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 supply 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:allocation-depth metadata context accessible through NBD_OPT_SET_META_CONTEXT.

-B, --bitmap=NAME
    If filename has a qcow2 persistent bitmap NAME, expose that bitmap 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 snapshot.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), native (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 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), 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.

--handshake-limit=N
    Set the timeout for a client to successfully complete its handshake to N seconds (default 10), or 0 for no limit.

-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.

--tls-hostname=hostname
    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-hostname option 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 --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. This option also keeps the original STDERR stream open if the qemu-nbd process is daemonized due to other options like --fork or -c.

-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 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 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
Anthony Liguori <anthony@codemonkey.ws>

Copyright
2025, The QEMU Project Developers