QEMU Disk Network Block Device Server¶
qemu-nbd [OPTION]… filename
qemu-nbd -L [OPTION]…
qemu-nbd -d dev
Export a QEMU disk image using the NBD protocol.
- Bind a /dev/nbdX block device to a QEMU server (on Linux).
- As a client to query exports of a remote NBD server.
filename is a disk image filename, or a set of block
driver options if
--image-opts is specified.
dev is an NBD device.
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
secretobject, which is used to supply passwords and/or encryption keys, and the
tls-credsobject, which is used to supply TLS credentials for the
qemu-nbdserver or client.
TCP port to listen on as a server, or connect to as a client (default
The offset into the image.
The interface to bind to as a server, or connect to as a client (default
Use a unix socket with path PATH.
Treat filename as a set of image options, instead of a plain filename. If this flag is specified, the
-fflag should not be used, instead the
format=option should be set.
Force the use of the block driver for format FMT instead of auto-detecting.
Export the disk as read-only.
Expose allocation depth information via the
qemu:allocation-depthmetadata context accessible through NBD_OPT_SET_META_CONTEXT.
If filename has a qcow2 persistent bitmap NAME, expose that bitmap via the
qemu:dirty-bitmap:NAMEmetadata context accessible through NBD_OPT_SET_META_CONTEXT.
Use filename as an external snapshot, create a temporary file with
backing_file=filename, redirect the write to the temporary one.
Load an internal snapshot inside filename and export it as an read-only device, SNAPSHOT_PARAM format is
The cache mode to be used with the file. Valid values are:
unsafe. See the documentation of the emulator’s
-drive cache=...option for more info.
Set the asynchronous I/O mode between
native(Linux only), and
discard(also known as
unmap) requests are ignored or passed to the filesystem. DISCARD is one of
on). The default is
Control the automatic conversion of plain zero writes by the OS to driver-specific optimized zero write commands. DETECT_ZEROES is one of
unmapconverts a zero write to an unmap operation and can only be used if DISCARD is set to
unmap. The default is
Connect filename to NBD device DEV (Linux only).
Disconnect the device DEV (Linux only).
Allow up to NUM clients to share the device (default
1), 0 for unlimited.
Don’t exit on the last connection.
Set the NBD volume export name (default of a zero-length string).
Set the NBD volume export description, as a human-readable string.
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
Enable mandatory TLS encryption for the server by setting the ID of the TLS credentials object previously created with the
--objectoption; or provide the credentials needed for connecting as a client in list mode.
When validating an x509 certificate received over a TLS connection, the hostname that the NBD client used to connect will be checked against information in the server provided certificate. Sometimes it might be required to override the hostname used to perform this check. For example, if the NBD client is using a tunnel from localhost to connect to the remote server, the
--tls-hostnameoption should be used to set the officially expected hostname of the remote NBD server. This can also be used if accessing NBD over a UNIX socket where there is no inherent hostname available. This is only permitted when acting as a NBD client with the
Fork off the server process and exit the parent once the server is running.
Store the server’s process ID in the given file.
Specify the ID of a qauthz object previously created with the
--objectoption. This will be used to authorize connecting users against their x509 distinguished name.
Display this help and exit.
Display version information and exit.
Specify tracing options.
Immediately enable events matching PATTERN (either event name or a globbing pattern). This option is only available if QEMU has been compiled with the
ftracetracing backend. To specify multiple events or patterns, specify the
-traceoption multiple times.
-trace helpto print a list of names of trace points.
events=FILEImmediately enable events listed in FILE. The file must contain one event name (as listed in the
trace-events-allfile) per line; globbing patterns are accepted too. This option is only available if QEMU has been compiled with the
file=FILELog output traces to FILE. This option is only available if QEMU has been compiled with the
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 allow 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 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 \ --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 a /dev/nbd device generally requires root
privileges, and may also require the execution of
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