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@c man begin SYNOPSIS
@command{qemu-nbd} [OPTION]... @var{filename}
@command{qemu-nbd} @option{-L} [OPTION]...
@command{qemu-nbd} @option{-d} @var{dev}
@c man end
@end example
@c man begin 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.
@end itemize
@c man end
@c man begin OPTIONS
@var{filename} is a disk image filename, or a set of block
driver options if @option{--image-opts} is specified.
@var{dev} is an NBD device.
@table @option
@item --object type,id=@var{id},...props...
Define a new instance of the @var{type} object class identified by @var{id}.
See the @code{qemu(1)} manual page for full details of the properties
supported. The common object types that it makes sense to define are the
@code{secret} object, which is used to supply passwords and/or encryption
keys, and the @code{tls-creds} object, which is used to supply TLS
credentials for the qemu-nbd server or client.
@item -p, --port=@var{port}
The TCP port to listen on as a server, or connect to as a client
(default @samp{10809}).
@item -o, --offset=@var{offset}
The offset into the image.
@item -b, --bind=@var{iface}
The interface to bind to as a server, or connect to as a client
(default @samp{}).
@item -k, --socket=@var{path}
Use a unix socket with path @var{path}.
@item --image-opts
Treat @var{filename} as a set of image options, instead of a plain
filename. If this flag is specified, the @var{-f} flag should
not be used, instead the '@code{format=}' option should be set.
@item -f, --format=@var{fmt}
Force the use of the block driver for format @var{fmt} instead of
@item -r, --read-only
Export the disk as read-only.
@item -P, --partition=@var{num}
Deprecated: Only expose MBR partition @var{num}. Understands physical
partitions 1-4 and logical partition 5. New code should instead use
@option{--image-opts} with the raw driver wrapping a subset of the
original image.
@item -B, --bitmap=@var{name}
If @var{filename} has a qcow2 persistent bitmap @var{name}, expose
that bitmap via the ``qemu:dirty-bitmap:@var{name}'' context
accessible through NBD_OPT_SET_META_CONTEXT.
@item -s, --snapshot
Use @var{filename} as an external snapshot, create a temporary
file with backing_file=@var{filename}, redirect the write to
the temporary one.
@item -l, --load-snapshot=@var{snapshot_param}
Load an internal snapshot inside @var{filename} and export it
as an read-only device, @var{snapshot_param} format is
'[ID],[NAME]' or '[ID_OR_NAME]'
@item -n, --nocache
@itemx --cache=@var{cache}
The cache mode to be used with the file. See the documentation of
the emulator's @code{-drive cache=...} option for allowed values.
@item --aio=@var{aio}
Set the asynchronous I/O mode between @samp{threads} (the default)
and @samp{native} (Linux only).
@item --discard=@var{discard}
Control whether @dfn{discard} (also known as @dfn{trim} or @dfn{unmap})
requests are ignored or passed to the filesystem. @var{discard} is one of
@samp{ignore} (or @samp{off}), @samp{unmap} (or @samp{on}). The default is
@item --detect-zeroes=@var{detect-zeroes}
Control the automatic conversion of plain zero writes by the OS to
driver-specific optimized zero write commands. @var{detect-zeroes} is one of
@samp{off}, @samp{on} or @samp{unmap}. @samp{unmap}
converts a zero write to an unmap operation and can only be used if
@var{discard} is set to @samp{unmap}. The default is @samp{off}.
@item -c, --connect=@var{dev}
Connect @var{filename} to NBD device @var{dev} (Linux only).
@item -d, --disconnect
Disconnect the device @var{dev} (Linux only).
@item -e, --shared=@var{num}
Allow up to @var{num} clients to share the device (default
@samp{1}). Safe for readers, but for now, consistency is not
guaranteed between multiple writers.
@item -t, --persistent
Don't exit on the last connection.
@item -x, --export-name=@var{name}
Set the NBD volume export name (default of a zero-length string).
@item -D, --description=@var{description}
Set the NBD volume export description, as a human-readable
@item -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}, ...).
@item --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.
@item --fork
Fork off the server process and exit the parent once the server is running.
@item --pid-file=PATH
Store the server's process ID in the given file.
@item --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.
@item -v, --verbose
Display extra debugging information.
@item -h, --help
Display this help and exit.
@item -V, --version
Display version information and exit.
@item -T, --trace [[enable=]@var{pattern}][,events=@var{file}][,file=@var{file}]
@findex --trace
@include qemu-option-trace.texi
@end table
@c man end
@c man begin 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
qemu-nbd -f qcow2 file.qcow2
@end example
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,,,\
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
@end example
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
@end example
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 @code{modprobe nbd}
to enable the kernel NBD client module. @emph{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
@end example
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
@end example
@c man end
@setfilename qemu-nbd
@settitle QEMU Disk Network Block Device Server
@c man begin AUTHOR
Copyright (C) 2006 Anthony Liguori <>.
This is free software; see the source for copying conditions. There is NO
@c man end
@c man begin SEEALSO
qemu(1), qemu-img(1)
@c man end
@end ignore