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Julien Couvreur's programming blog and more

Ad blocking on iOS

Mobile browsers don’t support extensions like their desktop counterparts and most don’t have an ad blocker built-in. But it turns out that iOS (and probably Android and Windows Phone) supports good old “proxy auto-configuration” (PAC).
PAC is a mechanism by which the operating system uses a simple script file to choose when to use a proxy. The script receives the host and url of each web request and tells the operating system whether to connect directly (as normal) or instead use a proxy.
The trick is to use a blackhole proxy (which returns no content) for urls that are recognized as advertisement, based on a list of known domains and url patterns.

So I dug up an existing ad-blocking PAC file which seems somehow up-to-date (no-ads version 5.125 from John LoVerso), configured the blackhole proxy to Google’s DNS server ( port 53), and updated my iOS wi-fi settings to point to it. I tested in Chrome on iPhone and iPad and this method seems to work.

You can try this solution by following the instructions below, but please read the security considerations below first.
You should note that PAC only works for wi-fi in iOS, not on cellular or other connections.
Also, you should know that iOS 9 may have official support for ad blocking extensions. The details are not yet known.

How to install

On iOS, go to Settings > Wi-Fi and open the configuration for the wi-fi you’re connected to. At the bottom, switch the HTTP proxy option to “Auto” and copy and paste into the box.

PAC configuration in iOS

Security considerations

Configuring a PAC file into your operating system can be dangerous. If the PAC file is adversarial or was modified by a hacker, the attacker could send parts or all of your web traffic through a proxy of his choice.

What is typically recommended is for you to use your own copy of this file (you still have to host your copy securely).

The way I’m looking to solve this is to host the PAC file on a trusted CDN of immutable files. But I have not yet found an appropriate CDN.
This will allow you to review the contents of the PAC file you choose (it’s easy to check the code to see it only points to Google’s DNS servers as blackhole proxies) and have peace of mind that it cannot be surreptitiously updated.
On the other hand, this means you’ll have to update your OS settings if you want to use a newer version of the file.

Another approach I’m going to investigate to solve this security problem is trying to host the PAC file on the device itself. This would mean installing an iOS app containing a PAC file and referencing that file from the network settings of the OS. I’ll post an update once I try.

Any other ideas are appreciated.

Using Google DNS as blackhole

The idea of using Google DNS servers comes from the FAQ of Weblock, an iOS app which generates PAC files. The FAQ offers a good explanation for this choice:

  1. iOS requires dummy proxy to be a valid IP address accepting connections (so it’s not possible to use local IP address of your device, since there is no open port to connect to).
  2. It’s really responsive, fast and stable anywhere in the world.
  3. It’s NOT ABLE to handle HTTP/HTTPS traffic, since it’s a DNS server (it handles an entirely different protocol). It immediately closes the HTTP/HTTPS connection (which is perfect!).
  4. It’s widely recognized and well known IP, so you don’t have to be concerned about your privacy. We’re quite sure Google is not logging all web connection attempts made while blocking content from your device, since this dummy proxy is actually a DNS server supporting a different kind of requests.

Weblock also hosts some PAC files. Here’s a few I’ve seen referenced:, and

How computers work (part 1)

A while back, one of my nephews asked me how computers work. During our recent sailing trip, I spent some time with him to answer that question. I will share the introduction I gave him over the next few posts.
Feel free to drop a comment if something needs elaborating. A later post will list technical references for further reading.

“How do computers work?” can be answered at many levels, just like “How does biological life work?”. For life, you can explain that there is an ecosystem of plants and animals, the interaction of animals, the physical and behavioral attributes of individuals of a specie, the function of different organs in their body, the cellular structure of each organ, or the atomic structure of different types of cells.

For computers, there is a similar hierarchy of abstractions: ecosystem of computers (networks), individual computers formed of large components (screen, keyboard, proceesor, memory) and software, circuits forming processors and memory, and the atomic-size elements (transistors, wires) forming those circuits. At a high level, you plug the power, keyboard and screen. At a low level, electrons flow through circuits and light up the display.

I will mainly focus on the hardware part, starting from the bottom (transistor, see next post). We’ll build up to a processor by assembling simple and small components into more advanced and larger components. I won’t discuss the software part much (maybe later).
It is useful if you have been exposed to basics of software programming (mathematical primitives, conditionals, loops, variables, functions/modules), as the computer we will “build” will implement those instructions. I will also assume that you are familiar with logic (booleans, AND/OR/NOT operations) and binary representation of numbers (base-2/binary 110 is base-10/decimal 6; in binary 110 + 1010 = 10000 just like 6 + 10 = 16 in decimal).

Before laying the first brick, let me give an overview of what we are going to build. The processor is the central component of a computer. At every tick of the clock, it reads an instruction and executes it. In a sense, the processor is like a dull but diligent factory worker who follows a detailed recipe or script (1- “take a screw from shelf A”, 2- “take a part from shelf B”, 3- “attach the screw to the part”, 4- “put the result in a bin”, 5- “if no screws are left, call the supervisor”, and 6- “start over at step 1”).
The processor can execute different types of instructions, such as “read input number from memory A”, “add two numbers”, and “write output number into memory B”. The processor normally follows the instructions in order, but it can be instructed to jump to a different instruction (“jump to step X”, or “if number is zero, then jump to step Y, otherwise proceed to next instruction”).
Recipes are called algorithms. You can solve complex problems with a long list of such simple instructions, just like you can manufacture a car with a worker following a long list of small steps.

Next time, we’ll start our pyramid by looking at transistors. We’ll then assemble transistors to make simple logical components (AND, OR, NOT), followed by slightly more advanced arithmetic components (ADD). After that we’ll look at stateful components (used to store information) and build a simple processor and finally computer.

Getting videos out of iTunes U

I wanted to get a video out of iTunes U for viewing offline without iTunes or special apps.

The solution I found is to browse the iTunes U app in iOS and share a course by mail (see screenshot below). This sends out a link (like this one) which offers a web preview of the content. Luckily, the source of those preview pages also includes direct links to the media files.

Screenshot of sharing a course from iTunes U:

Screenshot of the email you receive:

mail screenshot

Look at the source code to find the video link:

    preview-album="Partnership for Urban Health Research - Media" 
    preview-artist="Judea Pearl" 
    preview-title="Theoretical Developments in Causal Inference"