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104 lines
3.1 KiB
104 lines
3.1 KiB
---
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title: Mounting disk images on linux
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aliases: /posts/2022-08-06-mounting-disk-images-on-linux
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summary: >
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I sometimes create full-disk-images of USB drives or hard drives and in
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rare cases I have to look into them. This is how I usually do that.
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date: 2022-08-06T21:02:21+02:00
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lastmod: 2024-09-28T23:48:06+0000
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categories: [computerstuff]
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tags: [linux, raspberry-pi]
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---
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Let's assume you created a disk image with `dd` on a linux computer like
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```console
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$ sudo dd if=/dev/sda of=disk.img bs=4M status=progress conv=fsync
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```
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There are several partitions in that image and we want to access the linux
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filesystem on it. For reference, I'll bring in some old backup I made from a
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Raspberry Pi. That backup is taken from a 8GB sdcard, which is 2.6GB compressed
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with `xz`.
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When uncompressed, look at the partition table with _fdisk_:
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```console
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$ fdisk -l disk.img
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Festplatte disk.img: 7,4 GiB, 7948206080 Bytes, 15523840 Sektoren
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Einheiten: Sektoren von 1 * 512 = 512 Bytes
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Sektorgröße (logisch/physikalisch): 512 Bytes / 512 Bytes
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E/A-Größe (minimal/optimal): 512 Bytes / 512 Bytes
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Festplattenbezeichnungstyp: dos
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Festplattenbezeichner: 0x1b7f4bbb
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Gerät Boot Anfang Ende Sektoren Größe Kn Typ
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disk.img1 8192 532479 524288 256M c W95 FAT32 (LBA)
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disk.img2 532480 15523839 14991360 7,1G 83 Linux
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```
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We will refer to this output later again.
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## Using losetup
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The output of _fdisk_ is not that important to us, unless we have an unknown
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disk image that we need to inspect first. I already know the partitions. The
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first is the FAT32 partition used for UEFI and the second is the root file system.
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### Creating and mounting the loop device
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```console
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$ sudo losetup --partscan --find --show disk.img
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/dev/loop1
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```
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The second line is the output of the program. I used losetup already today, so
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this is not _loop0_ but _loop1_. You may get `/dev/loop0` usually.
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Mount the new virtual loop device to the directory that you like. This is
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`~/tmp` in my case.
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```console
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$ sudo mount /dev/loop1p2 tmp
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```
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### Removing the loop device
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```console
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$ sudo umount tmp
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$ sudo losetup -d /dev/loop1
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```
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## Using `fdisk` and `mount`
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From the output above, we see that `532480` is the starting unit of the linux
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filesystem in this image file. Further above you see the Units (Einheiten):
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1 Unit is 1 sector of 512 Bytes.
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I use a german speaking computer, so you might look for _Start_ or _Offset_ or
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_Beginning_---you know what to look for...
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We calculate the needed offset like: `532480 * 512 = 272629760`
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And the resulting command is
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```console
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$ sudo mount -o loop,offset=272629760 disk.img tmp/
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```
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A remount is simple as
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```console
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$ sudo umount tmp
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```
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## When do you need this stuff
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I often create quick and dirty (big) card images from my Raspberry Pies.
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They are saved and easy to copy over to another storage (because they are
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a single file).
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If you have less space, `dd` is probably not the best method to create a disk
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backup. `partimage` for example creates images from partitions, but it only
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saves the used data from that partition. Those images are smaller.
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