docs/tools/qemu-nbd.rst

   1 QEMU Disk Network Block Device Server
   2 =====================================
   3
   4 Synopsis
   5 --------
   6
   7 **qemu-nbd** [*OPTION*]... *filename*
   8
   9 **qemu-nbd** -L [*OPTION*]...
  10
  11 **qemu-nbd** -d *dev*
  12
  13 Description
  14 -----------
  15
  16 Export a QEMU disk image using the NBD protocol.
  17
  18 Other uses:
  19
  20 - Bind a /dev/nbdX block device to a QEMU server (on Linux).
  21 - As a client to query exports of a remote NBD server.
  22
  23 Options
  24 -------
  25
  26 .. program:: qemu-nbd
  27
  28 *filename* is a disk image filename, or a set of block
  29 driver options if ``--image-opts`` is specified.
  30
  31 *dev* is an NBD device.
  32
  33 .. option:: --object type,id=ID,...props...
  34
  35   Define a new instance of the *type* object class identified by *ID*.
  36   See the :manpage:`qemu(1)` manual page for full details of the properties
  37   supported. The common object types that it makes sense to define are the
  38   ``secret`` object, which is used to supply passwords and/or encryption
  39   keys, and the ``tls-creds`` object, which is used to supply TLS
  40   credentials for the qemu-nbd server or client.
  41
  42 .. option:: -p, --port=PORT
  43
  44   TCP port to listen on as a server, or connect to as a client
  45   (default ``10809``).
  46
  47 .. option:: -o, --offset=OFFSET
  48
  49   The offset into the image.
  50
  51 .. option:: -b, --bind=IFACE
  52
  53   The interface to bind to as a server, or connect to as a client
  54   (default ``0.0.0.0``).
  55
  56 .. option:: -k, --socket=PATH
  57
  58   Use a unix socket with path *PATH*.
  59
  60 .. option:: --image-opts
  61
  62   Treat *filename* as a set of image options, instead of a plain
  63   filename. If this flag is specified, the ``-f`` flag should
  64   not be used, instead the :option:`format=` option should be set.
  65
  66 .. option:: -f, --format=FMT
  67
  68   Force the use of the block driver for format *FMT* instead of
  69   auto-detecting.
  70
  71 .. option:: -r, --read-only
  72
  73   Export the disk as read-only.
  74
  75 .. option:: -B, --bitmap=NAME
  76
  77   If *filename* has a qcow2 persistent bitmap *NAME*, expose
  78   that bitmap via the ``qemu:dirty-bitmap:NAME`` context
  79   accessible through NBD_OPT_SET_META_CONTEXT.
  80
  81 .. option:: -s, --snapshot
  82
  83   Use *filename* as an external snapshot, create a temporary
  84   file with ``backing_file=``\ *filename*, redirect the write to
  85   the temporary one.
  86
  87 .. option:: -l, --load-snapshot=SNAPSHOT_PARAM
  88
  89   Load an internal snapshot inside *filename* and export it
  90   as an read-only device, SNAPSHOT_PARAM format is
  91   ``snapshot.id=[ID],snapshot.name=[NAME]`` or ``[ID_OR_NAME]``
  92
  93 .. option:: --cache=CACHE
  94
  95   The cache mode to be used with the file.  See the documentation of
  96   the emulator's ``-drive cache=...`` option for allowed values.
  97
  98 .. option:: -n, --nocache
  99
 100   Equivalent to :option:`--cache=none`.
 101
 102 .. option:: --aio=AIO
 103
 104   Set the asynchronous I/O mode between ``threads`` (the default),
 105   ``native`` (Linux only), and ``io_uring`` (Linux 5.1+).
 106
 107 .. option:: --discard=DISCARD
 108
 109   Control whether ``discard`` (also known as ``trim`` or ``unmap``)
 110   requests are ignored or passed to the filesystem. *DISCARD* is one of
 111   ``ignore`` (or ``off``), ``unmap`` (or ``on``).  The default is
 112   ``ignore``.
 113
 114 .. option:: --detect-zeroes=DETECT_ZEROES
 115
 116   Control the automatic conversion of plain zero writes by the OS to
 117   driver-specific optimized zero write commands.  *DETECT_ZEROES* is one of
 118   ``off``, ``on``, or ``unmap``.  ``unmap``
 119   converts a zero write to an unmap operation and can only be used if
 120   *DISCARD* is set to ``unmap``.  The default is ``off``.
 121
 122 .. option:: -c, --connect=DEV
 123
 124   Connect *filename* to NBD device *DEV* (Linux only).
 125
 126 .. option:: -d, --disconnect
 127
 128   Disconnect the device *DEV* (Linux only).
 129
 130 .. option:: -e, --shared=NUM
 131
 132   Allow up to *NUM* clients to share the device (default
 133   ``1``). Safe for readers, but for now, consistency is not
 134   guaranteed between multiple writers.
 135
 136 .. option:: -t, --persistent
 137
 138   Don't exit on the last connection.
 139
 140 .. option:: -x, --export-name=NAME
 141
 142   Set the NBD volume export name (default of a zero-length string).
 143
 144 .. option:: -D, --description=DESCRIPTION
 145
 146   Set the NBD volume export description, as a human-readable
 147   string.
 148
 149 .. option:: -L, --list
 150
 151   Connect as a client and list all details about the exports exposed by
 152   a remote NBD server.  This enables list mode, and is incompatible
 153   with options that change behavior related to a specific export (such as
 154   :option:`--export-name`, :option:`--offset`, ...).
 155
 156 .. option:: --tls-creds=ID
 157
 158   Enable mandatory TLS encryption for the server by setting the ID
 159   of the TLS credentials object previously created with the --object
 160   option; or provide the credentials needed for connecting as a client
 161   in list mode.
 162
 163 .. option:: --fork
 164
 165   Fork off the server process and exit the parent once the server is running.
 166
 167 .. option:: --pid-file=PATH
 168
 169   Store the server's process ID in the given file.
 170
 171 .. option:: --tls-authz=ID
 172
 173   Specify the ID of a qauthz object previously created with the
 174   :option:`--object` option. This will be used to authorize connecting users
 175   against their x509 distinguished name.
 176
 177 .. option:: -v, --verbose
 178
 179   Display extra debugging information.
 180
 181 .. option:: -h, --help
 182
 183   Display this help and exit.
 184
 185 .. option:: -V, --version
 186
 187   Display version information and exit.
 188
 189 .. option:: -T, --trace [[enable=]PATTERN][,events=FILE][,file=FILE]
 190
 191   .. include:: ../qemu-option-trace.rst.inc
 192
 193 Examples
 194 --------
 195
 196 Start a server listening on port 10809 that exposes only the
 197 guest-visible contents of a qcow2 file, with no TLS encryption, and
 198 with the default export name (an empty string). The command is
 199 one-shot, and will block until the first successful client
 200 disconnects:
 201
 202 ::
 203
 204   qemu-nbd -f qcow2 file.qcow2
 205
 206 Start a long-running server listening with encryption on port 10810,
 207 and whitelist clients with a specific X.509 certificate to connect to
 208 a 1 megabyte subset of a raw file, using the export name 'subset':
 209
 210 ::
 211
 212   qemu-nbd \
 213     --object tls-creds-x509,id=tls0,endpoint=server,dir=/path/to/qemutls \
 214     --object 'authz-simple,id=auth0,identity=CN=laptop.example.com,,\
 215               O=Example Org,,L=London,,ST=London,,C=GB' \
 216     --tls-creds tls0 --tls-authz auth0 \
 217     -t -x subset -p 10810 \
 218     --image-opts driver=raw,offset=1M,size=1M,file.driver=file,file.filename=file.raw
 219
 220 Serve a read-only copy of a guest image over a Unix socket with as
 221 many as 5 simultaneous readers, with a persistent process forked as a
 222 daemon:
 223
 224 ::
 225
 226   qemu-nbd --fork --persistent --shared=5 --socket=/path/to/sock \
 227     --read-only --format=qcow2 file.qcow2
 228
 229 Expose the guest-visible contents of a qcow2 file via a block device
 230 /dev/nbd0 (and possibly creating /dev/nbd0p1 and friends for
 231 partitions found within), then disconnect the device when done.
 232 Access to bind qemu-nbd to an /dev/nbd device generally requires root
 233 privileges, and may also require the execution of ``modprobe nbd``
 234 to enable the kernel NBD client module.  *CAUTION*: Do not use
 235 this method to mount filesystems from an untrusted guest image - a
 236 malicious guest may have prepared the image to attempt to trigger
 237 kernel bugs in partition probing or file system mounting.
 238
 239 ::
 240
 241   qemu-nbd -c /dev/nbd0 -f qcow2 file.qcow2
 242   qemu-nbd -d /dev/nbd0
 243
 244 Query a remote server to see details about what export(s) it is
 245 serving on port 10809, and authenticating via PSK:
 246
 247 ::
 248
 249   qemu-nbd \
 250     --object tls-creds-psk,id=tls0,dir=/tmp/keys,username=eblake,endpoint=client \
 251     --tls-creds tls0 -L -b remote.example.com
 252
 253 See also
 254 --------
 255
 256 :manpage:`qemu(1)`, :manpage:`qemu-img(1)`