing-cookies
 allow-all-cookies  = -crunch-all-cookies -session-cookies-only
 allow-popups       = -filter{all-popups}
+block-as-image     = +block{Blocked as image.} +handle-as-image
-block-as-image     = -block

# These aliases define combinations of actions that are useful for
# certain types of sites:
#
fragile     = -block -crunch-all-cookies -filter -fast-redirects -hide-referrer
shop        = -crunch-all-cookies allow-popups

# Allow ads for selected useful free sites:
#
allow-ads   = -block -filter{banners-by-size} -filter{banners-by-link}

# Alias for specific file types that are text, but might have conflicting
# MIME types. We want the browser to force these to be text documents.
handle-as-text = -filter +-content-type-overwrite{text/plain} +-force-text-mode -hide-content-disposition

Say you have accounts on some sites that you visit regularly, and you don't want to have to log in manually each time. So you'd like to allow persistent cookies for these sites. The allow-all-cookies alias defined above does exactly that, i.e. it disables crunching of cookies in any direction, and the processing of cookies to make them only temporary.

{ allow-all-cookies }
 sourceforge.net
 .yahoo.com
 .msdn.microsoft.com
 .redhat.com

Your bank is allergic to some filter, but you don't know which, so you disable them all:

{ -filter }
 .your-home-banking-site.com

Some file types you may not want to filter for various reasons:

# Technical documentation is likely to contain strings that might
# erroneously get altered by the JavaScript-oriented filters:
#
.tldp.org
/(.*/)?selfhtml/

# And this stupid host sends streaming video with a wrong MIME type,
# so that Privoxy thinks it is getting HTML and starts filtering:
#
stupid-server.example.com/

Example of a simple block action. Say you've seen an ad on your favourite page on example.com that you want to get rid of. You have right-clicked the image, selected "copy image location" and pasted the URL below while removing the leading http://, into a { +block{} } section. Note that { +handle-as-image } need not be specified, since all URLs ending in .gif will be tagged as images by the general rules as set in default.action anyway:

{ +block{Nasty ads.} }
 www.example.com/nasty-ads/sponsor\.gif
 another.example.net/more/junk/here/

The URLs of dynamically generated banners, especially from large banner farms, often don't use the well-known image file name extensions, which makes it impossible for Privoxy to guess the file type just by looking at the URL. You can use the +block-as-image alias defined above for these cases. Note that objects which match this rule but then turn out NOT to be an image are typically rendered as a "broken image" icon by the browser. Use cautiously.

{ +block-as-image }
 .doubleclick.net
 .fastclick.net
 /Realmedia/ads/
 ar.atwola.com/

Now you noticed that the default configuration breaks Forbes Magazine, but you were too lazy to find out which action is the culprit, and you were again too lazy to give feedback, so you just used the fragile alias on the site, and -- whoa! -- it worked. The fragile aliases disables those actions that are most likely to break a site. Also, good for testing purposes to see if it is Privoxy that is causing the problem or not. We later find other regular sites that misbehave, and add those to our personalized list of troublemakers:

{ fragile }
 .forbes.com
 webmail.example.com
 .mybank.com

You like the "fun" text replacements in default.filter, but it is disabled in the distributed actions file. So you'd like to turn it on in your private, update-safe config, once and for all:

{ +filter{fun} }
 / # For ALL sites!

Note that the above is not really a good idea: There are exceptions to the filters in default.action for things that really shouldn't be filtered, like code on CVS->Web interfaces. Since user.action has the last word, these exceptions won't be valid for the "fun" filtering specified here.

You might also worry about how your favourite free websites are funded, and find that they rely on displaying banner advertisements to survive. So you might want to specifically allow banners for those sites that you feel provide value to you:

{ allow-ads }
 .sourceforge.net
 .slashdot.org
 .osdn.net

Note that allow-ads has been aliased to -block, -filter{banners-by-size}, and -filter{banners-by-link} above.

Invoke another alias here to force an over-ride of the MIME type application/x-sh which typically would open a download type dialog. In my case, I want to look at the shell script, and then I can save it should I choose to.

{ handle-as-text }
 /.*\.sh$

user.action is generally the best place to define exceptions and additions to the default policies of default.action. Some actions are safe to have their default policies set here though. So let's set a default policy to have a "blank" image as opposed to the checkerboard pattern for ALL sites. "/" of course matches all URL paths and patterns:

{ +set-image-blocker{blank} }
/ # ALL sites