Miscellaneous

Privoxy Frequently Asked Questions
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# 4. Miscellaneous

### 4.1. How much does Privoxy slow my browsing down? This has to add extra time to browsing.

How much of an impact depends on many things, including the CPU of the host system, how aggressive the configuration is, which specific actions are being triggered, the size of the page, the bandwidth of the connection, etc.

Overall, it should not slow you down any in real terms, and may actually help speed things up since ads, banners and other junk are not typically being retrieved and displayed. The actual processing time required by Privoxy itself for each page, is relatively small in the overall scheme of things, and happens very quickly. This is typically more than offset by time saved not downloading and rendering ad images and other junk content (if ad blocking is being used).

"Filtering" content via the `filter` or `deanimate-gifs` actions may cause a perceived slowdown, since the entire document needs to be buffered before displaying. And on very large documents, filtering may have some measurable impact. How much depends on the page size, the actual definition of the filter(s), etc. See below. Most other actions have little to no impact on speed.

Also, when filtering is enabled but zlib support isn't available, compression is often disabled (see prevent-compression). This can have an impact on speed as well, although it's probably smaller than you might think. Again, the page size, etc. will determine how much of an impact.

### 4.2. I notice considerable delays in page requests. What's wrong?

If you use any `filter` action, such as filtering banners by size, web-bugs etc, or the `deanimate-gifs` action, the entire document must be loaded into memory in order for the filtering mechanism to work, and nothing is sent to the browser during this time.

The loading time typically does not really change much in real numbers, but the feeling is different, because most browsers are able to start rendering incomplete content, giving the user a feeling of "it works". This effect is more noticeable on slower dialup connections. Extremely large documents may have some impact on the time to load the page where there is filtering being done. But overall, the difference should be very minimal. If there is a big impact, then probably some other situation is contributing (like anti-virus software).

Filtering is automatically disabled for inappropriate MIME types. But note that if the web server mis-reports the MIME type, then content that should not be filtered, could be. Privoxy only knows how to differentiate filterable content because of the MIME type as reported by the server, or because of some configuration setting that enables/disables filtering.

### 4.3. What are "http://config.privoxy.org/" and "http://p.p/"?

http://config.privoxy.org/ is the address of Privoxy's built-in user interface, and http://p.p/ is a shortcut for it.

Since Privoxy sits between your web browser and the Internet, it can simply intercept requests for these addresses and answer them with its built-in "web server".

This also makes for a good test for your browser configuration: If entering the URL http://config.privoxy.org/ takes you to a page saying "This is Privoxy ...", everything is OK. If you get a page saying "Privoxy is not working" instead, then your browser didn't use Privoxy for the request, hence it could not be intercepted, and you have accessed the real web site at config.privoxy.org.

### 4.4. How can I submit new ads, or report problems?

Please see the Contact section for various ways to interact with the developers.

### 4.5. If I do submit missed ads, will they be included in future updates?

Whether such submissions are eventually included in the `default.action` configuration file depends on how significant the issue is. We of course want to address any potential problem with major, high-profile sites such as Google, Yahoo, etc. Any site with global or regional reach, has a good chance of being a candidate. But at the other end of the spectrum are any number of smaller, low-profile sites such as for local clubs or schools. Since their reach and impact are much less, they are best handled by inclusion in the user's `user.action`, and thus would be unlikely to be included.

### 4.6. Why doesn't anyone answer my support request?

Rest assured that it has been read and considered. Why it is not answered, could be for various reasons, including no one has a good answer for it, no one has had time to yet investigate it thoroughly, it has been reported numerous times already, or because not enough information was provided to help us help you. Your efforts are not wasted, and we do appreciate them.

### 4.7. How can I hide my IP address?

If you run both the browser and Privoxy locally, you cannot hide your IP address with Privoxy or ultimately any other software alone. The server needs to know your IP address so that it knows where to send the responses back.

There are many publicly usable "anonymous" proxies out there, which provide a further level of indirection between you and the web server.

However, these proxies are called "anonymous" because you don't need to authenticate, not because they would offer any real anonymity. Most of them will log your IP address and make it available to the authorities in case you violate the law of the country they run in. In fact you can't even rule out that some of them only exist to *collect* information on (those suspicious) people with a more than average preference for privacy.

If you want to hide your IP address from most adversaries, you should consider chaining Privoxy with Tor. The configuration details can be found in How do I use Privoxy together with Tor section just below.

### 4.8. Can Privoxy guarantee I am anonymous?

No. Your chances of remaining anonymous are improved, but unless you chain Privoxy with Tor or a similar proxy and know what you're doing when it comes to configuring the rest of your system, you should assume that everything you do on the Web can be traced back to you.

Privoxy can remove various information about you, and allows you more freedom to decide which sites you can trust, and what details you want to reveal. But it neither hides your IP address, nor can it guarantee that the rest of the system behaves correctly. There are several possibilities how a web sites can find out who you are, even if you are using a strict Privoxy configuration and chained it with Tor.

Most of Privoxy's privacy-enhancing features can be easily subverted by an insecure browser configuration, therefore you should use a browser that can be configured to only execute code from trusted sites, and be careful which sites you trust. For example there is no point in having Privoxy modify the User-Agent header, if websites can get all the information they want through JavaScript, ActiveX, Flash, Java etc.

A few browsers disclose the user's email address in certain situations, such as when transferring a file by FTP. Privoxy does not filter FTP. If you need this feature, or are concerned about the mail handler of your browser disclosing your email address, you might consider products such as NSClean.

Browsers available only as binaries could use non-standard headers to give out any information they can have access to: see the manufacturer's license agreement. It's impossible to anticipate and prevent every breach of privacy that might occur. The professionally paranoid prefer browsers available as source code, because anticipating their behavior is easier. Trust the source, Luke!

### 4.9. A test site says I am not using a Proxy.

Good! Actually, they are probably testing for some other kinds of proxies. Hiding yourself completely would require additional steps.

### 4.10. How do I use Privoxy together with Tor?

Before you configure Privoxy to use Tor, please follow the User Manual chapters 2. Installation and 5. Startup to make sure Privoxy itself is setup correctly.

If it is, refer to Tor's extensive documentation to learn how to install Tor, and make sure Tor's logfile says that "Tor has successfully opened a circuit" and it "looks like client functionality is working".

If either Tor or Privoxy isn't working, their combination most likely will neither. Testing them on their own will also help you to direct problem reports to the right audience. If Privoxy isn't working, don't bother the Tor developers. If Tor isn't working, don't send bug reports to the Privoxy Team.

If you verified that Privoxy and Tor are working, it is time to connect them. As far as Privoxy is concerned, Tor is just another proxy that can be reached by socks4 or socks4a. Most likely you are interested in Tor to increase your anonymity level, therefore you should use socks4a, to make sure DNS requests are done through Tor and thus invisible to your local network.

Since Privoxy 3.0.5, its main configuration file is already prepared for Tor, if you are using a default Tor configuration and run it on the same system as Privoxy, you just have to edit the forwarding section and uncomment the line:

    #        forward-socks4a             /     127.0.0.1:9050 .

This is enough to reach the Internet, but additionally you might want to uncomment the following forward rules, to make sure your local network is still reachable through Privoxy:

    #        forward         192.168.*.*/     .
    #        forward            10.*.*.*/     .
    #        forward           127.*.*.*/     .

Unencrypted connections to systems in these address ranges will be as (un)secure as the local network is, but the alternative is that your browser can't reach the network at all. Then again, that may actually be desired and if you don't know for sure that your browser has to be able to reach the local network, there's no reason to allow it.

If you want your browser to be able to reach servers in your local network by using their names, you will need additional exceptions that look like this:

    #        forward           localhost/     .

Save the modified configuration file and open http://config.privoxy.org/show-status/ in your browser, confirm that Privoxy has reloaded its configuration and that there are no other forward lines, unless you know that you need them. If everything looks good, refer to Tor Faq 4.2 to learn how to verify that you are really using Tor.

Afterward, please take the time to at least skim through the rest of Tor's documentation. Make sure you understand what Tor does, why it is no replacement for application level security, and why you probably don't want to use it for unencrypted logins.

### 4.11. Might some things break because header information or content is being altered?

Definitely. It is common for sites to use browser type, browser version, HTTP header content, and various other techniques in order to dynamically decide what to display and how to display it. What you see, and what I see, might be very different. There are many, many ways that this can be handled, so having hard and fast rules, is tricky.

The "User-Agent" is sometimes used in this way to identify the browser, and adjust content accordingly.

Also, different browsers use different encodings of non-English characters, certain web servers convert pages on-the-fly according to the User Agent header. Giving a "User Agent" with the wrong operating system or browser manufacturer causes some sites in these languages to be garbled; Surfers to Eastern European sites should change it to something closer. And then some page access counters work by looking at the "Referer" header; they may fail or break if unavailable. The weather maps of Intellicast have been blocked by their server when no "Referer" or cookie is provided, is another example. (But you can forge both headers without giving information away). There are many other ways things can go wrong when trying to fool a web server. The results of which could inadvertently cause pages to load incorrectly, partially, or even not at all. And there may be no obvious clues as to just what went wrong, or why. Nowhere will there be a message that says "Turn off `fast-redirects` or else! "

Similar thoughts apply to modifying JavaScript, and, to a lesser degree, HTML elements.

If you have problems with a site, you will have to adjust your configuration accordingly. Cookies are probably the most likely adjustment that may be required, but by no means the only one.

### 4.12. Can Privoxy act as a "caching" proxy to speed up web browsing?

No, it does not have this ability at all. You want something like Squid or Polipo for this. And, yes, before you ask, Privoxy can co-exist with other kinds of proxies like Squid. See the forwarding chapter in the user manual for details.

### 4.13. What about as a firewall? Can Privoxy protect me?

Not in the way you mean, or in the way some firewall vendors claim they can. Privoxy can help protect your privacy, but can't protect your system from intrusion attempts. It is, of course, perfectly possible to use both.

### 4.14. I have large empty spaces / a checkerboard pattern now where ads used to be. Why?

It is technically possible to eliminate banners and ads in a way that frees their allocated page space. This could easily be done by blocking with Privoxy's filters, and eliminating the entire image references from the HTML page source.

But, this would consume considerably more CPU resources (IOW, slow things down), would likely destroy the layout of some web pages which rely on the banners utilizing a certain amount of page space, and might fail in other cases, where the screen space is reserved (e.g. by HTML tables for instance). Also, making ads and banners disappear without any trace complicates troubleshooting, and would sooner or later be problematic.

The better alternative is to instead let them stay, and block the resulting requests for the banners themselves as is now the case. This leaves either empty space, or the familiar checkerboard pattern.

So the developers won't support this in the default configuration, but you can of course define appropriate filters yourself to achieve this.

### 4.15. How can Privoxy filter Secure (HTTPS) URLs?

Since secure HTTP connections are encrypted SSL sessions between your browser and the secure site, and are meant to be reliably secure, there is little that Privoxy can do but hand the raw gibberish data though from one end to the other unprocessed.

The only exception to this is blocking by host patterns, as the client needs to tell Privoxy the name of the remote server, so that Privoxy can establish the connection. If that name matches a host-only pattern, the connection will be blocked.

As far as ad blocking is concerned, this is less of a restriction than it may seem, since ad sources are often identifiable by the host name, and often the banners to be placed in an encrypted page come unencrypted nonetheless for efficiency reasons, which exposes them to the full power of Privoxy's ad blocking.

"Content cookies" (those that are embedded in the actual HTML or JS page content, see `filter{content-cookies}`), in an SSL transaction will be impossible to block under these conditions. Fortunately, this does not seem to be a very common scenario since most cookies come by traditional means.

### 4.16. Privoxy runs as a "server". How secure is it? Do I need to take any special precautions?

On Unix-like systems, Privoxy can run as a non-privileged user, which is how we recommend it be run. Also, by default Privoxy listens to requests from "localhost" only.

The server aspect of Privoxy is not itself directly exposed to the Internet in this configuration. If you want to have Privoxy serve as a LAN proxy, this will have to be opened up to allow for LAN requests. In this case, we'd recommend you specify only the LAN gateway address, e.g. 192.168.1.1, in the main Privoxy configuration file and check all access control and security options. All LAN hosts can then use this as their proxy address in the browser proxy configuration, but Privoxy will not listen on any external interfaces. ACLs can be defined in addition, and using a firewall is always good too. Better safe than sorry.

### 4.17. Can I temporarily disable Privoxy?

Privoxy doesn't have a transparent proxy mode, but you can toggle off blocking and content filtering.

The easiest way to do that is to point your browser to the remote toggle URL: http://config.privoxy.org/toggle.

See the Bookmarklets section of the User Manual for an easy way to access this feature. Note that this is a feature that may need to be enabled in the main `config` file.

### 4.18. When "disabled" is Privoxy totally out of the picture?

No, this just means all optional filtering and actions are disabled. Privoxy is still acting as a proxy, but just doing less of the things that Privoxy would normally be expected to do. It is still a "middle-man" in the interaction between your browser and web sites. See below to bypass the proxy.

### 4.19. How can I tell Privoxy to totally ignore certain sites?

Bypassing a proxy, or proxying based on arbitrary criteria, is purely a browser configuration issue, not a Privoxy issue. Modern browsers typically do have settings for not proxying certain sites. Check your browser's help files.

### 4.20. My logs show Privoxy "crunches" ads, but also its own internal CGI pages. What is a "crunch"?

A "crunch" simply means Privoxy intercepted something, nothing more. Often this is indeed ads or banners, but Privoxy uses the same mechanism for trapping requests for its own internal pages. For instance, a request for Privoxy's configuration page at: http://config.privoxy.org, is intercepted (i.e. it does not go out to the 'net), and the familiar CGI configuration is returned to the browser, and the log consequently will show a "crunch".

Since version 3.0.7, Privoxy will also log the crunch reason. If you are using an older version you might want to upgrade.

### 4.21. Can Privoxy effect files that I download from a webserver? FTP server?

From the webserver's perspective, there is no difference between viewing a document (i.e. a page), and downloading a file. The same is true of Privoxy. If there is a match for a `block` pattern, it will still be blocked, and of course this is obvious.

Filtering is potentially more of a concern since the results are not always so obvious, and the effects of filtering are there whether the file is simply viewed, or downloaded. And potentially whether the content is some obnoxious advertisement, or Mr. Jimmy's latest/greatest source code jewel. Of course, one of these presumably is "bad" content that we don't want, and the other is "good" content that we do want. Privoxy is blind to the differences, and can only distinguish "good from bad" by the configuration parameters we give it.

Privoxy knows the differences in files according to the "Content Type" as reported by the webserver. If this is reported accurately (e.g. "application/zip" for a zip archive), then Privoxy knows to ignore these where appropriate. Privoxy potentially can filter HTML as well as plain text documents, subject to configuration parameters of course. Also, documents that are of an unknown type (generally assumed to be "text/plain") can be filtered, as will those that might be incorrectly reported by the webserver. If such a file is a downloaded file that is intended to be saved to disk, then any content that might have been altered by filtering, will be saved too, for these (probably rare) cases.

Note that versions later than 3.0.2 do NOT filter document types reported as "text/plain". Prior to this, Privoxy did filter this document type.

In short, filtering is "ON" if a) the content type as reported by the webserver is appropriate and b) the configuration allows it (or at least does not disallow it). That's it. There is no magic cookie anywhere to say this is "good" and this is "bad". It's the configuration that lets it all happen or not.

If you download text files, you probably do not want these to be filtered, particularly if the content is source code, or other critical content. Source code sometimes might be mistaken for Javascript (i.e. the kind that might open a pop-up window). It is recommended to turn off filtering for download sites (particularly if the content may be plain text files and you are using version 3.0.2 or earlier) in your `user.action` file. And also, for any site or page where making any changes at all to the content is to be avoided.

Privoxy does not do FTP at all, only HTTP and HTTPS (SSL) protocols.

### 4.22. I just downloaded a Perl script, and Privoxy altered it! Yikes, what is wrong!

Please read above.

### 4.23. Should I continue to use a "HOSTS" file for ad-blocking?

One time-tested technique to defeat common ads is to trick the local DNS system by giving a phony IP address for the ad generator in the local `HOSTS` file, typically using `127.0.0.1`, aka `localhost`. This effectively blocks the ad.

There is no reason to use this technique in conjunction with Privoxy. Privoxy does essentially the same thing, much more elegantly and with much more flexibility. A large `HOSTS` file, in fact, not only duplicates effort, but may get in the way and seriously slow down your system. It is recommended to remove such entries from your `HOSTS` file. If you think your hosts list is neglected by Privoxy's configuration, consider adding your list to your `user.action` file:

      { +block }
       www.ad.example1.com
       ad.example2.com
       ads.galore.example.com
       etc.example.com

### 4.24. Where can I find more information about Privoxy and related issues?

Other references and sites of interest to Privoxy users:

http://www.privoxy.org/, the Privoxy Home page.

http://www.privoxy.org/faq/, the Privoxy FAQ.

http://sourceforge.net/projects/ijbswa/, the Project Page for Privoxy on SourceForge.

http://config.privoxy.org/, the web-based user interface. Privoxy must be running for this to work. Shortcut: http://p.p/

http://sourceforge.net/tracker/?group_id=11118&atid=460288, to submit "misses" and other configuration related suggestions to the developers.

http://www.junkbusters.com/ht/en/cookies.html, an explanation how cookies are used to track web users.

http://www.junkbusters.com/ijb.html, the original Internet Junkbuster.

http://privacy.net/, a useful site to check what information about you is leaked while you browse the web.

http://www.squid-cache.org/, a popular caching proxy, which is often used together with Privoxy.

http://www.pps.jussieu.fr/~jch/software/polipo/, Polipo is a caching proxy with advanced features like pipelining, multiplexing and caching of partial instances. In many setups it can be used as Squid replacement.

http://tor.eff.org/, Tor can help anonymize web browsing, web publishing, instant messaging, IRC, SSH, and other applications.

http://www.privoxy.org/developer-manual/, the Privoxy developer manual.

### 4.25. I've noticed that Privoxy changes "Microsoft" to "MicroSuck"! Why are you manipulating my browsing?

We're not. The text substitutions that you are seeing are disabled in the default configuration as shipped. You have either manually activated the "`fun`" filter which is clearly labeled "Text replacements for subversive browsing fun!" or you are using an older Privoxy version and have implicitly activated it by choosing the "Advanced" profile in the web-based editor. Please upgrade.

### 4.26. Does Privoxy produce "valid" HTML (or XHTML)?

Privoxy generates HTML in both its own "templates", and possibly whenever there are text substitutions via a Privoxy filter. While this should always conform to the HTML 4.01 specifications, it has not been validated against this or any other standard.

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