nbdkit-nbd-plugin - Man Page

nbdkit nbd plugin

Synopsis

 nbdkit nbd { socket=SOCKNAME | hostname=HOST [port=PORT] | [uri=]URI }
    [export=NAME] [retry=N] [shared=BOOL] [tls=MODE] [tls-certificates=DIR]
    [tls-verify=BOOL] [tls-username=NAME] [tls-psk=FILE]

Description

nbdkit-nbd-plugin is an NBD forwarding plugin for nbdkit(1).

It provides an NBD server that forwards all traffic as a client to another existing NBD server.  A primary usage of this setup is to alter the set of features available to the ultimate end client, without having to change the original server (for example, to convert between oldstyle and newstyle, or to add TLS support where the original server lacks it).  Use of this plugin along with nbdkit filters (adding --filter to the nbdkit command line) makes it possible to apply any nbdkit filter to any other NBD server.

Remember that when using this plugin as a bridge between an encrypted and a non-encrypted endpoint, it is best to preserve encryption over TCP and use plaintext only on a Unix socket.

Parameters

One of socket, hostname or uri must be provided to designate the server. The server can speak either new or old style protocol. uri= is a magic config key and may be omitted in most cases.  See “Magic parameters” in nbdkit(1).

The following parameters are available whether or not the plugin was compiled against libnbd:

socket=SOCKNAME

Connect to the NBD server located at the Unix socket SOCKNAME.

hostname=HOST

Connect to the NBD server at the given remote HOST using a TCP socket.

port=PORT

When hostname is supplied, use PORT instead of the default port 10809.

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.

retry=N

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=BOOL

If this parameter is false (default), the plugin will open a distinct connection to the server for each client making a connection to nbdkit, and the remote server does not have to be started until immediately before the first nbdkit client.  If this parameter is set to true, the plugin will open a single connection to the server when nbdkit is first started (the delay parameter may be necessary to coordinate timing of the remote server startup), and all clients to nbdkit will share that single connection.

The following parameters are only available if the plugin was compiled against libnbd:

uri=URI

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) or a Unix socket (such as nbd+unix:///export?socket=/path/to/sock).  Remember to use proper shell quoting to prevent URI from accidentally being handled as a shell glob. 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.

tls=MODE

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

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

tls-verify=BOOL

This defaults to true; setting it 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

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

tls-psk=FILE

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.

Examples

Expose the contents of an export served by an old style server over a Unix socket to TCP network clients that only want to consume encrypted data. Use --exit-with-parent to clean up nbdkit at the same time that the old server exits.

 ( sock=`mktemp -u`
   nbdkit --exit-with-parent --tls=require nbd socket=$sock &
   exec /path/to/oldserver --socket=$sock )

 ┌────────────┐   TLS    ┌────────┐  plaintext  ┌────────────┐
 │ new client │ ────────▶│ nbdkit │ ───────────▶│ old server │
 └────────────┘   TCP    └────────┘    Unix     └────────────┘

Combine nbdkit's partition filter with qemu-nbd's ability to visit qcow2 files (nbdkit does not have a native qcow2 plugin), performing the same task as the deprecated qemu-nbd -P 1 -f qcow2 /path/to/image.qcow2 command. Allow multiple clients, even though qemu-nbd without -t normally quits after the first client, and utilize a 5-second retry to give qemu-nbd time to create the socket:

 ( sock=`mktemp -u`
   nbdkit --exit-with-parent --filter=partition nbd \
     nbd+unix:///\?socket=$sock shared=1 retry=5 partition=1 &
   exec qemu-nbd -k $sock -f qcow2 /path/to/image.qcow2 )

Conversely, expose the contents of export foo from a new style server with encrypted data to a client that can only consume unencrypted old style. Use --run to clean up nbdkit at the time the client exits.  In general, note that it is best to keep the plaintext connection limited to a Unix socket on the local machine.

 nbdkit -U - -o --tls=off nbd hostname=example.com export=foo tls=require \
  --run '/path/to/oldclient --socket=$unixsocket'

 ┌────────────┐  plaintext  ┌────────┐   TLS    ┌────────────┐
 │ old client │ ───────────▶│ nbdkit │ ────────▶│ new server │
 └────────────┘    Unix     └────────┘   TCP    └────────────┘

Learn which features are provided by libnbd by inspecting the libnbd_* lines:

 nbdkit --dump-plugin nbd

Files

$plugindir/nbdkit-nbd-plugin.so

The plugin.

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

Version

nbdkit-nbd-plugin first appeared in nbdkit 1.2.

See Also

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

Authors

Eric Blake

License

Redistribution and use in source and binary forms, with or without modification, are permitted provided that the following conditions are met:

THIS SOFTWARE IS PROVIDED BY RED HAT AND CONTRIBUTORS ''AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL RED HAT OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.

Referenced By

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

2020-05-10 nbdkit-1.21.3