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) 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).

   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

        [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

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
        }

LOGGING

       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.

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.  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())'

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), nbdsh(1).

AUTHORS

       Eric Blake

       Richard W.M. Jones

       Pino Toscano

COPYRIGHT

       Copyright (C) 2013-2020 Red Hat Inc.

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