iTunes
How to fix iTunes errors.Apple

When iTunes fails to communicate with the Apple update server, you get the following error messages:

  • Error 17
  • Error 1639
  • Errors 3000-3020
  • Error 3194
  • Errors 3100-3999
  • The device is not eligible for the requested build

In such cases, install the latest version of iTunes and try to update or, restore your iOS device. If that does not resolve the issue, check out the following potential fixes.

Check for host files

Mac:

  • Quit iTunes
  • Launch Terminal
  • Execute the following command and press Enter/Return

Sudo nano / private/etc/hosts

  • Enter password to sign in to your computer when prompted and press Enter/Return
  • The terminal will display Hosts file
  • If you see any entries for gs.apple.com then either remove or comment in the entry
  • Go to gs.apple.com entry using arrow keys and add # symbol and a space at the beginning of gs.apple.com entry
  • Press Control -O to save the file
  • Press Enter key when asked for the file name and Control- X to exit the editor
  • Finally restart your PC

Windows

  • Do a swipe from the right edge of the screen, tap Search>> type run>> then tap Run icon.

Or, if you are using a mouse, point to the top right corner of the screen, move the mouse pointer down>> click Search>> type run>> click Run icon

  • Type %WinDir%System32DriversEtc in the Open box and click OK
  • Doing so will open Etc folder, double click the Hosts file to edit it
  • If you see entries for gs.apple.com then remove it and save the Hosts file

Now you can try to restore your iOS device. Error 3194 will, hopefully, not show up.

The error usually occurs if you have used Redsn0w jailbreak tool, which adds the entry to the Hosts file so that iTunes connects to Cydia servers instead of Apple update servers.

Disable firewall or, anti-virus software: Disable firewall or, any anti-virus software on your PC temporarily as they could prevent iTunes from connecting to Apple servers.

Check iOS software version: You might get an error message that reads, "This device isn't eligible for the requested build", while trying to update or, downgrade to a firmware which Apple has stopped signing. Recently, Apple has stopped signing iOS 8.1.2 or lower firmware.

Connectivity issue: On rare occasions a connectivity issue with the update server could be the reason iTunes Error 3194 crops up.

If none of the aforementioned fixes work out, try to update or restore your iOS device to some other PC, having a different internet connection.

[Source: Apple via iPhone hacks]