Luckily, a couple weeks ago, I learned about launchctl. It is now the primary way in OSX to launch system daemons instead of init scripts or xinetd. It turns out, using launchctl, as root, you can execute a program in another context using its bsexec subcommand! If you use the loginwindow process id’s context you’d like to run your program in, it will now run just as if you were physically sitting at the keyboard.  The command:

isightcapture image.jpg

which would before fail if you ssh’d into a remote machine now becomes:

export PID=`ps auxww |grep loginwindow |grep -v grep |awk ‘{print $2}’`
launchctl bsexec $PID isightcapture image.jpg

Bamo! Now you’re breakin’ the law in style!

So, since I’ve got my crappy little image uploader working once again, I thought I’d share it with the rest of the world. I’m using sleepwatcher, isightcapture, and some other craziness that I hacked together to overlay some good information on a picture from your webcam and upload it to a website of your choice!

If you know what you’re doing, you should be able to follow my instructions just fine, if you don’t know what you’re doing, you might possibly blow up your mac and kill a kitten.  You have been warned!

First, download the following file (I’m going to assume it will download into your Downloads folder):

http://tompohl.com/files/locateme.tgz

Create a directory on your web server to host the supporting scripts and uploaded images for example:

mkdir /var/www/html/locateme

There are 10 types of people in this world.  Those who understand binary and those who don’t.

Let’s talk about numbering systems.  Specifically Decimal (base10), Hexadecimal (base16) and Binary (base2).

First off, what is this base stuff that you’re talking about.  The base number in a numbering system is the number of values represented by a single digit.

Decimal – Base 10

This is the numbering system that everyone should know.  Each digit can have up to 10 values 0,1,2,3,4,5,6,7,8,9.  You might be saying “Wait a minute! Why is it called base 10 if its highest number for a digit is 9?” I’d say, fantastic question! Here’s the secret: nothing is really something. For each additional digit added in a number, you increase the total number of values that you can represent by a factor of 10. That is:

1 digit  (0 to 9) = 10 values =  10 = 10^1

2 digits (00 to 99) = 100 values = 10 x 10 = 10^2

3 digits (000 to 999) = 1000 values = 10 x 10 x 10 = 10^3

Now, don’t get freaked out when I say 3 digits can be 000 to 999 because 000 is the same as 0 is the same as 0^3. Just don’t confuse it with 0^0 which would be 1 (I already told you, nothing is really something).

So, if you take the base of your numbering system (10 in this case) and take it to the power of the number of digits you’re using, you’ll get the total number of values you can represent with that number.  Note: the highest number you can represent is 1 less than the total number of values you could represent because nothing is really something!

Example math:

 1  (overflow aka carry the 1)
  9
+ 1
---
 10

Hexadecimal – Base 16

I’ve included hexadecimal in this discussion simply because it’s cool and you’ll find it in a lot of places on the web.  A single digit can represent up to 16 different values 0,1,2,3,4,5,6,7,8,9,A,B,C,D,E,F. That’s right, you just got your alphabet in my numbers and I just got my numbers in your alphabet! In this notation, A = 10, B = 11, and so forth up to F = 15. A lot of times, when you see a number that is represented in hex (the short name for hexadecimal) they might tip you off by placing a 0x before the value.  This is important BECAUSE once we get into multiple digits, a hex number without any letters looks just like a decimal number, but its value totally different!

1 digit (0 to F) = 16 values = 16 = 16^1

2 digits (00 to FF) = 256 values = 16 x 16 = 16^2

3 digits (000 to FFF) = 4096 values = 16 x 16 x 16 = 16^3

Example math (15 + 1 = 16 right? Here’s how you do it in hex):

 1  (overflow aka carry the 1)
  F
+ 1
---
 10

Binary – Base 2

OK now the whole reason for this entry. We’re talking binary 1’s and 0’s the language of LOVEly computers. In binary, there are only 2 values, 0 and 1, off and on. I’m only going to deal with unsigned binary numbers (no negative numbers) because in my future entries, I’ll be explaining things like masks and bytes in regards to TCP/IP networking. Besides, I’m a generally positive guy and try to not dwell on the negative. On to the numbers!

1 digit (0 and 1) = 2 values = 2 = 2^1

2 digits (00 through 11) = 4 values = 2 x 2 = 2^2

3 digits (000 through 111) = 8 values = 2 x 2 x 2 = 2^3

8 digits (00000000 through 11111111) = 256 values = 2 x 2 x 2 x 2 x 2 x 2 x 2 x 2 = 2^8

Example math (1 + 1 = 2 of course! Now, let’s see what that looks like in binary):

 1  (overflow aka carry the 1)
  1
+ 1
---
 10

Ahh yes! There it is! The punch line to our joke! Feel free to laugh OR cry OR just pitty me for regurgitating this stuff from memory.

Bring on your questions! Do you need more examples? You might be asking yourself, “But in the world does this have to do with the OpenVPN configuration that Tom’s working towards?” For that, you’ll have to keep coming back to find out!

Actually, a couple people have asked me to write up some instructions on getting remote access to a home / office network so they can use remote resources like Apple’s Time Capsule or iTunes music shares.

I’ve decided to rise to the challenge, BUT I feel that I need to lay a solid base of knowledge before I tackle explaining my solution.  My goal is to help you make smart decisions about designing your network.

Over the next several posts, I plan to cover the following topics:

1. Numbering systems (Decimal, Binary, Hexadecimal)
2. Bits, Nibbles, Bytes, Words
3. TCP/IP Networking
4. Using OpenVPN to access your network remotely

To some / most of you, a lot of this will be boring or old news to you, but please keep with it! If anything, correct where I’m wrong, call me out when you think I’m full of it, and most certainly, ask for clarification when you just don’t understand what I’m talking about.

Let’s DO this!