How to Resolve iPhone 4S Error 21

Posted by Mayank on November 22, 2012 in iPad,iPhone

Trevor W. reported this issue:

The 2 errors I’m getting are iPhone Error 4S Error 21 and iPhone Error 4S Error 1600. If I fix one error, it just jumps to the other. Any help would be appreciated, just got this thing not too long ago.

It is most likely that you would have configure the software to allow the following domains to send and receive data from the Internet for some special purpose (like downloading a newer OS upgrade when all Apple servers are heavily burdened).

How to Resolve iPhone 4S Error 21

You can easily fix the iPhone Errore 21 or iPhone 4S error 1600 aby temporarily blocking Apple’s firmware signing services. Error 21 or Error 1600 are usually triggered when an unsigned firmware is used, often during an iOS downgrade or even during some restores.Follow the following easy steps to resolve iPhone error 21.
  • Quit iTunes
  • Locate your hosts file. If you’re using Mac OS X, it’s in /etc/hosts and if you’re using Windows, it’s in C:\windows\system32\drivers\etc\hosts
  • Open the hosts file with Administrator privileges
  • Comment whichever of the following lines are present in the hosts file: or and

In many cases, iPhone Error 21 or iPhone 4S errore 1600 will get resolved just by updating the hosts file and restarting you computer. If the issue remains unresolved, you need to do a few more steps after updating the hosts file:

  • Connect your iPhone or iPod or iPad to the same computer
  • Launch iTunes
  • Put the iPhone/iPad/iPod into DFU mode by turning the phone off and then holding down the sleep and power button for 10 seconds, then release the power button but hold onto the home button until iTunes tells you that the device is now in recover mode
  • Use the iTunes Restore feature as usual with the iOS device

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