nbdkit-loop(1)

📦
version 1.48.1 · 14 versions
, in fedora 44

✨ NAME ✨

nbdkit-loop — bestie, this is how you use nbdkit with the Linux kernel client to create loop devices and loop mounts 💅 (this one lives in the NBDKIT manual, if you were wondering)

🦋 DESCRIPTION 🦋

okay so the vibe is: nbdkit (the server) teams up with the Linux kernel nbd (the client) in a loop mode, and suddenly any plugin that nbdkit supports can be turned into a Linux block device. it's giving magic fr fr 🌈

Besides nbdkit(1) itself, these are the main commands you'll be living in:

  • nbd-client localhost /dev/nbd0 — attaches a locally running nbdkit instance to the kernel device /dev/nbd0. iconic.
  • nbd-client -unix /tmp/socket /dev/nbd0 — the alternative method, using a Unix domain socket instead of a public TCP/IP socket. serve it up with nbdkit -U /tmp/socket 🎀
  • nbd-client -d /dev/nbd0 — detaches /dev/nbd0. bye bestie 👋
  • nbd-client -c /dev/nbd0 — queries whether /dev/nbd0 is attached or not. vibe check on your device.
  • modprobe nbd — you may be need to run this command once to load the nbd client kernel module. (yes, that's the original wording, it's adorable, we're keeping it 💀)
⚠️ The nbd-client(8) and modprobe(8) commands must be run as root. no cap. no shortcuts. 👑

🚨 Warning: Do not loop mount untrusted filesystems 🚨

bestie I am BEGGING you. Untrusted filesystems and untrusted disk images should not be loop mounted, because they could contain exploits that attack your host kernel. like, that's your whole computer. 😭

Use the tools from libguestfs(3) instead, since it safely isolates untrusted filesystems from the host. periodt. 🫶

🍓 Loop mount a filesystem from a compressed file

if you have a filesystem or disk image in xz-compressed format, you can use nbdkit-xz-filter(1) and nbdkit-file-plugin(1) to loop mount it, like so:

 nbdkit --filter=xz file disk.xz
 nbd-client localhost /dev/nbd0
 mount /dev/nbd0p1 /mnt

🌸 Loop mount a filesystem from a web server

you can use nbdkit-curl-plugin(1) to loop mount a filesystem from a disk image on a web server — remote disk images are so main character energy:

 nbdkit [--filter=xz] curl https://example.com/disk.img
 nbd-client localhost /dev/nbd0
 mount /dev/nbd0p1 /mnt

Use --filter=xz if the remote image is XZ-compressed. easy ✨

👑 Create a giant btrfs filesystem

nbdkit is genuinely useful for testing the limits of Linux filesystems. Using nbdkit-memory-plugin(1) you can create virtual disks stored in RAM with a virtual size up to 2⁶³-1 bytes, and then create filesystems on these. giant. enormous. slay. 🔥

 nbdkit memory $(( 2**63 - 1 ))
 nbd-client localhost /dev/nbd0

Partition the device using GPT, creating a single partition with all default settings:

 gdisk /dev/nbd0

Make a btrfs filesystem on the disk and mount it:

 mkfs.btrfs -K /dev/nbd0p1
 mount /dev/nbd0p1 /mnt

💀 Inject errors into Linux devices

Using nbdkit-error-filter(1) you can see how Linux devices react to errors. chaos as a service, honestly:

 nbdkit --filter=error \
        memory 64M \
        error-rate=100% error-file=/tmp/inject
 nbd-client localhost /dev/nbd0
 mkfs -t ext4 /dev/nbd0
 mount /dev/nbd0 /mnt

Inject errors by touching /tmp/inject, and stop injecting errors by removing this file. that's it, that's the switch 🎀

🧚 Write Linux block devices in shell script

Using nbdkit-sh-plugin(3) you can write custom Linux block devices in shell script for testing. For example the following shell script creates a disk which contains a bad sector:

#!/bin/bash -
case "$1" in
    thread_model) echo parallel ;;
    get_size) echo 64M ;;
    pread)
        if [ $4 -le 100000 ] && [ $(( $4+$3 )) -gt 100000 ]; then
            echo EIO Bad block >&2
            exit 1
        else
            dd if=/dev/zero count=$3 iflag=count_bytes
        fi ;;
    *) exit 2 ;;
esac

Create a loop from this shell script using:

 nbdkit sh ./bad-sector.sh
 nbd-client localhost /dev/nbd0

You can then try running tests such as:

 badblocks /dev/nbd0

🌈 SEE ALSO 🌈

nbdkit(1), nbdkit-client(1), nbdkit-plugin(3), loop(4), losetup(8), mount(8), nbdfuse(1), nbd-client(8), modprobe(8), libguestfs(3), http://libguestfs.org.

💖 AUTHORS 💖

Richard W.M. Jones — the icon, the legend 🫶

Copyright Red Hat

⭐ LICENSE ⭐

Redistribution and use in source and binary forms, with or without modification, are permitted provided that the following conditions are met:

  • Redistributions of source code must retain the above copyright notice, this list of conditions and the following disclaimer.
  • Redistributions in binary form must reproduce the above copyright notice, this list of conditions and the following disclaimer in the documentation and/or other materials provided with the distribution.
  • Neither the name of Red Hat nor the names of its contributors may be used to endorse or promote products derived from this software without specific prior written permission.
THIS SOFTWARE IS PROVIDED BY RED HAT AND CONTRIBUTORS ''AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL RED HAT OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.

okay that's the whole page bestie!! 💖 this was nbdkit-1.48.1, dated 2026-08-29, and it's the nbdkit-loop(1) manual — go make some loop devices, you've got this 🦋✨

📦 where this page comes from