===================================== 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,... 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:: -A, --allocation-depth Expose allocation depth information via the ``qemu:allocation-depth`` metadata context accessible through NBD_OPT_SET_META_CONTEXT. .. option:: -B, --bitmap=NAME If *filename* has a qcow2 persistent bitmap *NAME*, expose that bitmap via the ``qemu:dirty-bitmap:NAME`` metadata 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. Valid values are: ``none``, ``writeback`` (the default), ``writethrough``, ``directsync`` and ``unsafe``. See the documentation of the emulator's ``-drive cache=...`` option for more info. .. 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``), 0 for unlimited. 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 a /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)`