Disk Image Formats

iPhone Firmware (ipsw) files contain .dmg disk images beside other files.

The main filesystem .dmg disk image (the largest .dmg file) is decrypted to an UDZO .dmg disk image.

UDZO Image Format

The UDZO Image is an UDIF zlib-compressed read-only image which can be mounted on Mac OS X. UDZO Images can be converted to read/write UDRW Images with the hdiutil on Mac OSX:

hdiutil convert -format UDRW -o udrw.dmg udzo.dmg

UDRW Image Format

The UDRW Image is an UDIF read/write image which can be mounted on Mac OS X. UDRW Images can be converted to zlib-compressed read-only UDZO Images with the hdiutil on Mac OSX:

hdiutil convert -format UDZO udrw.dmg -o udzo.dmg

The resulted UDZO Image file can be checked with the asr utility:

asr -imagescan udzo.dmg