How to solve the "this device isn' t eligible for the requested build" error during iPhone upgrade

From: http://tonyking2010.iteye.com/blog/1263416

After the upgrade of iturns to 10.5, my 3gs prompts that there is an update, which is to be updated to 5.0.1. But when I update it, I find that it cannot be updated, and the error is: this device isn't eligible for the requested build.

I cannot recover to the previous backup. My cell phone is jailbroken. I got it online and finally solved it.
Method:
Mac environment !!!
1. To enable terminal, Su admin, and Su root, you must enter the password
2. VI/private/etc/hosts
3. Find the last line to see if there is any gs.apple.com
4. Add a comment in front of the comment #
5. Save and use iturns to upgrade your phone.
I will not talk about how to use VI. It has nothing to do with this article.
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It is simpler to determine whether there are any methods:
Access: http://gs.apple.com in a browser
If an accessible page is displayed, the hosts file contains
If 404 tells you that the page does not exist, it indicates that the name does not exist.
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The following is an English website:
Thankfully the solution is simple and falls into one of two areas:

The IOS version that's been downloaded isn't the right one for the device.
There's an entry in your host file redirecting Apple traffic to another location.
In most cases, it'll be the second option. As a quick test on the machine where you're re trying to do a restore put the following into a web browser
Http://gs.apple.com if an apple 404 page (page not found) is displayed then you shoshould be good to restore and therefore you shoshould re-download the required firmware, reboot both your computer and the iPhone and try again. if an index page is displayed (like
Displayed below) then it means you 've got entries in your host file you need to remove.

The process for removal on Mac and Windows machines is slightly different but we 've detailed them below:

Mac OS X

1. on your Mac, open your Applications folder.

2. navigate to the utilities folder.

3. Open terminal.

4. Type "sudo nano/etc/hosts" (without quotes) and press Enter.

5. enter your password

6. Use the down arrow key to find the gs.apple.com entries. Once the cursor is in front, make sure you comment out the line (s) by entering # in front
Of the text.

7. Save the file by pressing Control + O.

8. Exit the nano editor by pressing Control + X.

9. Restore your idevice.

Windows

1. Press start, then programs, then accessories

2. Launch notepad or WordPad

3. Click Open from file menu.

4. Browse to Windows/system32/Drivers/etc

5. In "files of Type:" select all documents

6. Open "hosts"

7. Delete all line that have gs.apple.com or similar

8. Hit save

9. Restore your device