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QEMU-NBD(8) QEMU QEMU-NBD(8)

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,...
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 sup-
ply TLS credentials for the qemu-nbd server or client.

-p, --port=PORT
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.

-A, --allocation-depth
Expose allocation depth information via the qemu:alloca-
tion-depth metadata context accessible through
NBD_OPT_SET_META_CONTEXT.

-B, --bitmap=NAME
If filename has a qcow2 persistent bitmap NAME, expose that bit-
map via the qemu:dirty-bitmap:NAME metadata 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 snap-
shot.id=[ID],snapshot.name=[NAME] or [ID_OR_NAME]

--cache=CACHE
The cache mode to be used with the file. Valid values are: none,
writeback (the default), writethrough, directsync and unsafe.
See the documentation of the emulator's -drive cache=... option
for more info.

-n, --nocache
Equivalent to --cache=none.

--aio=AIO
Set the asynchronous I/O mode between threads (the default), na-
tive (Linux only), and io_uring (Linux 5.1+).

--discard=DISCARD
Control whether discard (also known as trim or unmap) requests
are ignored or passed to the filesystem. DISCARD is one of ig-
nore (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), 0 for
unlimited.

-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 ex-
posed by a remote NBD server. This enables list mode, and is
incompatible with options that change behavior related to a spe-
cific 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 connect-
ing as a client in list mode.

--tls-hostname=hostname
When validating an x509 certificate received over a TLS connec-
tion, 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-hostname option should be used to set the officially ex-
pected 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 --list option.

--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 con-
tain 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-vis-
ible contents of a qcow2 file, with no TLS encryption, and with the de-
fault 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 dae-
mon:

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 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