QEMU Disk Network Block Device Server — QEMU documentation

# 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. 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.

--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.

-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 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)