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:

 nbdinfo nbd+vsock:///

 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.

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