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 pro- gram (see nbdkit-captive(1). It is also possible to run nbdkit as a stand- alone service, which is what this page describes. SOCKET ACTIVATION nbdkit supports socket activation (sometimes called systemd socket activa- tion). 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 end- ing 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 sys- temd.socket(5) for more information. Also create a service unit (eg. /etc/systemd/system/nbdkit.service) con- taining: [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 regu- lar 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/VirtioV- sock 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 num- ber. 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 con- nect 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-exit- when-filter(1), nbdkit-ip-filter(1), nbdkit-limit-filter(1), systemd(1), systemd.socket(5), inetd(8), xinetd(8), syslog(3), rsyslogd(8), jour- nalctl(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 modifi- cation, 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 no- tice, this list of conditions and the following disclaimer in the docu- mentation 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 EX- PRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DIS- CLAIMED. IN NO EVENT SHALL RED HAT OR CONTRIBUTORS BE LIABLE FOR ANY DI- RECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SER- VICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABIL- ITY, 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 DAM- AGE. nbdkit-1.46.2 2026-08-30 nbdkit-service(1)
NAME | DESCRIPTION | SOCKET ACTIVATION | INETD AND XINETD | AF_VSOCK | ENVIRONMENT VARIABLES | SEE ALSO | AUTHORS | COPYRIGHT | LICENSE
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