1 QEMU Disk Network Block Device Server
2 =====================================
7 **qemu-nbd** [*OPTION*]... *filename*
9 **qemu-nbd** -L [*OPTION*]...
16 Export a QEMU disk image using the NBD protocol.
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.
28 *filename* is a disk image filename, or a set of block
29 driver options if ``--image-opts`` is specified.
31 *dev* is an NBD device.
33 .. option:: --object type,id=ID,...props...
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.
42 .. option:: -p, --port=PORT
44 TCP port to listen on as a server, or connect to as a client
47 .. option:: -o, --offset=OFFSET
49 The offset into the image.
51 .. option:: -b, --bind=IFACE
53 The interface to bind to as a server, or connect to as a client
54 (default ``0.0.0.0``).
56 .. option:: -k, --socket=PATH
58 Use a unix socket with path *PATH*.
60 .. option:: --image-opts
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.
66 .. option:: -f, --format=FMT
68 Force the use of the block driver for format *FMT* instead of
71 .. option:: -r, --read-only
73 Export the disk as read-only.
75 .. option:: -P, --partition=NUM
77 Deprecated: Only expose MBR partition *NUM*. Understands physical
78 partitions 1-4 and logical partition 5. New code should instead use
79 :option:`--image-opts` with the raw driver wrapping a subset of the
82 .. option:: -B, --bitmap=NAME
84 If *filename* has a qcow2 persistent bitmap *NAME*, expose
85 that bitmap via the ``qemu:dirty-bitmap:NAME`` context
86 accessible through NBD_OPT_SET_META_CONTEXT.
88 .. option:: -s, --snapshot
90 Use *filename* as an external snapshot, create a temporary
91 file with ``backing_file=``\ *filename*, redirect the write to
94 .. option:: -l, --load-snapshot=SNAPSHOT_PARAM
96 Load an internal snapshot inside *filename* and export it
97 as an read-only device, SNAPSHOT_PARAM format is
98 ``snapshot.id=[ID],snapshot.name=[NAME]`` or ``[ID_OR_NAME]``
100 .. option:: --cache=CACHE
102 The cache mode to be used with the file. See the documentation of
103 the emulator's ``-drive cache=...`` option for allowed values.
105 .. option:: -n, --nocache
107 Equivalent to :option:`--cache=none`.
109 .. option:: --aio=AIO
111 Set the asynchronous I/O mode between ``threads`` (the default)
112 and ``native`` (Linux only).
114 .. option:: --discard=DISCARD
116 Control whether ``discard`` (also known as ``trim`` or ``unmap``)
117 requests are ignored or passed to the filesystem. *DISCARD* is one of
118 ``ignore`` (or ``off``), ``unmap`` (or ``on``). The default is
121 .. option:: --detect-zeroes=DETECT_ZEROES
123 Control the automatic conversion of plain zero writes by the OS to
124 driver-specific optimized zero write commands. *DETECT_ZEROES* is one of
125 ``off``, ``on``, or ``unmap``. ``unmap``
126 converts a zero write to an unmap operation and can only be used if
127 *DISCARD* is set to ``unmap``. The default is ``off``.
129 .. option:: -c, --connect=DEV
131 Connect *filename* to NBD device *DEV* (Linux only).
133 .. option:: -d, --disconnect
135 Disconnect the device *DEV* (Linux only).
137 .. option:: -e, --shared=NUM
139 Allow up to *NUM* clients to share the device (default
140 ``1``). Safe for readers, but for now, consistency is not
141 guaranteed between multiple writers.
143 .. option:: -t, --persistent
145 Don't exit on the last connection.
147 .. option:: -x, --export-name=NAME
149 Set the NBD volume export name (default of a zero-length string).
151 .. option:: -D, --description=DESCRIPTION
153 Set the NBD volume export description, as a human-readable
156 .. option:: -L, --list
158 Connect as a client and list all details about the exports exposed by
159 a remote NBD server. This enables list mode, and is incompatible
160 with options that change behavior related to a specific export (such as
161 :option:`--export-name`, :option:`--offset`, ...).
163 .. option:: --tls-creds=ID
165 Enable mandatory TLS encryption for the server by setting the ID
166 of the TLS credentials object previously created with the --object
167 option; or provide the credentials needed for connecting as a client
172 Fork off the server process and exit the parent once the server is running.
174 .. option:: --pid-file=PATH
176 Store the server's process ID in the given file.
178 .. option:: --tls-authz=ID
180 Specify the ID of a qauthz object previously created with the
181 :option:`--object` option. This will be used to authorize connecting users
182 against their x509 distinguished name.
184 .. option:: -v, --verbose
186 Display extra debugging information.
188 .. option:: -h, --help
190 Display this help and exit.
192 .. option:: -V, --version
194 Display version information and exit.
196 .. option:: -T, --trace [[enable=]PATTERN][,events=FILE][,file=FILE]
198 .. include:: qemu-option-trace.rst.inc
203 Start a server listening on port 10809 that exposes only the
204 guest-visible contents of a qcow2 file, with no TLS encryption, and
205 with the default export name (an empty string). The command is
206 one-shot, and will block until the first successful client
211 qemu-nbd -f qcow2 file.qcow2
213 Start a long-running server listening with encryption on port 10810,
214 and whitelist clients with a specific X.509 certificate to connect to
215 a 1 megabyte subset of a raw file, using the export name 'subset':
220 --object tls-creds-x509,id=tls0,endpoint=server,dir=/path/to/qemutls \
221 --object 'authz-simple,id=auth0,identity=CN=laptop.example.com,,\
222 O=Example Org,,L=London,,ST=London,,C=GB' \
223 --tls-creds tls0 --tls-authz auth0 \
224 -t -x subset -p 10810 \
225 --image-opts driver=raw,offset=1M,size=1M,file.driver=file,file.filename=file.raw
227 Serve a read-only copy of just the first MBR partition of a guest
228 image over a Unix socket with as many as 5 simultaneous readers, with
229 a persistent process forked as a daemon:
233 qemu-nbd --fork --persistent --shared=5 --socket=/path/to/sock \
234 --partition=1 --read-only --format=qcow2 file.qcow2
236 Expose the guest-visible contents of a qcow2 file via a block device
237 /dev/nbd0 (and possibly creating /dev/nbd0p1 and friends for
238 partitions found within), then disconnect the device when done.
239 Access to bind qemu-nbd to an /dev/nbd device generally requires root
240 privileges, and may also require the execution of ``modprobe nbd``
241 to enable the kernel NBD client module. *CAUTION*: Do not use
242 this method to mount filesystems from an untrusted guest image - a
243 malicious guest may have prepared the image to attempt to trigger
244 kernel bugs in partition probing or file system mounting.
248 qemu-nbd -c /dev/nbd0 -f qcow2 file.qcow2
249 qemu-nbd -d /dev/nbd0
251 Query a remote server to see details about what export(s) it is
252 serving on port 10809, and authenticating via PSK:
257 --object tls-creds-psk,id=tls0,dir=/tmp/keys,username=eblake,endpoint=client \
258 --tls-creds tls0 -L -b remote.example.com
263 :manpage:`qemu(1)`, :manpage:`qemu-img(1)`