DOKK / manpages / debian 11 / nbdkit / nbdkit-service.1.en
nbdkit-service(1) NBDKIT nbdkit-service(1)

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

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.

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) passes in pre-opened file descriptors. Socket activation lets you serve infrequent NBD requests using a superserver without needing nbdkit to be running the whole time.

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 is illegal and will cause an error. Also in this mode nbdkit does not fork into the background (ie. -f is implied).

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
 
 [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

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

Error messages from nbdkit can be sent to standard error (--log=stderr), or to the system log (--log=syslog), or can be discarded completely (--log=null, not recommended for normal use).

The default, if --log is not specified on the command line, is to send error messages to stderr, unless nbdkit forks into the background in which case they are sent to syslog.

In detail:

When running from the command line in the foreground.

When using systemd socket activation.

Using --log=stderr forces all messages to go to standard error.

When running from the command line, forked into the background.

Using --log=syslog forces all messages to go to the system log.

Debug messages (-v/--verbose) always go to standard error and are never sent to the system log.

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. Only libnbd supports "AF_VSOCK" NBD client connections at the time of writing (2019). 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())'

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

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), syslog(3), rsyslogd(8), journalctl(1), nbdsh(1).

Eric Blake

Richard W.M. Jones

Pino Toscano

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2021-01-20 nbdkit-1.24.1