Guide to Update iPhone 4 to iOS 4.2.1 While Preserving Old 01.59.00 Baseband for Unlock

Written By Sam on 6 January 2011
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Since the baseband requirements were changed in iOS 4.2.1, updating iPhone 4 to iOS 4.2.1 presented some problems as the baseband would get updated too and there would be no way to go back. Now, there is a way out and one can preserve the old 01.59.00 baseband while upgrading to iOS 4.2.1 and you can then unlock the phone with Ultrasn0w 1.2. As on date, this method works only on iPhone 4. Before proceeding, backup everything on the iPhone. Then:

  1. Download iOs 4.2.1 for iPhone 4.
  2. This step involves fooling itunes into thinking it is pointed to Apple servers while you change over to Saurik’s server. This is done by- going to C:WindowsSystem32driversetc. Then locating “hosts” file.
    – Mac users can go to “/etc/” directory using “go->go to folder” from Finder
    – Opening “hosts” file in Notepad(windows) or TextEdit(Mac) and adding “74.208.105.171 gs.apple.comline” line
    – Save this “hosts” file
  3. Start iTunes, In the sidebar click on iOS device icon, press+hold left “alt”(option) button on Mac or left “shift” in Windows on the keyboard andclick on “restore” (not Update or check for update) button in the iTunes screen and release this button. When iTunes prompts for a location for iOS 4.2.1 firmware file select the .ipsw file and click on “open” to restore the iPhone.
  4. During this process of restoring to iOS 4.2.1 you may see a 1013 error. Simply ignore it. The firmware should be upgraded without upgrading the baseband now.
  5. The iPhone will hang in the Recovery Mode, with “connect to iTunes” on the screen. Download TinyUmbrella, run it and then click “exit recovery” button. The iPhone should restart in a normal mode. Go to settings->about on the iPhone, scroll to check your model firmware (baseband) and it should display 01.59.00. Note this is unlockable with UltraSn0w. Now the iPhone 4 on 4.2.1 in the tethered mode can be jailbroken using Redsn0w 0.9.6 and untethered with Redsn0w 0..7. Next, when the device is jailbroken, it can be unlocked with Ultrasn0w 1.2 on iOS 4.2.1 with 01.59.00 baseband untouched.

Notes: Notepad should be started with administrator rights (right click notepad, select “run as administrator” option. Mac users should ensure they have full access while saving the host file. If there still is a problem copy the Host file to the desktop, change it as in the above steps and copy it back to the “/etc/” directory.

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