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

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