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The following line shows the the four partition table entries of 16 bytes each. See here for a specification of the format. 

$ sudo hd ubuntu-16.10-desktop-amd64.iso -s 446 -n 64
000001be  80 00 01 00 00 5e e0 ff  00 00 00 00 00 80 2f 00  |.....^......../.|
000001ce  00 fe ff ff ef fe ff ff  54 24 2f 00 c0 12 00 00  |........T$/.....|
000001de  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
*
000001fe

The latter three are all zeros, while the first entry is the interesting one. Byte 0 (counting from 0) has value 0x80 which means that it does in fact describe "an active partition". Byte 4 describes the partition type, fx 0x0B is FAT32. However 0 is an invalid value. According to this source:

00 : -- Should NOT be used in an actual table entry! It does not indicate an unknown type, but rather an empty entry; in which case, all other fields in that 16-byte entry should be zero-filled as well.

So according to what I can read, the MBR data is invalid. What is going on?

I did this with Ubuntu 16.04 (Yakkety), downloaded via this link, and the file has md5sum 3f50877c05121f7fd8544bef2d722824.

Mads Skjern
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  • Can you provide the exact link you got the ISO from? Also I don't think the second to fourth partition file tables are always strictly enforced when the first is flagged as boot. Nonetheless it will be fun to poke around the exact same one you acquired. – WinEunuuchs2Unix Oct 25 '16 at 17:08
  • Take your time and just put the exact link in your question. I'll look at it tonight after work. – WinEunuuchs2Unix Oct 25 '16 at 17:14
  • Also see: http://askubuntu.com/q/153833/158442 – muru Oct 26 '16 at 06:05

1 Answers1

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I don't have much idea about partition systems, but I believe the reason is that it's not a plain MBR filesystem. Using parted:

$ parted ubuntu-16.10-desktop-amd64.iso p 
Warning: The driver descriptor says the physical block size is 2048 bytes, but Linux says it is 512 bytes.
Ignore/Cancel? Ignore                                                     
Model:  (file)
Disk /home/muru/ubuntu-16.10-desktop-amd64.iso: 6375MB
Sector size (logical/physical): 2048B/512B
Partition Table: mac
Disk Flags: 

Number  Start   End     Size    File system  Name   Flags
 1      2048B   6143B   4096B                Apple
 2      1582MB  1584MB  2458kB               EFI

Note: Partition Table: mac. And the Wikipedia article for Apple Partition Map says:

Some hybrid discs contain both an ISO 9660 primary volume descriptor and an Apple Partition Map, thus allowing the disc to work on different types of computers, including Apple systems.

I don't know why parted thinks the file is 6GB in size, though. However, file agrees that there's something involving Apple Partition Map here:

$ file ubuntu-16.10-desktop-amd64.iso -k | fold -w 80                        
ubuntu-16.10-desktop-amd64.iso: DOS/MBR boot sector ISO 9660 CD-ROM filesystem d
ata (DOS/MBR boot sector) 'Ubuntu 16.10 amd64' (bootable); partition 2 : ID=0xef
, start-CHS (0x3ff,254,63), end-CHS (0x3ff,254,63), startsector 3089492, 4800 se
ctors DOS/MBR boot sector ISO 9660 CD-ROM filesystem data (DOS/MBR boot sector) 
'Ubuntu 16.10 amd64' (bootable) Apple Driver Map, blocksize 2048, blockcount 370
08, devtype 0, devid 0, descriptors 0, contains[@0x800]: Apple Partition Map, ma
p block count 2, start block 1, block count 2, name Apple, type Apple_partition_
map ISO 9660 CD-ROM filesystem data, contains[@0x1000]: Apple Partition Map, map
 block count 2, start block 772373, block count 1200, name EFI, type Apple_HFS I
SO 9660 CD-ROM filesystem data, contains[@0x1800]: ISO 9660 CD-ROM filesystem da
ta FoxPro FPT, blocks size 37008, next free block index 1163003904\012- data
muru
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  • parted (well at least gparted) won't read an UEFI formatted USB flash drive / thumb drive properly. It drove me nuts a couple weeks ago and had to use dd to write 128 byte of zeros over the MBR so I could use gparted to reformat it and create a new Live USB. Which I had to do again and again and again until I told myself I didn't need one. – WinEunuuchs2Unix Oct 25 '16 at 23:40