Apple launched iOS 4.0.2 to fix the exploit used by jailbreakme and stopped signing earlier firmware restore files (40.1 and before). So if you have iPhone 4 or iPhone 3Gs, you can’t restore the firmware to earlier version except for 4.0.2.
If anyone accidentally updated their iPhone 3Gs or iPhone 4 to iOS firmware 4.0.2, they can now downgrade it to 4.0.1 or below.
Please note, this hack only works for those who saved their SHSH blob to cydia server before the release of iOS 4.0.1 via Cydia “make my life easier” button.
The trick is to add Saurik’s server address to your system’s hosts file. It will make iTunes connect to his server instead of Apple, and making it think it’s connected to Apple server while restoring your iPhone iOS. This allows iTunes to access signatures already stored by Cydia “on file” feature.
Make sure you saved shsh blob and read the instructions completely and carefully before attempting to start the process.
Changing Hosts file in Windows
- Open notepad by right clicking and selecting open as Administrator
- Click File > Open and browse to C:/windows/System32/drivers/etc folder
- Add this line at the end of the hosts.txt file: 126.96.36.199 gs.apple.com
- save the hosts.txt file.
Now you can put your iPhone in dfu mode and restore back to iOS 4.0.1 or lower versions.
Changing Hosts file in Mac OS X
- Navigate to Finder, Click to select Go to Folder from the Go menu and Enter /etc and click Go button
- Right click on hosts file and open with Text Edit
- Add this line at the end of hosts.txt file: 188.8.131.52 gs.apple.com
- Save the hosts.txt file (save it as new file on your desktop named hosts.txt and then drag it to etc folder, replacing previous hosts.txt file)
Now you can put your iPhone in dfu mode and restore back to iOS 4.0.1 or lower versions from Mac OS X.