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QEMU Disk Network Block Device Server
**qemu-nbd** [*OPTION*]... *filename*
**qemu-nbd** -L [*OPTION*]...
**qemu-nbd** -d *dev*
Export a QEMU disk image using the NBD protocol.
Other uses:
- Bind a /dev/nbdX block device to a QEMU server (on Linux).
- As a client to query exports of a remote NBD server.
.. program:: qemu-nbd
*filename* is a disk image filename, or a set of block
driver options if ``--image-opts`` is specified.
*dev* is an NBD device.
.. option:: --object type,id=ID,...props...
Define a new instance of the *type* object class identified by *ID*.
See the :manpage:`qemu(1)` manual page for full details of the properties
supported. The common object types that it makes sense to define are the
``secret`` object, which is used to supply passwords and/or encryption
keys, and the ``tls-creds`` object, which is used to supply TLS
credentials for the qemu-nbd server or client.
.. option:: -p, --port=PORT
TCP port to listen on as a server, or connect to as a client
(default ``10809``).
.. option:: -o, --offset=OFFSET
The offset into the image.
.. option:: -b, --bind=IFACE
The interface to bind to as a server, or connect to as a client
(default ````).
.. option:: -k, --socket=PATH
Use a unix socket with path *PATH*.
.. option:: --image-opts
Treat *filename* as a set of image options, instead of a plain
filename. If this flag is specified, the ``-f`` flag should
not be used, instead the :option:`format=` option should be set.
.. option:: -f, --format=FMT
Force the use of the block driver for format *FMT* instead of
.. option:: -r, --read-only
Export the disk as read-only.
.. option:: -B, --bitmap=NAME
If *filename* has a qcow2 persistent bitmap *NAME*, expose
that bitmap via the ``qemu:dirty-bitmap:NAME`` context
accessible through NBD_OPT_SET_META_CONTEXT.
.. option:: -s, --snapshot
Use *filename* as an external snapshot, create a temporary
file with ``backing_file=``\ *filename*, redirect the write to
the temporary one.
.. option:: -l, --load-snapshot=SNAPSHOT_PARAM
Load an internal snapshot inside *filename* and export it
as an read-only device, SNAPSHOT_PARAM format is
``[ID],[NAME]`` or ``[ID_OR_NAME]``
.. option:: --cache=CACHE
The cache mode to be used with the file. See the documentation of
the emulator's ``-drive cache=...`` option for allowed values.
.. option:: -n, --nocache
Equivalent to :option:`--cache=none`.
.. option:: --aio=AIO
Set the asynchronous I/O mode between ``threads`` (the default),
``native`` (Linux only), and ``io_uring`` (Linux 5.1+).
.. option:: --discard=DISCARD
Control whether ``discard`` (also known as ``trim`` or ``unmap``)
requests are ignored or passed to the filesystem. *DISCARD* is one of
``ignore`` (or ``off``), ``unmap`` (or ``on``). The default is
.. option:: --detect-zeroes=DETECT_ZEROES
Control the automatic conversion of plain zero writes by the OS to
driver-specific optimized zero write commands. *DETECT_ZEROES* is one of
``off``, ``on``, or ``unmap``. ``unmap``
converts a zero write to an unmap operation and can only be used if
*DISCARD* is set to ``unmap``. The default is ``off``.
.. option:: -c, --connect=DEV
Connect *filename* to NBD device *DEV* (Linux only).
.. option:: -d, --disconnect
Disconnect the device *DEV* (Linux only).
.. option:: -e, --shared=NUM
Allow up to *NUM* clients to share the device (default
``1``). Safe for readers, but for now, consistency is not
guaranteed between multiple writers.
.. option:: -t, --persistent
Don't exit on the last connection.
.. option:: -x, --export-name=NAME
Set the NBD volume export name (default of a zero-length string).
.. option:: -D, --description=DESCRIPTION
Set the NBD volume export description, as a human-readable
.. option:: -L, --list
Connect as a client and list all details about the exports exposed by
a remote NBD server. This enables list mode, and is incompatible
with options that change behavior related to a specific export (such as
:option:`--export-name`, :option:`--offset`, ...).
.. option:: --tls-creds=ID
Enable mandatory TLS encryption for the server by setting the ID
of the TLS credentials object previously created with the --object
option; or provide the credentials needed for connecting as a client
in list mode.
.. option:: --fork
Fork off the server process and exit the parent once the server is running.
.. option:: --pid-file=PATH
Store the server's process ID in the given file.
.. option:: --tls-authz=ID
Specify the ID of a qauthz object previously created with the
:option:`--object` option. This will be used to authorize connecting users
against their x509 distinguished name.
.. option:: -v, --verbose
Display extra debugging information.
.. option:: -h, --help
Display this help and exit.
.. option:: -V, --version
Display version information and exit.
.. option:: -T, --trace [[enable=]PATTERN][,events=FILE][,file=FILE]
.. include:: ../
Start a server listening on port 10809 that exposes only the
guest-visible contents of a qcow2 file, with no TLS encryption, and
with the default export name (an empty string). The command is
one-shot, and will block until the first successful client
qemu-nbd -f qcow2 file.qcow2
Start a long-running server listening with encryption on port 10810,
and whitelist clients with a specific X.509 certificate to connect to
a 1 megabyte subset of a raw file, using the export name 'subset':
qemu-nbd \
--object tls-creds-x509,id=tls0,endpoint=server,dir=/path/to/qemutls \
--object 'authz-simple,id=auth0,,,\
O=Example Org,,L=London,,ST=London,,C=GB' \
--tls-creds tls0 --tls-authz auth0 \
-t -x subset -p 10810 \
--image-opts driver=raw,offset=1M,size=1M,file.driver=file,file.filename=file.raw
Serve a read-only copy of a guest image over a Unix socket with as
many as 5 simultaneous readers, with a persistent process forked as a
qemu-nbd --fork --persistent --shared=5 --socket=/path/to/sock \
--read-only --format=qcow2 file.qcow2
Expose the guest-visible contents of a qcow2 file via a block device
/dev/nbd0 (and possibly creating /dev/nbd0p1 and friends for
partitions found within), then disconnect the device when done.
Access to bind qemu-nbd to an /dev/nbd device generally requires root
privileges, and may also require the execution of ``modprobe nbd``
to enable the kernel NBD client module. *CAUTION*: Do not use
this method to mount filesystems from an untrusted guest image - a
malicious guest may have prepared the image to attempt to trigger
kernel bugs in partition probing or file system mounting.
qemu-nbd -c /dev/nbd0 -f qcow2 file.qcow2
qemu-nbd -d /dev/nbd0
Query a remote server to see details about what export(s) it is
serving on port 10809, and authenticating via PSK:
qemu-nbd \
--object tls-creds-psk,id=tls0,dir=/tmp/keys,username=eblake,endpoint=client \
--tls-creds tls0 -L -b
See also
:manpage:`qemu(1)`, :manpage:`qemu-img(1)`