nbdkit-service - Man Page

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 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).

 [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

 [Service]
 ExecStart=/usr/sbin/nbdkit file /path/to/serve
 # Optional settings to run as non-root:
 #User=nbd
 #Group=nbd

 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);
 }

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
 }

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:

Messages go to standard error (stderr):

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.

Messages go to the system log (syslog):

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.  For more about this protocol, see https://wiki.qemu.org/Features/VirtioVsock

 nbdkit --vsock [--port PORT] memory 1G

 qemu ... -device vhost-vsock-pci,id=vhost-vsock-pci0

 <vsock/>

 modprobe -r vmw_vsock_vmci_transport

 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), inetd(8), xinetd(8), syslog(3), rsyslogd(8), journalctl(1), libnbd(3), nbd_connect_systemd_socket_activation(3), nbdsh(1).

Eric Blake

Richard W.M. Jones

Pino Toscano

Copyright Red Hat

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.

THIS SOFTWARE IS PROVIDED BY RED HAT AND CONTRIBUTORS ''AS IS'' 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.

nbdkit(1), nbdkit-client(1), nbdkit-exitlast-filter(1), nbdkit-exitwhen-filter(1), nbdkit-nbd-plugin(1), nbdkit-ondemand-plugin(1), nbdkit-tmpdisk-plugin(1).