Preventing Baseband Update

Step 1: Swap Ramdisks

Open the IPSW in a ZIP Editor and replace the Restore Ramdisk and the Update Ramdisk names with each other then follow Step 2
This is done to bypass baseband check

Step 2: Edit options.plist

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 the SystemPartitionSize in your plist file and leave it)

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
<plist version="1.0">
<dict>
	<key>UpdateBaseband</key>
        <false/>
</dict>
</plist>

5. Reencrypt the restore ramdisk
6. Repack the IPSW
7. Prepare device for custom firmware using redsn0w Just enter pwned DFU option
8. Restore IPSW to iTunes in pwned DFU Mode

This won't work with original IPSW. Pwned DFU mode doesn't patch sigchecks in iBSS, so the ramdisk won't load. You need to load patched iBSS/iBEC for this to work.

TinyUmbrella/Cydia Method for iPhone 4 (Doesnt work on iOS 4.2.1)

The iPhone 4 needs baseband signature to update the baseband.Pointing the hosts file to Saurik's server or running TinyUmbrella will allow this request for signature to be ignored thus preventing baseband update

Warning : This might only work if Cydia/TinyUmbrella accepts the latest firmware SHSH

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 firmwares SHSH
2. Use the restore button in iTunes to update firmware

Custom Firmware wont work with TinyUmbrella/Cydia Method