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nbdkit-service(1)		    NBDKIT		     nbdkit-service(1)

NAME
       nbdkit-service -	running	nbdkit as a service, and systemd socket
       activation

DESCRIPTION
       Most people start nbdkit	from the command line or run it	from another
       program (see nbdkit-captive(1).	It is also possible to run nbdkit as a
       standalone service, which is what this page describes.

SOCKET ACTIVATION
       nbdkit supports socket activation (sometimes called systemd socket
       activation).  This is a simple protocol where instead of	nbdkit itself
       opening the listening socket(s),	the parent process (typically systemd
       or libnbd) passes in pre-opened file descriptors.

       One use for socket activation is	to serve infrequent NBD	requests using
       a superserver without needing nbdkit to be running the whole time.
       Another use is to run nbdkit from a controlling libnbd process.

       Socket activation is triggered when both	the "LISTEN_FDS" and
       "LISTEN_PID" environment	variables are set.  In this mode using -i, -p,
       --run, -s or -U flags on	the command line will cause an error.  Also in
       this mode nbdkit	does not fork into the background (ie. -f is implied).

   Using socket	activation with	systemd
       To use nbdkit with socket activation from systemd, create a unit	file
       ending in ".socket" (eg.	/etc/systemd/system/nbdkit.socket) containing:

	[Unit]
	Description=NBDKit Network Block Device	server

	[Socket]
	ListenStream=10809
	# Optional settings to detect dead clients:
	#KeepAlive=true
	#KeepAliveTimeSec=60
	#KeepAliveIntervalSec=10
	#KeepAliveProbes=5

	[Install]
	WantedBy=sockets.target

       There are various formats for the "ListenStream"	key.  See
       systemd.socket(5) for more information.

       Also create a service unit (eg. /etc/systemd/system/nbdkit.service)
       containing:

	[Service]
	ExecStart=/usr/sbin/nbdkit file	/path/to/serve
	# Optional settings to run as non-root:
	#User=nbd
	#Group=nbd

       For more	information on systemd and socket activation, see
       http://0pointer.de/blog/projects/socket-activation.html

   Using socket	activation with	libnbd
       libnbd(3) nbd_connect_systemd_socket_activation(3) lets you run nbdkit
       from a program using libnbd.  An	example	is:

	struct nbd_handle *nbd = nbd_create ();
	if (nbd	== NULL) {
	  fprintf (stderr, "%s\n", nbd_get_error ());
	  exit (EXIT_FAILURE);
	}
	char *argv[] = {
	  "nbdkit", "--exit-with-parent", "memory", "1G", NULL
	};
	if (nbd_connect_systemd_socket_activation (nbd,
						   args) == -1)	{
	  fprintf (stderr, "%s\n", nbd_get_error ());
	  exit (EXIT_FAILURE);
	}

INETD AND XINETD
       You can run nbdkit from inetd(8)	or xinetd(8).  For inetd use:

	nbd stream tcp nowait root /usr/sbin/nbdkit nbdkit -s file /tmp/disk.img

       For xinetd create a file	/etc/xinetd.d/nbdkit containing:

	service	nbd
	{
	    socket_type	= stream
	    wait = no
	    user = root
	    server = /usr/sbin/nbdkit
	    server_args	= -s file /tmp/disk.img
	}

AF_VSOCK
       On Linux	nbdkit supports	the "AF_VSOCK" address family /	protocol.
       This allows you to serve	NBD devices into virtual machines without
       using a regular network connection.

       Note that this is different from	the usual case where you present NBD
       as a virtual block device to a guest (which the guest sees as something
       like a SATA or virtio-scsi disk).  With "AF_VSOCK" the virtual machine
       sees a raw NBD socket which it can connect to by	opening	an "AF_VSOCK"
       connection.  For	more about this	protocol, see
       https://wiki.qemu.org/Features/VirtioVsock

   AF_VSOCK example
       To set up an "AF_VSOCK" server, use for example:

	nbdkit --vsock [--port PORT] memory 1G

       The optional -p/--port argument is used to change the "AF_VSOCK"	port
       number.	These port numbers exist in a different	namespace from TCP/IP
       port numbers.  Also unlike TCP, the port	numbers	are 32 bit.  The
       default port is 10809.

       The guest that wishes to	access nbdkit must be configured for
       virtio-vsock.  On the qemu command line use:

	qemu ... -device vhost-vsock-pci,id=vhost-vsock-pci0

       For libvirt add this element to the "<devices>" section:

	<vsock/>

       If you see the error "unable to open vhost-vsock	device"	then you may
       have to unload the VMCI transport on the	host:

	modprobe -r vmw_vsock_vmci_transport

       Once nbdkit and the guest are running, from inside the guest you	can
       connect to nbdkit on the	host using libnbd:

	nbdsh -c 'h.connect_vsock(2, 10809)' -c	'print(h.get_size())'

ENVIRONMENT VARIABLES
       "LISTEN_FDS"
       "LISTEN_PID"
	   If present in the environment when nbdkit starts up,	these trigger
	   "SOCKET ACTIVATION".

SEE ALSO
       nbdkit(1), nbdkit-client(1), nbdkit-exitlast-filter(1),
       nbdkit-exitwhen-filter(1), nbdkit-ip-filter(1), nbdkit-limit-filter(1),
       systemd(1), systemd.socket(5), inetd(8),	xinetd(8), syslog(3),
       rsyslogd(8), journalctl(1), libnbd(3),
       nbd_connect_systemd_socket_activation(3), nbdsh(1).

AUTHORS
       Eric Blake

       Richard W.M. Jones

       Pino Toscano

COPYRIGHT
       Copyright Red Hat

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

       •   Redistributions of source code must retain the above	copyright
	   notice, this	list of	conditions and the following disclaimer.

       •   Redistributions in binary form must reproduce the above copyright
	   notice, this	list of	conditions and the following disclaimer	in the
	   documentation and/or	other materials	provided with the
	   distribution.

       •   Neither the name of Red Hat nor the names of	its contributors may
	   be used to endorse or promote products derived from this software
	   without specific prior written permission.

       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.

nbdkit-1.46.0			  2026-03-04		     nbdkit-service(1)

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