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Before we get to the good stuff I should warn you that hacking your iPhone WILL void your warranty and following these instructions could very well get you a higher than normal bill from AT&T or whoever your carrier is that does not support iPhone tethering officially.  You could potentially screw up your phone and if you follow these steps, iPhone MVP and myself are NOT liable for anything you screw up.

This tutorial is for those of you who are NOT currently jailbroken.  I have a separate tutorial for those already jailbroken right here. After you complete these steps, you will be jailbroken, unlocked, and have tethering enabled.  This is everything you will ever need in your iPhone life!  Everything you need is just past the break!

So, you’re here because you’re probably on AT&T and the 3.1.2 update broke your tethering and you want it back.  Well, we’re going to show you how!

1. First, you will need to visit blackra1n.com and download the program for your PC or Mac.
2. Next, connect your iPhone to your computer.
3. This step is optional but I chose to do it, sync your iPhone with iTunes allowing it to do a full backup.
4. Run the blackra1n program.
   
5. Click the make it ra1n button and your iPhone should immediately reboot and display Geohot’s picture.  This step should take about a minute or a little less. DO NOT UNPLUG YOUR IPHONE! YOU WILL SCREW IT UP AND I AM NOT RESPONSIBLE! YOU HAVE BEEN WARNED!
6. Once your iPhone has rebooted, you are now fully jailbroken and ready to continue on.
7. Next, locate the blackra1n app on your home screen and open it up.
8. You will need to select the sn0w option which will unlock your iPhone and enable the next part of this hack.  After the sn0w option is finished running, go ahead and install Cydia as well.  Once both Cydia and sn0w are installed, go ahead and select Uninstall blackra1n.  You don’t HAVE to, but I chose to do so on my iPhone.
   
9. Now, open Mobile Safari and navigate to: tr.im/312tether with will redirect you to http://m.peacefulinsanity.com/Tether.mobileconfig.  I made the shortened URL for our readers, its much easier to type out than that long thing!
10. Mobile Safari will close and then open a prompt for you to install a new carrier profile.  Go ahead and tap on Install
    
11. Next you will get a prompt saying that the authenticity cannot be verified.  That is because this profile is unsigned so go ahead and hit replace.
    
12. This will take about 10 seconds, once done, hold down the home and power buttons until your iPhone reboots.  This reboot WILL take longer than normal, it is fine.
13. Once your iPhone is done rebooting, go to Settings > General > Network > Internet Tethering.  If this worked successfully, you should be presented with the following:
    
14. Go ahead and switch the slider to own and proceed to connect following the directions shown on the screen:
    
15. Congratulations! You have successfully enabled tethering on your 3.1.2 iPhone!  If this is your first jailbreak, read on for some more hints.

Now that you are jailbroken, unlocked, and have enabled tethering, I strongly suggest you go to Cydia and check out some of the great work that this community has done.  Some of my personal favorites include SBSettings, Categories, and Winterboard.  One app that is not currently released that I’m helping to alpha test, is the Stack app.  It is the same concept as Categories however it integrates into springboard and is a much cleaner and more Apple-like experience.  If you wish to pay the $.99 for it, CategoriesSB is a worthwhile add-on to Categories as well.