[rnhmjoj]

All you're asking is already possible with Privoxy[1], which is even stronger than a browser adblocker. It's a very old software: it used to be unmaintained and lacking some essential features, but thankfully the development resumed and is now fully fuctional again with the modern web.

It can be used as an adblocker based on domain, request path, HTTP headers, etc, but it can do much more. It can redirect requests (for example, replacing assets from a CDN with a local cache), modify headers (stripping or making cookies temporary, changing user agent, etc.) and even rewrite the content of web pages using regular expressions or any external program.

By default, it has only a basic configuration that blocks tracking and ads, but there are tools[2] that convert adblock rules to the Privoxy format, so it will be functionally equivalent to adblock.

It acts as a CONNECT proxy, so you can run it locally or on a router and if combined with a NAT rule, it can also work transparently (obviously, you need to manually trust a CA certificate for https).

[1]: https://www.privoxy.org/

[2]: https://github.com/essandess/adblock2privoxy

[alpaca128]

Can it let me pick any visible HTML element on a website to filter out just by clicking on it? Can it block content of a website without interfering with a `curl` request or a file download from a random messenger app?

All those things are trivial to do in a browser plugin but probably a total workaround-filled pain on any other layer in the system.

[rnhmjoj]

> Can it let me pick any visible HTML element on a website to filter out just by clicking on it?

The best it can offer is a CGI editor to change its configuration from the browser. I don't see how you could implement something like this: it's either interactive or a passive network element, but not both.

> Can it block content of a website without interfering with a `curl` request or a file download from a random messenger app?

This is really trivial, just don't proxy them (ie don't set the http_proxy variable).

[unicornporn]

Oh, Privoxy. Must've been 20 years since I used that. But, can it block certain elements with certain IDs? And do I have to disable DoH to make it work?

[rnhmjoj]

> can it block certain elements with certain IDs?

If you mean hiding an element, yes it's possible: you can either inject CSS into the page or write a filter to remove the HTML entirely. For example, adblock2privoxy generates both Privoxy rules to block requests and stylesheets to hide elements (you need a local webserver for this, though).

> And do I have to disable DoH to make it work?

It's Privoxy, not the browser, that will do the DNS queries. So, no: it will work regardless of DoH.

[danShumway]

A couple of criticisms:

1) The converter you link has 62 stars and hasn't been updated in 2 years. Additionally I'm seeing multiple issues about basic adblock rules not taking effect. Short version, I would not trust this repo to convert rules.

This is kind of exactly what I'm talking about with the difficulty of keeping pace with what is essentially a shared standard in the adblocking community. It's not enough to write one converter that gets updated every 2 years, in the space of those 2 years, Ublock Origin has expanded the syntax it supports. Adblocking is a cat-and-mouse game, there isn't a single set of features that can implemented once and then the software marked as "done".

2) Even assuming that converter does work (which I am doubtful of), Ublock Origin uses a superset of the adblock rules format, so you have to target what Ublock Origin supports, not just what adblockers in general do.

And obviously I'm not going to try and recreate those lists myself manually, I don't have the time or energy to do that. They have to be 100% consumable from upstream.

----

Okay, moving on to Privoxy itself:

1) On the community aspect again, I don't see dedicated Reddit groups devoted to finding every single broken website on this software. I don't see a public issue tracker. It seems to be following the old FOSS philosophy of developing software primarily on mailing lists someplace, which is fine for some software but not fine for something that is highly community dependent like adblocking.

You say the software is being actively developed again, I don't see any way to easily confirm that. I don't see any way to easily figure out how many people are using this and verifying that it works.

2) As far as I can tell, this doesn't support DoH. That is also kind of a dealbreaker for me, I don't want to make myself less secure in one area to make myself more secure in another. This is a solveable problem: if Privoxy was being set up as a local DoH server as well, and it was using DoH itself to query/cache results, then the issue would almost completely go away.

However, am I correct in guessing that Privoxy is also going to struggle in the future with encrypted SNI, or with the fact that my browser strips referrer headers from requests?

3) I'm looking at Privoxy's pattern documentation[0], and correct me if I'm wrong but it doesn't seem to support contextual blocking at all. In Ublock Origin I can do rules like:

  $script,third-party,domain=imgbox.com

My original criticism of DNS blocking in general was that it lacked context information, so it's just flat-out not acceptable for a Ublock Origin replacement to lack the ability to distinguish between a third-party request and a first-party request. That's critical functionality. Maybe I'm missing something here, but I've gone over the Actions and Template file documentation and I don't see the words "third-party" even mentioned anywhere.

4) Privoxy seems to lack the ability to block iframes, or at most it seems to have the ability to strip them from the HTML itself. That's not enough, sometimes iframes get dynamically created after a page is loaded, and modifying the HTML is not enough to block that.

5) I don't see any way to mark sites as trusted (probably related to point #3). So there doesn't seem to be a way for me to disable Privoxy when I'm on a specific site.

6) I don't see anything in the docs about CNAME unmasking. And CNAME cloaking isn't a theoretical attack, there are websites in the wild using that technique.

7) Browser integration also seems to be lacking. This isn't the biggest problem, I can tolerate annoyance, but it's a little bit of a quality of life issue.

8) And so on. Most of UBlock Origin's dynamic filtering syntax[1] seems to be unsupported. It's very possible I'm misreading the docs, or the docs are out of date or there's a trick to make it work, but if that's the case, that's also a problem, because then the docs need to be clearer.

----

None of that is to say that Privoxy is bad software. It's just to say that it doesn't seem like it's an all-in-one replacement for what my browser does.

We often do defense-in-depth in this area. You can set up a Piihole, or a firewall, or a proxy server to handle adblocking for devices and applications that don't expose interfaces like the browser does. That's a good idea. But the farther away from the context of the application that you get, the harder it is to do really detailed blocking based on that context.

This is something fundamental about adblocking that people don't always seem to understand -- it's not an either/or proposition, it's not like you set up a proxy server and all of your browser configs become useless. The proxy server just adds another layer of defense.

[0]: https://www.privoxy.org/user-manual/actions-file.html#AF-PAT...

[1]: https://github.com/gorhill/uBlock/wiki/Static-filter-syntax

[sildur]

It makes me happy to see privoxy back again. Time to set it up back! :)

[hutrdvnj]

Can Privoxy do Javascript based blocking, e.g. stub methods?

[rnhmjoj]

No, there's nothing javascript specific in Privoxy, but there's also nothing stopping you from implementing it. You could write filters to inject custom code (like greasemonkey userscripts) or modifying the scripts in tranport.