Provided by: nbdkit_1.36.3-1ubuntu10_amd64 
NAME
nbdkit - toolkit for creating Network Block Device (NBD) servers
SYNOPSIS
nbdkit [-4|--ipv4-only] [-6|--ipv6-only]
[-D|--debug PLUGIN|FILTER|nbdkit.FLAG=N]
[--exit-with-parent] [-e|--exportname EXPORTNAME]
[--filter=FILTER ...] [-f|--foreground]
[-g|--group GROUP] [-i|--ipaddr IPADDR]
[--log=stderr|syslog|null] [--mask-handshake=MASK]
[-n|--newstyle] [--no-sr] [-o|--oldstyle]
[-P|--pidfile PIDFILE] [-p|--port PORT]
[-r|--readonly] [--run 'COMMAND ARGS ...']
[--selinux-label=LABEL] [-s|--single] [--swap]
[-t|--threads THREADS] [--tls=off|on|require]
[--tls-certificates=/path/to/certificates]
[--tls-psk=/path/to/pskfile] [--tls-verify-peer]
[-U|--unix SOCKET|-] [-u|--user USER]
[-v|--verbose] [--vsock]
PLUGIN [[KEY=]VALUE [KEY=VALUE [...]]]
nbdkit --dump-config
nbdkit PLUGIN --dump-plugin
nbdkit --help
nbdkit [-V|--version]
DESCRIPTION
Network Block Device (NBD) is a network protocol for accessing block devices over the network. Block
devices are hard disks and things that behave like hard disks such as disk images and virtual machines.
nbdkit is both a toolkit for creating NBD servers from “unconventional” sources, and the name of an NBD
server. nbdkit ships with many plugins for performing common tasks like serving local files.
Plugins and filters
nbdkit is different from other NBD servers because you can easily create new Network Block Device sources
by writing a few glue functions, possibly in C, or perhaps in a high level language like Perl or Python.
The liberal licensing of nbdkit is meant to allow you to link nbdkit with proprietary libraries or to
include nbdkit in proprietary code.
If you want to write your own nbdkit plugin you should read nbdkit-plugin(3).
nbdkit also has a concept of filters which can be layered on top of plugins. Several filters are
provided with nbdkit and if you want to write your own you should read nbdkit-filter(3).
EXAMPLES
Basic file serving
• Serve file disk.img on port 10809 using nbdkit-file-plugin(1), and connect to it using guestfish(1):
nbdkit file disk.img
guestfish --rw --format=raw -a nbd://localhost
• Serve file disk.img on port 10809, requiring clients to use encrypted (TLS) connections:
nbdkit --tls=require file disk.img
Other nbdkit plugins
• Create a small disk containing test patterns using nbdkit-data-plugin(1):
nbdkit data ' ( 0x55 0xAA )*2048 '
• Forward an NBD connection to a remote server over HTTPS or SSH using nbdkit-curl-plugin(1) or
nbdkit-ssh-plugin(1):
nbdkit -r curl https://example.com/disk.img
nbdkit ssh host=example.com /var/tmp/disk.img
• Create a sparse 1 terabyte RAM disk using nbdkit-memory-plugin(1) and use it as a loop device
(nbdkit-loop(1)):
nbdkit memory 1T
nbd-client localhost /dev/nbd0
• Create a floppy disk image containing files from a local directory using nbdkit-floppy-plugin(1):
nbdkit floppy dir/
Combining plugins and filters
• Serve only the first partition from compressed disk image disk.img.xz, combining
nbdkit-partition-filter(1), nbdkit-xz-filter(1) and nbdkit-file-plugin(1).
nbdkit --filter=partition --filter=xz file disk.img.xz partition=1
To understand this command line:
plugin name and plugin parameter
│
┌───────┴──────┐
│ │
nbdkit --filter=partition --filter=xz file disk.img.xz partition=1
│ │ │
└──────────────┴────┬─────────────────────┘
│
filters and filter parameter
• Create a scratch, empty nbdkit device and inject errors and delays, for testing clients, using
nbdkit-memory-plugin(1), nbdkit-error-filter(1) and nbdkit-delay-filter(1):
nbdkit --filter=error --filter=delay memory 100M \
error-rate=10% rdelay=1 wdelay=1
Writing plugins in shell script
• Write a simple, custom plugin in shell script using nbdkit-sh-plugin(3):
nbdkit sh - <<'EOF'
case "$1" in
get_size) echo 1M ;;
pread) dd if=/dev/zero count=$3 iflag=count_bytes ;;
*) exit 2 ;;
esac
EOF
• The same example as above can be written entirely on the command line using nbdkit-eval-plugin(1):
nbdkit eval get_size='echo 1M' \
pread='dd if=/dev/zero count=$3 iflag=count_bytes'
Display information
Display information about nbdkit or a specific plugin:
nbdkit --help
nbdkit --version
nbdkit --dump-config
nbdkit example1 --help
nbdkit example1 --dump-plugin
GLOBAL OPTIONS
--help
Display brief command line usage information and exit.
-4
--ipv4-only
-6
--ipv6-only
When a non-numeric argument is passed to the -i option (such as a Fully Qualified Domain Name, or a
host name from "/etc/hosts"), restrict the name resolution to IPv4 or IPv6 addresses.
When the -i option is omitted, listen on only the IPv4 or IPv6 address of all interfaces (0.0.0.0 or
"::", respectively).
When both -4 and -6 options are present on the command line, the last one takes effect.
-D PLUGIN.FLAG=N
-D FILTER.FLAG=N
--debug PLUGIN.FLAG=N
--debug FILTER.FLAG=N
Set the plugin or filter Debug Flag called "FLAG" to the integer value "N". See "Debug Flags" in
nbdkit-plugin(3).
-D nbdkit.FLAG=N
--debug nbdkit.FLAG=N
(nbdkit ≥ 1.18)
Set the nbdkit server Debug Flag called "FLAG" to the integer value "N". See "SERVER DEBUG FLAGS"
below.
--dump-config
Dump out the compile-time configuration values and exit. See nbdkit-probing(1).
--dump-plugin
Dump out information about the plugin and exit. See nbdkit-probing(1).
--exit-with-parent
If the parent process exits, we exit. This can be used to avoid complicated cleanup or orphaned
nbdkit processes. There are some important caveats with this, see "EXIT WITH PARENT" in
nbdkit-captive(1).
An alternative to this is "CAPTIVE NBDKIT" in nbdkit-captive(1).
This option implies --foreground.
-e EXPORTNAME
--export=EXPORTNAME
--export-name=EXPORTNAME
--exportname=EXPORTNAME
Set a preferred exportname to expose in the shell environment created during --run. The use of this
option without --run has no effect. This option does not change what nbdkit advertises as a server,
but can aid in writing a captive client that wants to access particular content from a plugin that
differentiates content based on the client's choice of export name.
If not set, the --run environment is set to access the default exportname "" (empty string).
--filter=FILTER
Add a filter before the plugin. This option may be given one or more times to stack filters in front
of the plugin. They are processed in the order they appear on the command line. See "FILTERS" and
nbdkit-filter(3).
-f
--foreground
--no-fork
Don't fork into the background.
-g GROUP
--group=GROUP
Change group to "GROUP" after starting up. A group name or numeric group ID can be used.
The server needs sufficient permissions to be able to do this. Normally this would mean starting the
server up as root.
See also -u.
-i IPADDR
--ip-addr=IPADDR
--ipaddr=IPADDR
Listen on the specified interface. The default is to listen on all interfaces. See also -4, -6, and
-p.
--log=stderr
--log=syslog
--log=null
Send error messages to standard error (--log=stderr), or to the system log (--log=syslog), or discard
them completely (--log=null, not recommended for normal use).
The default is to send error messages to stderr, unless nbdkit forks into the background in which
case they are sent to syslog.
For more details see "LOGGING" in nbdkit-service(1).
--mask-handshake=MASK
This option can be used to mask off particular global features which are advertised during new-style
handshake (defaulting to all supported bits set). See nbdkit-protocol(1).
-n
--new-style
--newstyle
Use the newstyle NBD protocol. This is the default in nbdkit ≥ 1.3. In earlier versions the default
was oldstyle. See nbdkit-protocol(1).
--no-sr
Do not advertise structured replies. A client must request structured replies to take advantage of
block status and potential sparse reads; however, as structured reads are not a mandatory part of the
newstyle NBD protocol, this option can be used to debug client fallbacks for dealing with older
servers. See nbdkit-protocol(1).
-o
--old-style
--oldstyle
Use the oldstyle NBD protocol. This was the default in nbdkit ≤ 1.2, but now the default is
newstyle. Note this is incompatible with newer features such as export names and TLS. See
nbdkit-protocol(1).
-P PIDFILE
--pid-file=PIDFILE
--pidfile=PIDFILE
Write "PIDFILE" (containing the process ID of the server) after nbdkit becomes ready to accept
connections.
If the file already exists, it is overwritten. nbdkit does not delete the file when it exits.
-p PORT
--port=PORT
Change the TCP/IP port number on which nbdkit serves requests. The default is 10809. See also -i.
-r
--read-only
--readonly
The export will be read-only. If a client writes, then it will get an error.
Note that some plugins inherently don't support writes. With those plugins the -r option is added
implicitly.
nbdkit-cow-filter(1) can be placed over read-only plugins to provide copy-on-write (or "snapshot")
functionality. If you are using qemu as a client then it also supports snapshots.
--run 'COMMAND ARGS ...'
Run nbdkit as a captive subprocess of the command. When the command exits, nbdkit is killed. See
"CAPTIVE NBDKIT" in nbdkit-captive(1).
Note that the command is executed by /bin/sh. On some platforms like Debian this might not be a
full-featured shell.
This option implies --foreground.
In nbdkit ≤ 1.34 you normally had to add -U -, otherwise nbdkit would use a TCP/IP port which was not
what you wanted. In nbdkit ≥ 1.36, using --run implies -U -. If you want the old behaviour of
nbdkit then you must use the --port option explicitly.
--selinux-label=SOCKET-LABEL
Apply the SELinux label "SOCKET-LABEL" to the nbdkit listening socket.
The common — perhaps only — use of this option is to allow libvirt guests which are using SELinux and
sVirt confinement to access nbdkit Unix domain sockets. The example below shows how to do this.
Note that the socket and filesystem labels are different.
nbdkit -U /tmp/sock --selinux-label=system_u:object_r:svirt_socket_t:s0 ...
chcon system_u:object_r:svirt_image_t:s0 /tmp/sock
-s
--single
--stdin
Don't fork. Handle a single NBD connection on stdin/stdout. After stdin closes, the server exits.
You can use this option to run nbdkit from inetd or similar superservers; or just for testing; or if
you want to run nbdkit in a non-conventional way. Note that if you want to run nbdkit from systemd,
then it may be better to use "SOCKET ACTIVATION" in nbdkit-service(1) instead of this option.
This option implies --foreground.
--swap
(nbdkit ≥ 1.18)
Specifies that the NBD device will be used as swap space loop mounted on the same machine which is
running nbdkit. To avoid deadlocks this locks the whole nbdkit process into memory using
mlockall(2). This may require additional permissions, such as starting the server as root or raising
the "RLIMIT_MEMLOCK" (ulimit(1) -l) limit on the process.
-t THREADS
--threads=THREADS
Set the number of threads to be used per connection, which in turn controls the number of outstanding
requests that can be processed at once. Only matters for plugins with thread_model=parallel (where
it defaults to 16). To force serialized behavior (useful if the client is not prepared for out-of-
order responses), set this to 1.
--tls=off
--tls=on
--tls=require
Disable, enable or require TLS (authentication and encryption support). See nbdkit-tls(1).
--tls-certificates=/path/to/certificates
Set the path to the TLS certificates directory. If not specified, some built-in paths are checked.
See nbdkit-tls(1) for more details.
--tls-psk=/path/to/pskfile
Set the path to the pre-shared keys (PSK) file. If used, this overrides certificate authentication.
There is no built-in path. See nbdkit-tls(1) for more details.
--tls-verify-peer
Enables TLS client certificate verification. The default is not to check the client's certificate.
-U SOCKET
--unix=SOCKET
-U -
--unix -
Accept connections on the Unix domain socket "SOCKET" (which is a path).
nbdkit creates this socket, but it will probably have incorrect permissions (too permissive). If it
is a problem that some unauthorized user could connect to this socket between the time that nbdkit
starts up and the authorized user connects, then put the socket into a directory that has restrictive
permissions.
nbdkit does not delete the socket file when it exits. The caller should delete the socket file after
use (else if you try to start nbdkit up again you will get an "Address already in use" error).
If the socket name is - then nbdkit generates a randomly named private socket. This is implied by
the --run option. See also "CAPTIVE NBDKIT" in nbdkit-captive(1).
-u USER
--user=USER
Change user to "USER" after starting up. A user name or numeric user ID can be used.
The server needs sufficient permissions to be able to do this. Normally this would mean starting the
server up as root.
See also -g.
-v
--verbose
Enable verbose messages.
It's a good idea to use -f as well so the process does not fork into the background (but not
required).
-V
--version
Print the version number of nbdkit and exit.
The --dump-config option provides separate major and minor numbers and may be easier to parse from
shell scripts.
--vsock
(nbdkit ≥ 1.16)
Use the AF_VSOCK protocol (instead of TCP/IP). You must use this in conjunction with -p/--port. See
"AF_VSOCK" in nbdkit-service(1).
PLUGIN NAME
You can give the full path to the plugin, like this:
nbdkit $libdir/nbdkit/plugins/nbdkit-file-plugin.so [...]
but it is usually more convenient to use this equivalent syntax:
nbdkit file [...]
$libdir is set at compile time. To print it out, do:
nbdkit --dump-config
PLUGIN CONFIGURATION
After specifying the plugin name you can (optionally, it depends on the plugin) give plugin configuration
on the command line in the form of "key=value". For example:
nbdkit file file=disk.img
To list all the options supported by a plugin, do:
nbdkit --help file
To dump information about a plugin, do:
nbdkit file --dump-plugin
Magic parameters
Some plugins declare a special "magic config key". This is a key which is assumed if no "key=" part is
present. For example:
nbdkit file disk.img
is assumed to be "file=disk.img" because the file plugin declares "file" as its magic config key. There
can be ambiguity in the parsing of magic config keys if the value might look like a "key=value". If
there could be ambiguity then modify the value, eg. by prefixing it with "./"
There is also a special exception for plugins which do not declare a magic config key, but where the
first plugin argument does not contain an '=' character: it is assumed to be "script=value". This is
used by scripting language plugins:
nbdkit perl foo.pl [args...]
has the same meaning as:
nbdkit perl script=foo.pl [args...]
Shebang scripts
You can use "#!" to run nbdkit plugins written in most scripting languages. The file should be
executable. For example:
#!/usr/sbin/nbdkit perl
sub open {
# etc
}
(see nbdkit-perl-plugin(3) for a full example).
SERVER DEBUG FLAGS
As well as enabling or disabling debugging in the server using --verbose you can control extra debugging
in the server using the -D nbdkit.* flags listed in this section. Note these flags are an internal
implementation detail of the server and may be changed or removed at any time in the future.
-D nbdkit.backend.controlpath=0
-D nbdkit.backend.controlpath=1
-D nbdkit.backend.datapath=0
-D nbdkit.backend.datapath=1
These flags control the verbosity of nbdkit backend debugging messages (the ones which show every
request processed by the server). The default for both settings is 1 (normal debugging) but you can
set them to 0 to suppress these messages.
-D nbdkit.backend.datapath=0 is the more useful setting which lets you suppress messages about pread,
pwrite, zero, trim, etc. commands. When transferring large amounts of data these messages are
numerous and not usually very interesting.
-D nbdkit.backend.controlpath=0 suppresses the non-datapath commands (config, open, close, can_write,
etc.)
-D nbdkit.environ=1
Print nbdkit's environment variables in the debug output at start up. This is insecure because
environment variables may contain both sensitive and user-controlled information, so it should not be
used routinely. But it is useful for tracking down problems related to environment variables.
-D nbdkit.tls.log=N
Enable TLS logging. "N" can be in the range 0 (no logging) to 99. See
gnutls_global_set_log_level(3).
-D nbdkit.tls.session=1
Print additional information about the TLS session, such as the type of authentication and
encryption, and client certificate information.
SIGNALS
nbdkit responds to the following signals:
"SIGINT"
"SIGQUIT"
"SIGTERM"
The server exits cleanly.
"SIGPIPE"
This signal is ignored.
ENVIRONMENT VARIABLES
"LISTEN_FDS"
"LISTEN_PID"
If present in the environment when nbdkit starts up, these trigger "SOCKET ACTIVATION" in
nbdkit-service(1).
SEE ALSO
Other topics
nbdkit-captive(1) — Run nbdkit under another process and have it reliably cleaned up.
nbdkit-client(1) — How to mount NBD filesystems on a client machine.
nbdkit-loop(1) — Use nbdkit with the Linux kernel client to create loop devices and loop mounts.
nbdkit-probing(1) — How to probe for nbdkit configuration and plugins.
nbdkit-protocol(1) — Which parts of the NBD protocol nbdkit supports.
nbdkit-security(1) — Lists past security issues in nbdkit.
nbdkit-service(1) — Running nbdkit as a service, and systemd socket activation.
nbdkit-tls(1) — Authentication and encryption of NBD connections (sometimes incorrectly called "SSL").
Plugins
nbdkit-blkio-plugin(1), nbdkit-cdi-plugin(1), nbdkit-curl-plugin(1), nbdkit-data-plugin(1),
nbdkit-eval-plugin(1), nbdkit-example1-plugin(1), nbdkit-example2-plugin(1), nbdkit-example3-plugin(1),
nbdkit-example4-plugin(1), nbdkit-file-plugin(1), nbdkit-floppy-plugin(1), nbdkit-full-plugin(1),
nbdkit-guestfs-plugin(1), nbdkit-info-plugin(1), nbdkit-iso-plugin(1), nbdkit-libvirt-plugin(1),
nbdkit-linuxdisk-plugin(1), nbdkit-memory-plugin(1), nbdkit-nbd-plugin(1), nbdkit-null-plugin(1),
nbdkit-ondemand-plugin(1), nbdkit-ones-plugin(1), nbdkit-partitioning-plugin(1),
nbdkit-pattern-plugin(1), nbdkit-random-plugin(1), nbdkit-S3-plugin(1), nbdkit-sparse-random-plugin(1),
nbdkit-split-plugin(1), nbdkit-ssh-plugin(1), nbdkit-tmpdisk-plugin(1), nbdkit-torrent-plugin(1),
nbdkit-vddk-plugin(1), nbdkit-zero-plugin(1) ; nbdkit-cc-plugin(3), nbdkit-golang-plugin(3),
nbdkit-lua-plugin(3), nbdkit-ocaml-plugin(3), nbdkit-perl-plugin(3), nbdkit-python-plugin(3),
nbdkit-ruby-plugin(3), nbdkit-rust-plugin(3), nbdkit-sh-plugin(3), nbdkit-tcl-plugin(3) .
Filters
nbdkit-blocksize-filter(1), nbdkit-blocksize-policy-filter(1), nbdkit-cache-filter(1),
nbdkit-cacheextents-filter(1), nbdkit-checkwrite-filter(1), nbdkit-cow-filter(1),
nbdkit-ddrescue-filter(1), nbdkit-delay-filter(1), nbdkit-error-filter(1), nbdkit-exitlast-filter(1),
nbdkit-exitwhen-filter(1), nbdkit-exportname-filter(1), nbdkit-ext2-filter(1),
nbdkit-extentlist-filter(1), nbdkit-evil-filter(1), nbdkit-fua-filter(1), nbdkit-gzip-filter(1),
nbdkit-ip-filter(1), nbdkit-limit-filter(1), nbdkit-log-filter(1), nbdkit-luks-filter(1),
nbdkit-multi-conn-filter(1), nbdkit-nocache-filter(1), nbdkit-noextents-filter(1),
nbdkit-nofilter-filter(1), nbdkit-noparallel-filter(1), nbdkit-nozero-filter(1), nbdkit-offset-filter(1),
nbdkit-partition-filter(1), nbdkit-pause-filter(1), nbdkit-protect-filter(1), nbdkit-qcow2dec-filter(1),
nbdkit-rate-filter(1), nbdkit-readahead-filter(1), nbdkit-retry-filter(1),
nbdkit-retry-request-filter(1), nbdkit-scan-filter(1), nbdkit-stats-filter(1), nbdkit-swab-filter(1),
nbdkit-tar-filter(1), nbdkit-tls-fallback-filter(1), nbdkit-truncate-filter(1), nbdkit-xz-filter(1) .
For developers
nbdkit-plugin(3), nbdkit-filter(3).
Writing plugins in other programming languages
nbdkit-cc-plugin(3), nbdkit-golang-plugin(3), nbdkit-lua-plugin(3), nbdkit-ocaml-plugin(3),
nbdkit-perl-plugin(3), nbdkit-python-plugin(3), nbdkit-ruby-plugin(3), nbdkit-rust-plugin(3),
nbdkit-sh-plugin(3), nbdkit-tcl-plugin(3) .
Release notes for previous releases of nbdkit
nbdkit-release-notes-1.36(1), nbdkit-release-notes-1.34(1), nbdkit-release-notes-1.32(1),
nbdkit-release-notes-1.30(1), nbdkit-release-notes-1.28(1), nbdkit-release-notes-1.26(1),
nbdkit-release-notes-1.24(1), nbdkit-release-notes-1.22(1), nbdkit-release-notes-1.20(1),
nbdkit-release-notes-1.18(1), nbdkit-release-notes-1.16(1), nbdkit-release-notes-1.14(1),
nbdkit-release-notes-1.12(1), nbdkit-release-notes-1.10(1), nbdkit-release-notes-1.8(1),
nbdkit-release-notes-1.6(1), nbdkit-release-notes-1.4(1).
NBD clients
guestfish(1), libnbd(3), nbd-client(1), nbdcopy(1), nbdfuse(1), nbdinfo(1), nbdsh(1), qemu(1).
nbdkit links
http://gitlab.com/nbdkit/nbdkit — Source code.
Other NBD servers
qemu-nbd(1), nbd-server(1), https://github.com/bignaux/lwNBD, https://bitbucket.org/hirofuchi/xnbd.
Documentation for the NBD protocol
https://github.com/NetworkBlockDevice/nbd/blob/master/doc/proto.md, https://nbd.sourceforge.io/.
Similar protocols
https://en.wikipedia.org/wiki/iSCSI, https://en.wikipedia.org/wiki/ATA_over_Ethernet,
https://en.wikipedia.org/wiki/Fibre_Channel_over_Ethernet.
Other manual pages of interest
gnutls_priority_init(3), qemu-img(1), psktool(1), systemd.socket(5).
AUTHORS
Eric Blake
Laszlo Ersek
Richard W.M. Jones
Yann E. MORIN
Nikolaus Rath
François Revol
Nir Soffer
Alan Somers
Pino Toscano
COPYRIGHT
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
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• 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,
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ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR
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DAMAGE.
nbdkit-1.36.3 2024-03-31 nbdkit(1)