qemu/docs/tools/qemu-nbd.rst

QEMU Disk Network Block Device Server
=====================================

Synopsis
--------

**qemu-nbd** [*OPTION*]... *filename*

**qemu-nbd** -L [*OPTION*]...

**qemu-nbd** -d *dev*

Description
-----------

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.

Options
-------

.. 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 ``0.0.0.0``).

.. 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
  auto-detecting.

.. 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
  ``snapshot.id=[ID],snapshot.name=[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
  ``ignore``.

.. 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
  string.

.. 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:: ../qemu-option-trace.rst.inc

Examples
--------

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
disconnects:

::

  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,identity=CN=laptop.example.com,,\
              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
daemon:

::

  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 remote.example.com

See also
--------

:manpage:`qemu(1)`, :manpage:`qemu-img(1)`