How To Jailbreak iOS 4.2.1 On iPhone, iPad, Or iPod Touch

Jailbreak iOS 4.2.1 on iPhone, iPad, or iPod Touch at your own risk:

1. Download the redsn0w software: Mac or Windows version (more mirrors are listed on the Dev Team blog)

2. Upgrade to iTunes 10.1 if you haven't already.

3. Install the iOS 4.2.1 firmware via iTunes as usual.

4. Run the redsn0w software executable.

5. Redsn0w will ask you to select your IPSW file for your current firmware. Point it to one of the following, depending on your OS:

• Mac OS X: /~/Library/iTunes/iPad Software Updates (or iPhone/iPod Software Updates, as the case may be)
• Windows 7 or Vista: C:\[username]\AppData\Roaming\Apple Computer\iTunes\iPad Software Updates (or iPhone/iPod Software Updates)
• Windows XP: C:\Documents and Settings\[username]\Application Data\Apple Computer\iTunes\iPad Software Updates (or iPhone/iPod Software Updates)

6. Redsn0w should now say that the IPSW file was successfully identified. On the following screen, check "Install Cydia" then click "Next."

7. With the iOS device powered off and plugged into the computer, click "Next" again.

8. Redsn0w will now provide instructions on how to put your device into DFU mode. This is a combination of key presses, that are as follows: Hold the Power button for 1 second, then without releasing it, hold the Home button for 10 seconds. Finally, release Power but keep holding Home for 15 seconds.

9. One in DFU mode, the jailbreak will proceed automatically.

The iPhone Dev Team's redsn0w jailbreak tool is what's known as a "tethered" jailbreak, at least on newer Apple devices. This means is that if your phone ever loses power or you need to reboot your device, you have to connect it to your computer (i.e. "tether" it) in order to keep the device jailbroken. Failure to do so will result in the device resetting to an un-jailbroken state. If, for some reason, you aren't able to reboot the device while tethered, you can still recover your jailbreak by connecting it again to your computer and then booting it using redsn0w.

Redsn0w jailbreak works on the iPhone 4, iPod Touch 3 and 4, iPad and iPod Touch 2 (MC model) it will be tethered. On iPhone 3G, iPhone 3GS (older bootroom), and iPod Touch 2 (older model) it's not tethered.

[Source: readwriteweb.com]