nbd-client - connect to a server running nbd-server(1), to use its
exported block device
nbd-client host [ port ] nbd-device [ -sdp ] [ -swap ] [ -persist ] [
-nofork ] [ -systemd-mark ] [ -block-size block size ] [ -timeout
seconds ] [ -name name ]
nbd-client -d nbd-device
nbd-client -c nbd-device
nbd-client -l host [ port ]
With nbd-client, you can connect to a server running nbd-server, thus
using raw diskspace from that server as a blockdevice on the local
To do this, support from the Linux Kernel is necessary, in the form of
the Network Block Device (NBD). When you have that, either in the
kernel, or as a module, you can connect to an NBD server and use its
exported file through a block special file with major mode 43.
Optionally, long options can also be specified with two leading dashes.
The following options are supported:
-block-size block size
-b Use a blocksize of "block size". Default is 1024; allowed values
are either 512, 1024, 2048 or 4096
host The hostname or IP address of the machine running nbd-server.
Since 2.9.15, the NBD utilities support IPv6.
-t Set the connection timeout to "seconds". For this to work, you
need a kernel with support for the NBD_SET_TIMEOUT ioctl; this
was introduced into Linus' tree on 2007-10-11, and will be part
of kernel 2.6.24.
port The TCP port on which nbd-server is running at the server.
For the deprecated oldstyle protocol, passing a port number is
required. In the oldstyle protocol, exports are defined by the
port on which they are running.
For the newstyle protocol, the port number defaults to 10809,
the IANA-assigned port number for the NBD protocol. The
newstyle protocol is selected automatically by nbd-client when
one of the -list or -name options are used.
The block special file this nbd-client should connect to.
-c Check whether the specified nbd device is connected.
If the device is connected, nbd-client will exit with an exit
state of 0 and print the PID of the nbd-client instance that
connected it to stdout.
If the device is not connected or does not exist (for example
because the nbd module was not loaded), nbd-client will exit
with an exit state of 1 and not print anything on stdout.
If an error occurred, nbd-client will exit with an exit state of
2, and not print anything on stdout either.
-d Disconnect the specified nbd device from the server
-l Ask the server for a list of available exports. If the server is
exporting over IPv6 as well as over IPv4, this will list all
exports twice; otherwise, it should list them all only once.
Note that this option only works with nbd-server processes
running version 3.1 or above, and must be enabled in server
configuration (with the "allowlist" option) before it can be
-p When this option is specified, nbd-client will immediately try
to reconnect an nbd device if the connection ever drops
unexpectedly due to a lost server or something similar.
-S Connect to the server using the Socket Direct Protocol (SDP),
rather than IP. See nbd-server(5) for details.
-s Specifies that this NBD device will be used as swapspace. This
option attempts to prevent deadlocks by performing mlockall()
and adjusting the oom-killer score at an appropriate time. It
does not however guarantee that such deadlocks can be avoided.
-m The systemd init system requires that processes which should not
be killed at shutdown time be marked appropriately by replacing
the first letter of their argv with an '@' sign.
This option will cause nbd-client to do so.
Note that this only works if nbd-client is run from an initrd;
i.e., systemd will ignore such a mark if run from a systemd unit
file or from the command line.
-n Specifies that the NBD client should not detach and daemonize
itself. This is mostly useful for debugging.
Note that nbd-client will still fork once to trigger an update
to the device node's partition table. It is not possible to
-N Specifies the name of the export that we want to use. Required
if the port is not specified; changes the default port for
newstyle negotiation from 10809 in the other case.
When this option is specified, nbd-client uses the newstyle
version of the negotiation protocol. This version is much more
flexible than the oldstyle negotiation, and should be used for
Some examples of nbd-client usage:
· To connect to a server running on port 2000 at host
"server.domain.com", using the client's block special file
nbd-client server.domain.com 2000 /dev/nbd0
· To connect to a server running on port 2001 at host
"swapserver.domain.com", using the client's block special file
"/dev/nbd1", for swap purposes:
nbd-client swapserver.domain.com 2001 /dev/nbd1 -swap
· To disconnect the above connection again (after making sure the block
special file is not in use anymore):
nbd-client -d /dev/nbd1
The NBD kernel module and the NBD tools have been written by Pavel
The kernel module is now maintained by Paul Clements
(Paul.Clements@steeleye.com), while the userland tools are maintained
by Wouter Verhelst (email@example.com)
This manual page was written by Wouter Verhelst (<firstname.lastname@example.org>)
for the Debian GNU/Linux system (but may be used by others).
Permission is granted to copy, distribute and/or modify this document
under the terms of the GNU General Public License, version 2, as
published by the Free Software Foundation.