Where is partitions in MBR?

Question

I found lot of examples of MBR records on various systems, and they all have same structure, for example - as here.

So - first 512 bytes must have a Code area, Disc signature and Table of partitions.

My disk layout is:

$ lsblk                                                                                                                                                                                                    
NAME                       MAJ:MIN RM   SIZE RO TYPE MOUNTPOINT
sda                          8:0    0 596,2G  0 disk 
├─sda1                       8:1    0     2G  0 part /boot
├─sda2                       8:2    0    16G  0 part [SWAP]
├─sda3                       8:3    0   500G  0 part 
│ ├─kubuntu_vg-root (dm-0) 252:0    0    30G  0 lvm  /
│ └─kubuntu_vg-home (dm-1) 252:1    0   470G  0 lvm  /home
└─sda4                       8:4    0  78,2G  0 part /media/setevoy/4A50CF6C50CF5D77
sr0                         11:0    1  1024M  0 rom

Bootable device is sda4 here:

/dev/sda4   *  1086326784  1250260991    81967104    7  HPFS/NTFS/exFAT

And

 sda1 = /boot
 sda2 = swap
 sda3 = LVM physical volume with /home

Now, if I make:

$ sudo dd if=/dev/sda of=/tmp/mymbr bs=512 count=1

and run file on it - I can't see any similar to another MBR's googled:

$ file /tmp/mymbr 
/tmp/mymbr: x86 boot sector

While an "correct" (aka - googled) example display partitions, disks's IDs etc:

# file mbr.bin
mbr.bin: x86 boot sector;
partition 1: ID=0×83, active, starthead 1, startsector 63, 40949622 sectors;
partition 2: ID=0×82, starthead 254, startsector 40949685, 2088450 sectors;
partition 3: ID=0x8e, starthead 254, startsector 43038135, 74172105 sectors, code offset 0×48

Size is correct:

$ ls -lh /tmp/mymbr
-rw-r--r-- 1 root root 512 жов  6 11:18 /tmp/mymbr

using hexdump - I can see some data in file, including partitions (466 + 16 bytes):

$ hexdump -C -s 446 /tmp/mymbr 
000001be  00 20 21 00 83 35 70 05  00 08 00 00 00 00 40 00  |. !..5p.......@.|
000001ce  00 35 71 05 82 df 72 2d  00 08 40 00 00 00 00 02  |.5q...r-..@.....|
000001de  00 df 73 2d 83 b6 12 24  00 08 40 02 00 00 80 3e  |..s-...$..@....>|
000001ee  80 fe ff ff 07 fe ff ff  00 08 c0 40 00 70 c5 09  |[email protected]..|
000001fe  55 aa

But what's wrong with file?

Maybe version?

$ file --version
file-5.14
magic file from /etc/magic:/usr/share/misc/magic

UPD 1

$ sudo fdisk -ls /dev/sda

Disk /dev/sda: 640.1 GB, 640135028736 bytes
255 heads, 63 sectors/track, 77825 cylinders, total 1250263728 sectors
Units = sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disk identifier: 0x6fa60981

   Device Boot      Start         End      Blocks   Id  System
/dev/sda1            2048     4196351     2097152   83  Linux
/dev/sda2         4196352    37750783    16777216   82  Linux swap / Solaris
/dev/sda3        37750784  1086326783   524288000   83  Linux
/dev/sda4   *  1086326784  1250260991    81967104    7  HPFS/NTFS/exFAT

What does `fdisk -l /dev/sda` say? In particular, does it mention a disklabel type? — Stephen Kitt, Oct 06 '16 at 09:05
Yes - `fdisk` display all my partitions with types (Lin/Lix-swap/Lix/NTFS) and `sda4` marked as bootable (output added to this Q). — setevoy, Oct 06 '16 at 09:11
@rudimeier `-k` (_Don't stop at the first match, keep going._) made it! Thanks. Could you add it as answer? — setevoy, Oct 06 '16 at 09:20

score 3 · Accepted Answer · answered Oct 06 '16 at 09:28

You may have luck to see more information including partitions using

file -k # Don't stop at the first match, keep going.

Though on my systems it also does not show the partition table. (file 5.15 and 5.19).

But I wonder why file should print the partition table at all. IMO file is supposed to show what kind of data is it, not the the whole content. Better trust fdisk -l or other well known disk tools.

Where is partitions in MBR?

1 Answers1