QEMU-NBD.8(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,...props...
    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
    The 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.

-P, --partition=num
    Deprecated: Only expose MBR partition num. Understands physical partitions 1-4 and logical partition 5. New code should instead use --image-opts with the raw driver wrapping a subset of the original image.

-B, --bitmap=name
    If filename has a qcow2 persistent bitmap name, expose that bitmap via the "qemu:dirty-bitmap:name" 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]'

-n, --nocache
--cache=cache
    The cache mode to be used with the file. See the documentation of the emulator's "-drive cache=..." option for allowed values.

--aio=aio
    Set the asynchronous I/O mode between threads (the default) and native (Linux only).

--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). 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.

--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 just the first MBR partition 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 \
          --partition=1 --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 an /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
Copyright (C) 2006 Anthony Liguori <anthony@codemonkey.ws>. This is free software; see the source for copying conditions. There is NO warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.