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.

       -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

       2024, The QEMU Project Developers