35e087de08
Signed-off-by: Denis V. Lunev <den@openvz.org> CC: Eric Blake <eblake@redhat.com> CC: Vladimir Sementsov-Ogievskiy <vsementsov@yandex-team.ru> Message-ID: <20230906093210.339585-8-den@openvz.org> Reviewed-by: Eric Blake <eblake@redhat.com> [eblake: Wording improvement] Signed-off-by: Eric Blake <eblake@redhat.com>
280 lines
8.4 KiB
ReStructuredText
280 lines
8.4 KiB
ReStructuredText
=====================================
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QEMU Disk Network Block Device Server
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=====================================
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Synopsis
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--------
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**qemu-nbd** [*OPTION*]... *filename*
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**qemu-nbd** -L [*OPTION*]...
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**qemu-nbd** -d *dev*
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Description
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-----------
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Export a QEMU disk image using the NBD protocol.
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Other uses:
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- Bind a /dev/nbdX block device to a QEMU server (on Linux).
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- As a client to query exports of a remote NBD server.
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Options
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-------
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.. program:: qemu-nbd
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*filename* is a disk image filename, or a set of block
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driver options if :option:`--image-opts` is specified.
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*dev* is an NBD device.
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.. option:: --object type,id=ID,...
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Define a new instance of the *type* object class identified by *ID*.
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See the :manpage:`qemu(1)` manual page for full details of the properties
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supported. The common object types that it makes sense to define are the
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``secret`` object, which is used to supply passwords and/or encryption
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keys, and the ``tls-creds`` object, which is used to supply TLS
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credentials for the ``qemu-nbd`` server or client.
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.. option:: -p, --port=PORT
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TCP port to listen on as a server, or connect to as a client
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(default ``10809``).
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.. option:: -o, --offset=OFFSET
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The offset into the image.
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.. option:: -b, --bind=IFACE
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The interface to bind to as a server, or connect to as a client
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(default ``0.0.0.0``).
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.. option:: -k, --socket=PATH
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Use a unix socket with path *PATH*.
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.. option:: --image-opts
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Treat *filename* as a set of image options, instead of a plain
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filename. If this flag is specified, the ``-f`` flag should
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not be used, instead the :option:`format=` option should be set.
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.. option:: -f, --format=FMT
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Force the use of the block driver for format *FMT* instead of
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auto-detecting.
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.. option:: -r, --read-only
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Export the disk as read-only.
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.. option:: -A, --allocation-depth
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Expose allocation depth information via the
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``qemu:allocation-depth`` metadata context accessible through
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NBD_OPT_SET_META_CONTEXT.
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.. option:: -B, --bitmap=NAME
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If *filename* has a qcow2 persistent bitmap *NAME*, expose
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that bitmap via the ``qemu:dirty-bitmap:NAME`` metadata context
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accessible through NBD_OPT_SET_META_CONTEXT.
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.. option:: -s, --snapshot
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Use *filename* as an external snapshot, create a temporary
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file with ``backing_file=``\ *filename*, redirect the write to
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the temporary one.
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.. option:: -l, --load-snapshot=SNAPSHOT_PARAM
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Load an internal snapshot inside *filename* and export it
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as an read-only device, SNAPSHOT_PARAM format is
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``snapshot.id=[ID],snapshot.name=[NAME]`` or ``[ID_OR_NAME]``
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.. option:: --cache=CACHE
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The cache mode to be used with the file. Valid values are:
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``none``, ``writeback`` (the default), ``writethrough``,
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``directsync`` and ``unsafe``. See the documentation of
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the emulator's ``-drive cache=...`` option for more info.
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.. option:: -n, --nocache
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Equivalent to :option:`--cache=none`.
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.. option:: --aio=AIO
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Set the asynchronous I/O mode between ``threads`` (the default),
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``native`` (Linux only), and ``io_uring`` (Linux 5.1+).
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.. option:: --discard=DISCARD
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Control whether ``discard`` (also known as ``trim`` or ``unmap``)
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requests are ignored or passed to the filesystem. *DISCARD* is one of
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``ignore`` (or ``off``), ``unmap`` (or ``on``). The default is
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``ignore``.
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.. option:: --detect-zeroes=DETECT_ZEROES
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Control the automatic conversion of plain zero writes by the OS to
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driver-specific optimized zero write commands. *DETECT_ZEROES* is one of
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``off``, ``on``, or ``unmap``. ``unmap``
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converts a zero write to an unmap operation and can only be used if
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*DISCARD* is set to ``unmap``. The default is ``off``.
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.. option:: -c, --connect=DEV
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Connect *filename* to NBD device *DEV* (Linux only).
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.. option:: -d, --disconnect
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Disconnect the device *DEV* (Linux only).
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.. option:: -e, --shared=NUM
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Allow up to *NUM* clients to share the device (default
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``1``), 0 for unlimited.
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.. option:: -t, --persistent
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Don't exit on the last connection.
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.. option:: -x, --export-name=NAME
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Set the NBD volume export name (default of a zero-length string).
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.. option:: -D, --description=DESCRIPTION
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Set the NBD volume export description, as a human-readable
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string.
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.. option:: -L, --list
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Connect as a client and list all details about the exports exposed by
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a remote NBD server. This enables list mode, and is incompatible
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with options that change behavior related to a specific export (such as
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:option:`--export-name`, :option:`--offset`, ...).
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.. option:: --tls-creds=ID
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Enable mandatory TLS encryption for the server by setting the ID
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of the TLS credentials object previously created with the
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:option:`--object` option; or provide the credentials needed for
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connecting as a client in list mode.
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.. option:: --tls-hostname=hostname
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When validating an x509 certificate received over a TLS connection,
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the hostname that the NBD client used to connect will be checked
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against information in the server provided certificate. Sometimes
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it might be required to override the hostname used to perform this
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check. For example, if the NBD client is using a tunnel from localhost
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to connect to the remote server, the :option:`--tls-hostname` option should
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be used to set the officially expected hostname of the remote NBD
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server. This can also be used if accessing NBD over a UNIX socket
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where there is no inherent hostname available. This is only permitted
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when acting as a NBD client with the :option:`--list` option.
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.. option:: --fork
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Fork off the server process and exit the parent once the server is running.
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.. option:: --pid-file=PATH
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Store the server's process ID in the given file.
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.. option:: --tls-authz=ID
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Specify the ID of a qauthz object previously created with the
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:option:`--object` option. This will be used to authorize connecting users
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against their x509 distinguished name.
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.. option:: -v, --verbose
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Display extra debugging information. This option also keeps the original
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*STDERR* stream open if the ``qemu-nbd`` process is daemonized due to
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other options like :option:`--fork` or :option:`-c`.
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.. option:: -h, --help
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Display this help and exit.
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.. option:: -V, --version
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Display version information and exit.
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.. option:: -T, --trace [[enable=]PATTERN][,events=FILE][,file=FILE]
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.. include:: ../qemu-option-trace.rst.inc
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Examples
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--------
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Start a server listening on port 10809 that exposes only the
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guest-visible contents of a qcow2 file, with no TLS encryption, and
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with the default export name (an empty string). The command is
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one-shot, and will block until the first successful client
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disconnects:
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::
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qemu-nbd -f qcow2 file.qcow2
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Start a long-running server listening with encryption on port 10810,
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and allow clients with a specific X.509 certificate to connect to
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a 1 megabyte subset of a raw file, using the export name 'subset':
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::
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qemu-nbd \
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--object tls-creds-x509,id=tls0,endpoint=server,dir=/path/to/qemutls \
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--object 'authz-simple,id=auth0,identity=CN=laptop.example.com,,\
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O=Example Org,,L=London,,ST=London,,C=GB' \
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--tls-creds tls0 --tls-authz auth0 \
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-t -x subset -p 10810 \
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--image-opts driver=raw,offset=1M,size=1M,file.driver=file,file.filename=file.raw
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Serve a read-only copy of a guest image over a Unix socket with as
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many as 5 simultaneous readers, with a persistent process forked as a
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daemon:
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::
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qemu-nbd --fork --persistent --shared=5 --socket=/path/to/sock \
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--read-only --format=qcow2 file.qcow2
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Expose the guest-visible contents of a qcow2 file via a block device
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/dev/nbd0 (and possibly creating /dev/nbd0p1 and friends for
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partitions found within), then disconnect the device when done.
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Access to bind ``qemu-nbd`` to a /dev/nbd device generally requires root
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privileges, and may also require the execution of ``modprobe nbd``
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to enable the kernel NBD client module. *CAUTION*: Do not use
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this method to mount filesystems from an untrusted guest image - a
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malicious guest may have prepared the image to attempt to trigger
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kernel bugs in partition probing or file system mounting.
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::
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qemu-nbd -c /dev/nbd0 -f qcow2 file.qcow2
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qemu-nbd -d /dev/nbd0
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Query a remote server to see details about what export(s) it is
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serving on port 10809, and authenticating via PSK:
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::
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qemu-nbd \
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--object tls-creds-psk,id=tls0,dir=/tmp/keys,username=eblake,endpoint=client \
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--tls-creds tls0 -L -b remote.example.com
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See also
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--------
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:manpage:`qemu(1)`, :manpage:`qemu-img(1)`
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