nbdkit-nbd-plugin - Man Page

 nbdkit nbd { command=COMMAND [arg=ARG [...]] |
              hostname=HOST [port=PORT] |
              vhost-cid=CID [port=PORT] |
              socket=SOCKNAME |
              socket-fd=FD |
              [uri=]URI }
            [dynamic-export=BOOL] [export=NAME] [retry=N] [shared=BOOL]
            [tls=MODE] [tls-certificates=DIR] [tls-verify=BOOL]
            [tls-username=NAME] [tls-psk=FILE]

nbdkit-nbd-plugin is a plugin for nbdkit(1) that lets you forward the connection to another NBD server.  There are several uses for this plugin:

• Adjust the set of features seen by the ultimate NBD client without having to change the original server.  For example, convert between the oldstyle and newstyle protocols, or add TLS support if the original server lacks it.
• Apply nbdkit filters to any other NBD server.
• With qemu-nbd(8), read and write qcow2 files with nbdkit (see also nbdkit-qcow2dec-filter(1)).

One of socket, hostname (optionally with port), vsock (optionally with port), uri, socket-fd or command must be given to specify which NBD server to forward to:

command=COMMAND

arg=ARG

(nbdkit ≥ 1.22)

Run an NBD server, usually qemu-nbd(8), as an external command. See "Examples" below.

COMMAND is the program to run, followed by zero or more arg=ARG parameters for each argument.  For example:

 nbdkit nbd command=qemu-nbd arg=-f arg=qcow2 arg=$PWD/disk.qcow2

would run the command qemu-nbd -f qcow2 $PWD/disk.qcow2.  Because nbdkit may change directory before running the command, you may need to ensure that all file paths used in parameters (like the disk name above) are absolute paths.

This uses the libnbd API nbd_connect_systemd_socket_activation(3). This option implies shared=true.

hostname=HOST

port=PORT

(nbdkit ≥ 1.14)

Connect to the NBD server at the remote HOST using a TCP socket. The optional port parameter overrides the default port (10809), and may be a 16-bit number or a non-numeric string to look up the well-known port for a service name.

vsock=CID

port=PORT

(nbdkit ≥ 1.22)

Connect to the NBD server at the given vsock CID (for example, in a guest VM, using the cid 2 will connect to a server in the host).  The optional port parameter overrides the default port (10809), and must be a 32-bit number.  This only works for platforms with the AF_VSOCK family of sockets and libnbd new enough to use it; nbdkit --dump-plugin nbd will contain libnbd_vsock=1 if this is the case.  For more details on AF_VSOCK, see "AF_VSOCK" in nbdkit-service(1).

socket=SOCKNAME

Connect to the NBD server using Unix domain socket SOCKNAME.

socket-fd=FD

(nbdkit ≥ 1.22)

Connect to the NBD server over a socket file descriptor inherited by nbdkit.  This uses the libnbd API nbd_connect_socket(3).  This option implies shared=true.

[uri=]URI

(nbdkit ≥ 1.14)

When uri is supplied, decode URI to determine the address to connect to.  A URI can specify a TCP connection (such as nbd://localhost:10809/export), a Unix socket (such as nbd+unix:///export?socket=/path/to/sock), or a vsock connection (such as nbd+vsock:///2:10809/export).  Remember you may need to quote the parameter to protect it from the shell.

The uri parameter is only available when the plugin was compiled against libnbd with URI support; nbdkit --dump-plugin nbd will contain libnbd_uri=1 if this is the case.

The export portion of the URI is ignored when using dynamic-export=true.

uri= is a magic config key and may be omitted in most cases.  See "Magic parameters" in nbdkit(1).

Other parameters control the NBD connection:

export=NAME

If this parameter is given, and the server speaks new style protocol, then connect to the named export instead of the default export (the empty string).  If uri is supplied, the export name should be embedded in the URI instead.  This is incompatible with dynamic-export=true.

retry=N

(nbdkit ≥ 1.14)

If the initial connection attempt to the server fails, retry up to N times more after a one-second delay between tries (default 0).

shared=false

shared=true

(nbdkit ≥ 1.14)

If using command or socket-fd modes then this defaults to true, otherwise false.

If false the plugin will open a new connection to the server for each client making a connection to nbdkit.  The remote server does not have to be started until immediately before the first nbdkit client connects.

If this parameter is set to true, the plugin will open a single connection to the server when nbdkit is first started (the retry parameter may be necessary to coordinate timing of the remote server startup), and all clients to nbdkit will share that single connection. This mode is incompatible with dynamic-export=true.

dynamic-export=false

dynamic-export=true

(nbdkit ≥ 1.24)

This parameter defaults to false, in which case all clients to nbdkit use the same export of the server, as set by export or uri, regardless of the client's export name request.  If it is set to true, nbdkit will pass the client's requested export name over to the final NBD server, which means clients requesting different export names can see different content when the server differentiates content by export name.  Dynamic exports prevent the use of shared mode, and thus are not usable with command or socket-fd.

If libnbd is new enough, dynamic export mode is able to advertise the same exports as listed by the server; nbdkit --dump-plugin nbd will contain libnbd_dynamic_list=1 if this is the case.  Regardless of what this plugin lists, it is also possible to use nbdkit-exportname-filter(1) to adjust what export names the client sees or uses as a default.

tls=off

tls=on

tls=require

(nbdkit ≥ 1.14)

Selects which TLS mode to use with the server.  If no other tls option is present, this defaults to off, where the client does not attempt encryption (and may be rejected by a server that requires it).  If omitted but another tls option is present, this defaults to on, where the client opportunistically attempts a TLS handshake, but will continue running unencrypted if the server does not support encryption.  If set to require or if the uri parameter is used with a scheme that requires encryption (such as nbds://host), then this requires an encrypted connection to the server.

The tls parameter is only available when the plugin was compiled against libnbd with TLS support; nbdkit --dump-plugin nbd will contain libnbd_tls=1 if this is the case.  Note the difference between --tls=... controlling what nbdkit serves, and tls=... controlling what the nbd plugin connects to as a client.

tls-certificates=DIR

(nbdkit ≥ 1.14)

This specifies the directory containing X.509 client certificates to present to the server.

tls-verify=false

(nbdkit ≥ 1.14)

Setting this parameter to false disables server name verification, which opens you to potential Man-in-the-Middle (MITM) attacks, but allows for a simpler setup for distributing certificates.

tls-username=NAME

(nbdkit ≥ 1.14)

If provided, this overrides the user name to present to the server alongside the certificate.

tls-psk=FILE

(nbdkit ≥ 1.14)

If provided, this is the filename containing the Pre-Shared Keys (PSK) to present to the server.  While this is easier to set up than X.509, it requires that the PSK file be transmitted over a secure channel.

You can learn which features are provided by libnbd by inspecting the libnbd_* lines in --dump-plugin output:

 $ nbdkit --dump-plugin nbd
 [...]
 libnbd_version=1.2.3
 libnbd_tls=1
 libnbd_uri=1

$plugindir/nbdkit-nbd-plugin.so

The plugin.

Use nbdkit --dump-config to find the location of $plugindir.

nbdkit-nbd-plugin first appeared in nbdkit 1.2.

nbdkit(1), nbdkit-captive(1), nbdkit-filter(3), nbdkit-exportname-filter(1), nbdkit-qcow2dec-filter(1), nbdkit-tls(1), nbdkit-plugin(3), libnbd(3), qemu-nbd(8).

Eric Blake

Copyright Red Hat

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nbdkit(1), nbdkit-blocksize-filter(1), nbdkit-plugin(3), nbdkit-release-notes-1.14(1), nbdkit-release-notes-1.20(1), nbdkit-release-notes-1.22(1), nbdkit-release-notes-1.30(1).