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.IX Title "nbdkit-service 1"
.TH nbdkit-service 1 2026-07-14 nbdkit-1.48.0 NBDKIT
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.nh
.SH NAME
nbdkit\-service \- running nbdkit as a service, and systemd socket activation
.SH DESCRIPTION
.IX Header "DESCRIPTION"
Most people start nbdkit from the command line or run it from another
program (see \fBnbdkit\-captive\fR\|(1).  It is also possible to run nbdkit
as a standalone service, which is what this page describes.
.SH "SOCKET ACTIVATION"
.IX Header "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.
.PP
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.
.PP
Socket activation is triggered when both the \f(CW\*(C`LISTEN_FDS\*(C'\fR and
\&\f(CW\*(C`LISTEN_PID\*(C'\fR environment variables are set.  In this mode using
\&\fI\-i\fR, \fI\-p\fR, \fI\-\-run\fR, \fI\-s\fR or \fI\-U\fR flags on the command line will
cause an error.  Also in this mode nbdkit does not fork into the
background (ie. \fI\-f\fR is implied).
.SS "Using socket activation with systemd"
.IX Subsection "Using socket activation with systemd"
To use nbdkit with socket activation from systemd, create a unit file
ending in \f(CW\*(C`.socket\*(C'\fR (eg. \fI/etc/systemd/system/nbdkit.socket\fR)
containing:
.PP
.Vb 2
\& [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
.Ve
.PP
There are various formats for the \f(CW\*(C`ListenStream\*(C'\fR key.  See
\&\fBsystemd.socket\fR\|(5) for more information.
.PP
Also create a service unit (eg. \fI/etc/systemd/system/nbdkit.service\fR)
containing:
.PP
.Vb 5
\& [Service]
\& ExecStart=/usr/sbin/nbdkit file /path/to/serve
\& # Optional settings to run as non\-root:
\& #User=nbd
\& #Group=nbd
.Ve
.PP
For more information on systemd and socket activation, see
http://0pointer.de/blog/projects/socket\-activation.html
.SS "Using socket activation with libnbd"
.IX Subsection "Using socket activation with libnbd"
\&\fBlibnbd\fR\|(3) \fBnbd_connect_systemd_socket_activation\fR\|(3) lets you run
nbdkit from a program using libnbd.  An example is:
.PP
.Vb 10
\& struct nbd_handle *nbd = nbd_create ();
\& if (nbd == NULL) {
\&   fprintf (stderr, "%s\en", 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\en", nbd_get_error ());
\&   exit (EXIT_FAILURE);
\& }
.Ve
.SH "INETD AND XINETD"
.IX Header "INETD AND XINETD"
You can run nbdkit from \fBinetd\fR\|(8) or \fBxinetd\fR\|(8).  For inetd use:
.PP
.Vb 1
\& nbd stream tcp nowait root /usr/sbin/nbdkit nbdkit \-s file /tmp/disk.img
.Ve
.PP
For xinetd create a file \fI/etc/xinetd.d/nbdkit\fR containing:
.PP
.Vb 8
\& service nbd
\& {
\&     socket_type = stream
\&     wait = no
\&     user = root
\&     server = /usr/sbin/nbdkit
\&     server_args = \-s file /tmp/disk.img
\& }
.Ve
.SH AF_VSOCK
.IX Header "AF_VSOCK"
On Linux nbdkit supports the \f(CW\*(C`AF_VSOCK\*(C'\fR address family / protocol.
This allows you to serve NBD devices into virtual machines without
using a regular network connection.
.PP
\&\fBNote\fR 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 \f(CW\*(C`AF_VSOCK\*(C'\fR the
virtual machine sees a raw NBD socket which it can connect to by
opening an \f(CW\*(C`AF_VSOCK\*(C'\fR connection.  For more about this protocol, see
https://wiki.qemu.org/Features/VirtioVsock
.SS "AF_VSOCK example"
.IX Subsection "AF_VSOCK example"
To set up an \f(CW\*(C`AF_VSOCK\*(C'\fR server, use for example:
.PP
.Vb 1
\& nbdkit \-\-vsock [\-\-port PORT] memory 1G
.Ve
.PP
The optional \fI\-p\fR/\fI\-\-port\fR argument is used to change the
\&\f(CW\*(C`AF_VSOCK\*(C'\fR 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.
.PP
The guest that wishes to access nbdkit must be configured for
virtio\-vsock.  On the qemu command line use:
.PP
.Vb 1
\& qemu ... \-device vhost\-vsock\-pci,id=vhost\-vsock\-pci0
.Ve
.PP
For libvirt add this element to the \f(CW\*(C`<devices>\*(C'\fR section:
.PP
.Vb 1
\& <vsock/>
.Ve
.PP
If you see the error \f(CW\*(C`unable to open vhost\-vsock device\*(C'\fR then you may
have to unload the VMCI transport on the host:
.PP
.Vb 1
\& modprobe \-r vmw_vsock_vmci_transport
.Ve
.PP
Once nbdkit and the guest are running, from inside the guest you can
connect to nbdkit on the host using libnbd:
.PP
.Vb 1
\& nbdinfo nbd+vsock:///
\&
\& nbdsh \-c \*(Aqh.connect_vsock(2, 10809)\*(Aq \-c \*(Aqprint(h.get_size())\*(Aq
.Ve
.SH "ENVIRONMENT VARIABLES"
.IX Header "ENVIRONMENT VARIABLES"
.ie n .IP """LISTEN_FDS""" 4
.el .IP \f(CWLISTEN_FDS\fR 4
.IX Item "LISTEN_FDS"
.PD 0
.ie n .IP """LISTEN_PID""" 4
.el .IP \f(CWLISTEN_PID\fR 4
.IX Item "LISTEN_PID"
.PD
If present in the environment when nbdkit starts up, these trigger
"SOCKET ACTIVATION".
.SH "SEE ALSO"
.IX Header "SEE ALSO"
\&\fBnbdkit\fR\|(1),
\&\fBnbdkit\-client\fR\|(1),
\&\fBnbdkit\-exitlast\-filter\fR\|(1),
\&\fBnbdkit\-exitwhen\-filter\fR\|(1),
\&\fBnbdkit\-ip\-filter\fR\|(1),
\&\fBnbdkit\-limit\-filter\fR\|(1),
\&\fBsystemd\fR\|(1),
\&\fBsystemd.socket\fR\|(5),
\&\fBinetd\fR\|(8),
\&\fBxinetd\fR\|(8),
\&\fBsyslog\fR\|(3),
\&\fBrsyslogd\fR\|(8),
\&\fBjournalctl\fR\|(1),
\&\fBlibnbd\fR\|(3),
\&\fBnbd_connect_systemd_socket_activation\fR\|(3),
\&\fBnbdsh\fR\|(1).
.SH AUTHORS
.IX Header "AUTHORS"
Eric Blake
.PP
Richard W.M. Jones
.PP
Pino Toscano
.SH COPYRIGHT
.IX Header "COPYRIGHT"
Copyright Red Hat
.SH LICENSE
.IX Header "LICENSE"
Redistribution and use in source and binary forms, with or without
modification, are permitted provided that the following conditions are
met:
.IP \(bu 4
Redistributions of source code must retain the above copyright
notice, this list of conditions and the following disclaimer.
.IP \(bu 4
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.
.IP \(bu 4
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.
.PP
THIS SOFTWARE IS PROVIDED BY RED HAT AND CONTRIBUTORS \*(Aq\*(AqAS IS\*(Aq\*(Aq 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.
