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NBD server with stable plugin ABI and permissive license

nbdkit is an NBD server. NBD — Network Block Device — is a protocol for accessing Block Devices (hard disks and disk-like things) over a Network.

The key features of nbdkit are:

  • Multithreaded NBD server written in C with good performance.
  • Minimal dependencies for the basic server.
  • Liberal license (BSD) allows nbdkit to be linked to proprietary libraries or included in proprietary code.
  • Well-documented, simple plugin API with a stable ABI guarantee. Lets you export “unconventional” block devices easily.
  • You can write plugins in C/C++, Go, Lua, Perl, Python, OCaml, Ruby, Rust, shell script or Tcl.
  • Filters can be stacked in front of plugins to transform the output.

For documentation, see the docs directory.

For plugins and filters, see the plugins and filters directories. Example plugins can be found in plugins/example*.

There are many NBD clients, but the nbdkit project has a companion client library called


This software is copyright (C) Red Hat Inc. and licensed under a BSD license. See LICENSE for details.

Building from source


  • Linux, macOS, Windows, FreeBSD, OpenBSD or Haiku
  • GCC or Clang
  • bash
  • GNU make

Recommended for TLS (authentication and encryption) support, but it is possible to build without it:

  • gnutls >= 3.3.0

For Windows support see the Windows section below.

For macOS support see the macOS section below.

Optional dependencies

To build the man pages, you will optionally need to install:

  • Perl
  • Pod::Man and Pod::Simple (Perl libraries)

For SELinux socket labelling support:

  • libselinux

For the gzip filter:

  • zlib

For the xz filter:

  • liblzma

For the memory plugin with allocator=zstd:

  • zstd

For the curl (HTTP/FTP) plugin:

  • libcurl

For the ssh plugin:

  • libssh >= 0.8.0 (this is a different library from libssh2 - that will not work)

For the iso plugin:

  • xorriso or genisoimage or mkisofs

For the floppy plugin:

  • iconv (on Linux this is built into glibc, on other systems it may be a separate library)

For the libvirt plugin:

  • libvirt

For the libguestfs plugin, and to run parts of the test suite:

  • libguestfs
  • guestfish (from libguestfs)

For the ext2 filter:

  • ext2fs
  • com_err

For the linuxdisk plugin:

  • mke2fs >= 1.42.10 (from e2fsprogs)

For the nbd plugin, to get URI and TLS support, and also to run parts of the test suite:

  • libnbd >= 0.9.8

For the bittorrent plugin:

For the blkio plugin:

For the containerized data importer (CDI) plugin:

  • podman
  • jq

For the Perl and example4 plugins:

  • perl interpreter
  • perl development libraries
  • perl modules ExtUtils::Embed

For the Python plugin:

  • python interpreter (version 3 only)
  • python development libraries
  • python unittest to run the test suite
  • boto3 is required to run the S3 plugin written in Python

For the OCaml plugin:

  • OCaml >= 4.03

For the Tcl plugin:

  • Tcl development library and headers

For the Lua plugin:

  • Lua development library and headers

For the Rust plugin:

  • cargo (other dependencies will be downloaded at build time)

To be able to write plugins in golang:

  • go >= 1.13

For bash tab completion:

  • bash-completion >= 1.99

To test for memory leaks (make check-valgrind):

  • valgrind program and development headers

For non-essential enhancements to the test suite:

  • expect
  • flake8
  • hexdump
  • ip, ss (from iproute package)
  • jq
  • losetup (from util-linux package)
  • mke2fs (from e2fsprogs)
  • nbdcopy, nbdinfo, nbdsh (from libnbd)
  • qemu-img, qemu-io, qemu-nbd (usually shipped with qemu)
  • sfdisk (from util-linux)
  • socat
  • stat (from coreutils)


To build from tarball:         To build from git:

                               autoreconf -i
./configure                    ./configure
make                           make
make check                     make check

On FreeBSD and OpenBSD which do not have GNU make by default you must use gmake instead of make.

To run nbdkit from the source directory, use the top level ./nbdkit wrapper. It will run nbdkit and plugins from the locally compiled directory:

$ ./nbdkit example1 -f -v
./server/nbdkit ./plugins/example1/.libs/ -f -v

Optionally run this command as root to install everything:

make install


Since nbdkit >= 1.16, only Python >= 3.6 is supported.

By default nbdkit uses the Python version of the Python interpreter called “python” on the current $PATH. If you have parallel versions of Python installed then you can choose a different version by setting the PYTHON variable when configuring. For example:

./configure PYTHON=/usr/bin/python3.8


Most recent (less than, say, 8 years old) versions of the OCaml compiler should work. By default the configure script should detect the bytecode and native code compilers and enable either one or both.

To enable OCaml warnings and turn them into hard errors (developers only):

./configure OCAMLOPTFLAGS="-warn-error +A-3"

Running the tests

To run the test suite:

make check

If there is a failure, look at the corresponding tests/*.log file for debug information.

A few tests require root privileges, and are skipped by default. To run them you must do:

make check-root

If you have the proprietary VDDK library, you can test nbdkit-vddk-plugin against the library like this:

make check-vddk vddkdir=vmware-vix-disklib-distrib

Running the benchmarks

To run benchmarks:

make bench

Download tarballs

Tarballs are available from:

Downstream packagers

If you are packaging nbdkit, use:

./configure --with-extra='...'

providing extra information about the distribution, and/or distro-specific versions. It helps us with troubleshooting bug reports. (Also, talk to us!)


Install the valgrind program and development headers.


./configure --enable-gcc-warnings --enable-valgrind

When testing use:

make check
make check-valgrind

For development ideas, see the TODO file.

The upstream git repository is:

Please send patches to the libguestfs mailing list:

For further information, see:

Address sanitizer (ASAN)

You can compile nbdkit with clang and ASAN with:

./configure CC=clang CXX=clang++ \
            CFLAGS="-O0 -g -fsanitize=address -fno-omit-frame-pointer" \
            --disable-linker-script \
make clean
ASAN_OPTIONS="allocator_may_return_null=1 detect_leaks=false" make check

Test coverage

./configure CFLAGS="--coverage -g" LDFLAGS="--coverage -g"
make clean
make check

lcov -c -d . -o
genhtml -o coverage

Open your browser and examine the coverage/ directory. At the time of writing (2020-04) test coverage of the server is reasonable, but things are much worse for certain plugins and filters.


nbdkit works on macOS and is mostly feature complete. Some tests may fail or be skipped.

We have tested macOS 12 (Monterey) using MacPorts. Other versions of macOS and Homebrew may work too.

We installed the following MacPorts packages:

  • autoconf
  • autoconf-archive
  • automake
  • bash
  • coreutils
  • gcc
  • gmake
  • gsed
  • jq
  • libtool
  • m4
  • ocaml
  • pkg-config
  • qemu
  • ranlib

We configured nbdkit using:

./configure --enable-gcc-warnings --disable-perl \
	    CFLAGS="-g -O2 -I/opt/local/include" \


Experimentally, the server can be compiled on Windows or cross-compiled from Linux using mingw-w64. Only a small subset of features are available. To find out what is missing read the TODO "Windows port".

Note that GCC or Clang is required even if you are compiling on Windows because of some C language extensions we use. MSVC will not work.

Cross-compiling from Linux to Windows

For the rest of this section we talk about cross-compiling for Windows using Linux and mingw-w64. At a minimum you will need:

  • mingw-w64 GCC
  • mingw-w64 dlfcn
  • mingw-w64 winpthreads
  • mingw-w64 gnutls (optional, but highly recommended)
  • wine (if you want to run it on Linux)

You may want to patch Wine with this patch which adds support for AF_UNIX sockets. It is needed to get most of the test suite to work, but if you don't care about the -U option then it is not needed.

Other mingw-w64 libraries may be installed which will add functionality (see full list of requirements above), but you may end up hitting areas we have not compiled or tested before.

To cross compile do:

mingw64-configure --disable-ocaml --disable-perl --disable-vddk

You can test if the server is working by doing:

./nbdkit.exe --dump-config

(This usually runs wine automatically. If not, you may need to prefix the command "wine nbdkit.exe ...")

To see which plugins and filters were compiled:

find plugins filters -name '*.dll'

You can run them under Wine without installing using eg:

./nbdkit.exe -f -v memory 1G

You can run the test suite in the usual way:

mingw64-make check

Running the cross-compiled binary on Windows

To test the binary on a real Windows system you will need to copy server/.libs/nbdkit.exe, any plugins and filters you need (plugins/*/.libs/*.dll and filters/*/.libs/*.dll), and many mingw DLLs (libdl.dll, libgnutls.dll, libwinpthread.dll, etc.).


  1. libtool creates files called nbdkit.exe in the top level directory and in server. These are the stripped binaries. The "real" binaries are located in hidden .libs/ subdirectories.

  2. The top level .libs/nbdkit.exe is the wrapper used to run nbdkit from the build directory, not the server binary. You need server/.libs/nbdkit.exe instead.

  3. To find the list of mingw DLLs I ran nbdkit.exe iteratively until Windows stopped complaining about missing libraries. You could also use a tool like Dependency Walker.

If you copy everything into a single directory on the Windows server then you should be able to run nbdkit normally, eg:

nbdkit.exe -f -v nbdkit-memory-plugin.dll 1G

You will probably need to open port 10809 on the Windows Firewall.