Preventing Baseband Update

Edit
UpdateBaseband Reencrypt the restore ramdisk Repack the IPSW Prepare device for custom firmware using redsn0w Restore the IPSW to iTunes in pwned DFU Mode using the appropriate method (Look at Restoring The Modified IPSW Section)  You must load a patched iBSS/iBEC for this to work. Using an original IPSW will not work, because redsn0w's pwned DFU Mode doesn't patch sigchecks in iBSS (which is loaded from the IPSW).
 * 1) Unpack custom IPSW
 * 2) Decrypt Restore Ramdisk using xpwntool and mount it
 * 3) Navigate to /usr/local/share/restore/
 * 4) Edit options.plist on the Restore Ramdisk (Ignore any other settings specified in the plist, don't edit them)

Idea
One could hook the function HookAMRestorePerformRecoveryModeRestore in the MobileDevice Library that iTunes uses, overriding the restore options CFDictionaryRef object. This would *THEORETICALLY* allow for not updating baseband without patches/jailbreak. A practical application for this is if someone absolutely needs to update to the latest firmware for work or other reasons, and can't as they must wait for PwnageTool to provide support.

Restoring The Modified IPSW
Firmwares like 4.2.1 and above have baseband checks on the Restore Ramdisk. If the modified IPSW is restored, iTunes will give an Error 1015 and the iDevice will be in a recovery mode loop which cannot be exited by TinyUmbrella or the 'setenv auto-boot true' command

Fortunately, the Update Ramdisk does not contain that baseband check so if the Update Method is used which is mentioned below, iTunes will give an Error 1013 and it can be exited by TinyUmbrella and the iRecovery command.

Update Method

 * Windows Users, Open iTunes. hold the Shift button and click Update then select the modified IPSW
 * Mac Users, Open iTunes, hold the Alt button and click Update then select the modified IPSW

Restore Method

 * Windows Users, Open iTunes. hold the Shift button and click Restore then select the modified IPSW
 * Mac Users, Open iTunes, hold the Alt button and click Restore then select the modified IPSW

TinyUmbrella/Cydia Server Method (iPhone 4)
The iPhone 4 requires a AT+XNONCE key signature from Apple in order to update the baseband. Pointing the hosts file to Cydia Server or running TinyUmbrella will allow this request for signature to be ignored, thus preventing a baseband update.


 * This only works if Cydia Server/TinyUmbrella accepts the firmware's SHSH.
 * This method 'works' with iOS 4.2.1, but in the restore ramdisk there is a baseband version check. If it doesn't match, it will crash before the Apple logo with the loading bar (the 2nd, first-boot one, not the restore one) appears. It will boot and crash again. The old 'Kick out of recovery mode' methods or "setenv auto-boot true" won't work, because it's not the problem that the auto-boot is false. So this method is actually not useful for iOS 4.2.1+.
 * greenpois0n RC6 will fix this as part of the jailbreaking process. The latest version of TinyUmbrella also features a new Fix Recovery function.
 * 1) Edit the hosts file and add the line "74.208.10.249 gs.apple.com" without the quotes, or run TinyUmbrella after saving the firmware's SHSH. If Cydia Server hasn't got your SHSH, but you have it locally, use TSS Server method in TinyUmbrella.
 * 2) Use the "Restore" button in iTunes to update if your firmware version is below 4.2 else use the "Update" button in iTunes to update.
 * 3) You will get Error 1013 and it can be easily bypassed by using the Exit Recovery Mode button in TinyUmbrella or typing 'setenv auto-boot true' and 'saveenv' in iRecovery

iH8Sn0w's Method
sn0wbreeze has a "Baseband Preservation Mode" since version 2.2. iH8sn0w confirmed that his method is not the same as the above mentioned methods.

This method can also be used on the iPhone 3GS and the iPhone 4 to downgrade from the 4.3 betas and above back to 4.2.1, as long as the device can be restored (and activated) to iOS 4.1 or an earlier version.

To get some more details about what sn0wbreeze with the option "Baseband Preservation Mode" does, here a short analysis - a comparison of the generated ipsw for iOS 4.3 (iPhone 4):

Root Filesystem (038-0688-006.dmg)
(Details to be added.)

options.plist
The file options.plist in the folder \ramdisk\usr\local\share\restore is changed. These are the changes:
 * first entry added: CreateFilesystemPartitions = true
 * changed value: SystemPartitionSize 1024 changed to 1050
 * added last entry: UpdateBaseband = false

ASR
In the folder \ramdisk\usr\sbin\ the file asr has been renamed to asr_orig and a patched file of size 180832 bytes added. The patches are: 0001321A: DF F8 0001321A: FD E7

0002BFC6: ED B7 6C AA D3 AF A0 B4 90 08 6D 63 51 46 17 0B 8A 40 8D C4 0002BFC6: 6E DB 54 D3 DB 60 A8 FB 74 47 28 EB 02 33 EA FA 20 B6 7E B5

0002C1BA: FA DE 0C 01 0002C1BA: F6 B6 D0 8F

Images
Two files were changed from the Apple logo to the iH8sn0w logo. Both are called applelogo.png and they are located in \ramdisk\usr\share\progressui\images-1x\ and in \ramdisk\usr\share\progressui\images-2x\.

Patches in iBSS
There are several patches in iBSS (first line is original, second line is the patched version): 000000FC: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 000000FC: 03 A2 13 68 1B B1 50 68 C8 50 08 32 F9 E7 70 47 E0 A9 8C 00 30 E0 00 20 A4 AC 92 00 06 9B 0B B1 A8 AC 92 00 00 23 04 D0 D8 D3 71 00 EA D1 01 20 E4 A9 8C 00 00 20 2D E0 5C DB 27 00 01 00012C26: FF F7 73 FE 00012C26: 00 20 00 20 00012C4C: FF F7 60 FE 00012C4C: 00 20 00 20 00012C70: FF F7 B6 FE 00012C70: 00 20 00 20 00012C98: FF F7 3A FE 00012C98: 00 20 00 20 00012CBA: FF F7 29 00 00012CBA: 00 20 00 20 00012CDE: FF F7 17 FE 00012CDE: 00 20 00 20 00012D02: FF F7 05 FE 00012D02: 00 20 00 20 00012D20: FF F7 F6 FD 00012D20: 00 20 00 20 00012D44: FF F7 E4 FD 00012D44: 00 20 00 20 0001345A: 08 F0 0D 0001345A: EC F7 4F 0001AD38: 4F F0 FF 30 0001AD38: 00 20 00 20