===================================== 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 :option:`--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. .. 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 :option:`--object` option; or provide the credentials needed for connecting as a client in list mode. .. option:: --tls-hostname=hostname When validating an x509 certificate received over a TLS connection, the hostname that the NBD client used to connect will be checked against information in the server provided certificate. Sometimes it might be required to override the hostname used to perform this check. For example, if the NBD client is using a tunnel from localhost to connect to the remote server, the :option:`--tls-hostname` option should be used to set the officially expected hostname of the remote NBD server. This can also be used if accessing NBD over a UNIX socket where there is no inherent hostname available. This is only permitted when acting as a NBD client with the :option:`--list` option. .. 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. This option also keeps the original *STDERR* stream open if the ``qemu-nbd`` process is daemonized due to other options like :option:`--fork` or :option:`-c`. .. 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 allow 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)`