1QEMU-NBD(8) QEMU QEMU-NBD(8)
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6 qemu-nbd - QEMU Disk Network Block Device Server
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9 qemu-nbd [OPTION]... filename
10 qemu-nbd -L [OPTION]...
12 qemu-nbd -d dev
14
16 Export a QEMU disk image using the NBD protocol.
17 Other uses:
19 • Bind a /dev/nbdX block device to a QEMU server (on Linux).
21 • As a client to query exports of a remote NBD server.
23
25 filename is a disk image filename, or a set of block driver options if
26 --image-opts is specified.
27 dev is an NBD device.
29 --object type,id=ID,...props...
31 Define a new instance of the type object class identified by ID.
32 See the qemu(1) manual page for full details of the properties
33 supported. The common object types that it makes sense to define
34 are the secret object, which is used to supply passwords and/or
35 encryption keys, and the tls-creds object, which is used to sup‐
36 ply TLS credentials for the qemu-nbd server or client.
37 -p, --port=PORT
39 TCP port to listen on as a server, or connect to as a client
40 (default 10809).
41 -o, --offset=OFFSET
43 The offset into the image.
44 -b, --bind=IFACE
46 The interface to bind to as a server, or connect to as a client
47 (default 0.0.0.0).
48 -k, --socket=PATH
50 Use a unix socket with path PATH.
51 --image-opts
53 Treat filename as a set of image options, instead of a plain
54 filename. If this flag is specified, the -f flag should not be
55 used, instead the format= option should be set.
56 -f, --format=FMT
58 Force the use of the block driver for format FMT instead of
59 auto-detecting.
60 -r, --read-only
62 Export the disk as read-only.
63 -A, --allocation-depth
65 Expose allocation depth information via the qemu:alloca‐
66 tion-depth metadata context accessible through
67 NBD_OPT_SET_META_CONTEXT.
68 -B, --bitmap=NAME
70 If filename has a qcow2 persistent bitmap NAME, expose that bit‐
71 map via the qemu:dirty-bitmap:NAME metadata context accessible
72 through NBD_OPT_SET_META_CONTEXT.
73 -s, --snapshot
75 Use filename as an external snapshot, create a temporary file
76 with backing_file=filename, redirect the write to the temporary
77 one.
78 -l, --load-snapshot=SNAPSHOT_PARAM
80 Load an internal snapshot inside filename and export it as an
81 read-only device, SNAPSHOT_PARAM format is snap‐
82 shot.id=[ID],snapshot.name=[NAME] or [ID_OR_NAME]
83 --cache=CACHE
85 The cache mode to be used with the file. See the documentation
86 of the emulator's -drive cache=... option for allowed values.
87 -n, --nocache
89 Equivalent to --cache=none.
90 --aio=AIO
92 Set the asynchronous I/O mode between threads (the default), na‐
93 tive (Linux only), and io_uring (Linux 5.1+).
94 --discard=DISCARD
96 Control whether discard (also known as trim or unmap) requests
97 are ignored or passed to the filesystem. DISCARD is one of ig‐
98 nore (or off), unmap (or on). The default is ignore.
99 --detect-zeroes=DETECT_ZEROES
101 Control the automatic conversion of plain zero writes by the OS
102 to driver-specific optimized zero write commands. DETECT_ZEROES
103 is one of off, on, or unmap. unmap converts a zero write to an
104 unmap operation and can only be used if DISCARD is set to unmap.
105 The default is off.
106 -c, --connect=DEV
108 Connect filename to NBD device DEV (Linux only).
109 -d, --disconnect
111 Disconnect the device DEV (Linux only).
112 -e, --shared=NUM
114 Allow up to NUM clients to share the device (default 1). Safe
115 for readers, but for now, consistency is not guaranteed between
116 multiple writers.
117 -t, --persistent
119 Don't exit on the last connection.
120 -x, --export-name=NAME
122 Set the NBD volume export name (default of a zero-length
123 string).
124 -D, --description=DESCRIPTION
126 Set the NBD volume export description, as a human-readable
127 string.
128 -L, --list
130 Connect as a client and list all details about the exports ex‐
131 posed by a remote NBD server. This enables list mode, and is
132 incompatible with options that change behavior related to a spe‐
133 cific export (such as --export-name, --offset, ...).
134 --tls-creds=ID
136 Enable mandatory TLS encryption for the server by setting the ID
137 of the TLS credentials object previously created with the --ob‐
138 ject option; or provide the credentials needed for connecting as
139 a client in list mode.
140 --fork Fork off the server process and exit the parent once the server
142 is running.
143 --pid-file=PATH
145 Store the server's process ID in the given file.
146 --tls-authz=ID
148 Specify the ID of a qauthz object previously created with the
149 --object option. This will be used to authorize connecting users
150 against their x509 distinguished name.
151 -v, --verbose
153 Display extra debugging information.
154 -h, --help
156 Display this help and exit.
157 -V, --version
159 Display version information and exit.
160 -T, --trace [[enable=]PATTERN][,events=FILE][,file=FILE]
162 Specify tracing options.
163 [enable=]PATTERN
165 Immediately enable events matching PATTERN (either event name
166 or a globbing pattern). This option is only available if
167 QEMU has been compiled with the simple, log or ftrace tracing
168 backend. To specify multiple events or patterns, specify the
169 -trace option multiple times.
170 Use -trace help to print a list of names of trace points.
172 events=FILE
174 Immediately enable events listed in FILE. The file must con‐
175 tain one event name (as listed in the trace-events-all file)
176 per line; globbing patterns are accepted too. This option is
177 only available if QEMU has been compiled with the simple, log
178 or ftrace tracing backend.
179 file=FILE
181 Log output traces to FILE. This option is only available if
182 QEMU has been compiled with the simple tracing backend.
183
185 Start a server listening on port 10809 that exposes only the guest-vis‐
186 ible contents of a qcow2 file, with no TLS encryption, and with the de‐
187 fault export name (an empty string). The command is one-shot, and will
188 block until the first successful client disconnects:
189 qemu-nbd -f qcow2 file.qcow2
191 Start a long-running server listening with encryption on port 10810,
193 and whitelist clients with a specific X.509 certificate to connect to a
194 1 megabyte subset of a raw file, using the export name 'subset':
195 qemu-nbd \
197 --object tls-creds-x509,id=tls0,endpoint=server,dir=/path/to/qemutls \
198 --object 'authz-simple,id=auth0,identity=CN=laptop.example.com,,\
199 O=Example Org,,L=London,,ST=London,,C=GB' \
200 --tls-creds tls0 --tls-authz auth0 \
201 -t -x subset -p 10810 \
202 --image-opts driver=raw,offset=1M,size=1M,file.driver=file,file.filename=file.raw
203 Serve a read-only copy of a guest image over a Unix socket with as many
205 as 5 simultaneous readers, with a persistent process forked as a dae‐
206 mon:
207 qemu-nbd --fork --persistent --shared=5 --socket=/path/to/sock \
209 --read-only --format=qcow2 file.qcow2
210 Expose the guest-visible contents of a qcow2 file via a block device
212 /dev/nbd0 (and possibly creating /dev/nbd0p1 and friends for partitions
213 found within), then disconnect the device when done. Access to bind
214 qemu-nbd to an /dev/nbd device generally requires root privileges, and
215 may also require the execution of modprobe nbd to enable the kernel NBD
216 client module. CAUTION: Do not use this method to mount filesystems
217 from an untrusted guest image - a malicious guest may have prepared the
218 image to attempt to trigger kernel bugs in partition probing or file
219 system mounting.
220 qemu-nbd -c /dev/nbd0 -f qcow2 file.qcow2
222 qemu-nbd -d /dev/nbd0
223 Query a remote server to see details about what export(s) it is serving
225 on port 10809, and authenticating via PSK:
226 qemu-nbd \
228 --object tls-creds-psk,id=tls0,dir=/tmp/keys,username=eblake,endpoint=client \
229 --tls-creds tls0 -L -b remote.example.com
230
232 qemu(1), qemu-img(1)
233
235 Anthony Liguori <anthony@codemonkey.ws>
236
238 2021, The QEMU Project Developers
2395.2.0 May 19, 2021 QEMU-NBD(8)