Provided by: qemu-utils_4.2-3ubuntu6.30_amd64 
NAME
qemu-nbd - 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
filename is a disk image filename, or a set of block driver options if --image-opts is specified.
dev is an NBD device.
--object type,id=id,...props...
Define a new instance of the type object class identified by id. See the 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.
-p, --port=port
The TCP port to listen on as a server, or connect to as a client (default 10809).
-o, --offset=offset
The offset into the image.
-b, --bind=iface
The interface to bind to as a server, or connect to as a client (default 0.0.0.0).
-k, --socket=path
Use a unix socket with path path.
--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 '"format="' option should be set.
-f, --format=fmt
Force the use of the block driver for format fmt instead of auto-detecting.
-r, --read-only
Export the disk as read-only.
-P, --partition=num
Deprecated: Only expose MBR partition num. Understands physical partitions 1-4 and logical partition
5. New code should instead use --image-opts with the raw driver wrapping a subset of the original
image.
-B, --bitmap=name
If filename has a qcow2 persistent bitmap name, expose that bitmap via the "qemu:dirty-bitmap:name"
context accessible through NBD_OPT_SET_META_CONTEXT.
-s, --snapshot
Use filename as an external snapshot, create a temporary file with backing_file=filename, redirect
the write to the temporary one.
-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]'
-n, --nocache
--cache=cache
The cache mode to be used with the file. See the documentation of the emulator's "-drive cache=..."
option for allowed values.
--aio=aio
Set the asynchronous I/O mode between threads (the default) and native (Linux only).
--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.
--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.
-c, --connect=dev
Connect filename to NBD device dev (Linux only).
-d, --disconnect
Disconnect the device dev (Linux only).
-e, --shared=num
Allow up to num clients to share the device (default 1). Safe for readers, but for now, consistency
is not guaranteed between multiple writers.
-t, --persistent
Don't exit on the last connection.
-x, --export-name=name
Set the NBD volume export name (default of a zero-length string).
-D, --description=description
Set the NBD volume export description, as a human-readable string.
-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 --export-name, --offset, ...).
--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.
--fork
Fork off the server process and exit the parent once the server is running.
--pid-file=PATH
Store the server's process ID in the given file.
--tls-authz=ID
Specify the ID of a qauthz object previously created with the --object option. This will be used to
authorize connecting users against their x509 distinguished name.
-v, --verbose
Display extra debugging information.
-h, --help
Display this help and exit.
-V, --version
Display version information and exit.
-T, --trace [[enable=]pattern][,events=file][,file=file]
Specify tracing options.
[enable=]pattern
Immediately enable events matching pattern (either event name or a globbing pattern). This
option is only available if QEMU has been compiled with the simple, log or ftrace tracing
backend. To specify multiple events or patterns, specify the -trace option multiple times.
Use "-trace help" to print a list of names of trace points.
events=file
Immediately enable events listed in file. The file must contain one event name (as listed in the
trace-events-all file) per line; globbing patterns are accepted too. This option is only
available if QEMU has been compiled with the simple, log or ftrace tracing backend.
file=file
Log output traces to file. This option is only available if QEMU has been compiled with the
simple tracing backend.
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 just the first MBR partition 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 \
--partition=1 --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 an /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
qemu(1), qemu-img(1)
AUTHOR
Copyright (C) 2006 Anthony Liguori <anthony@codemonkey.ws>. This is free software; see the source for copying conditions. There is NO warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. 2024-10-20 QEMU-NBD.8(8)