Ubuntu Manpage: nbd-client - connect to a server running nbd-server(1), to use its exported block

Provided by: nbd-client_3.16.2-1ubuntu0.2_amd64

NAME

       nbd-client  -  connect  to  a  server  running nbd-server(1), to use its    exported block
       device

SYNOPSIS

       nbd-client host [ port ] nbd-device [ -connections num ] [ -sdp ] [ -swap ] [ -persist ] [
       -nofork ] [ -systemd-mark ] [ -block-size block size ] [ -timeout seconds ] [ -name name ]
       [ -certfile certfile ] [ -keyfile keyfile ] [  -cacertfile  cacertfile  ]  [  -tlshostname
       hostname ]

       nbd-client  -unix  path  nbd-device [ -connections num ] [ -sdp ] [ -swap ] [ -persist ] [
       -nofork ] [ -systemd-mark ] [ -block-size block size ] [ -timeout seconds ] [ -name name ]

       nbd-client nbd-device

       nbd-client -d nbd-device

       nbd-client -c nbd-device

       nbd-client -l host [ port ]

DESCRIPTION

       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 client.

       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.

OPTIONS

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

-connections num

-C Use num connections to the server, to allow speeding up request handling, at the
cost of higher resource usage on the server. Use of this option requires kernel
support available first with Linux 4.9.

host The hostname or IP address of the machine running nbd-server. Since 2.9.15, the NBD
utilities support IPv6.

-timeout seconds

-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.

The port number defaults to 10809, the IANA-assigned port number for the NBD
protocol.

Previous versions of the nbd tools supported an older version of the negotiation
protocol known as "oldstyle". This protocol version is no longer supported as of
version 3.11 of the nbd support tools.

nbd-device
The block special file (/dev entry) which this nbd-client should connect to,
specified as a full path.

When the mode is used wherein no hostname or export name is specified, nbd-client
will look up the necessary configuration in the nbdtab file. For more information,
see nbdtab(5).

-check

-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.

-disconnect

-d Disconnect the specified nbd device from the server

-list

-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 used.

-persist

-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.

-sdp

-S Connect to the server using the Socket Direct Protocol (SDP), rather than IP. See
nbd-server(5) for details.

-swap

-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.

-systemd-mark

-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[0] 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.

-nofork

-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 disable this.

-name

-N Specifies the name of the export that we want to use. If not specified, nbd-client
will ask for a "default" export, if one exists on the server.

-unix

-u Connect to the server over a unix domain socket at path, rather than to a server
over a TCP socket. The server must be listening on the given socket.

TLS SUPPORT
Enabling any of the TLS-related options causes the client to use the NBD_OPT_STARTTLS
command to upgrade the connection to TLS. Since negotiating TLS support from userspace for
a kernel socket would be very involved (if passing keys to kernel space were even
possible, which it isn't), the way this is implemented is that the nbd-client process
creates a socketpair, one side of which it hands to the kernel, and the other side of
which is handed to an encrypting/decrypting proxy. This has the effect that all
communication will be encrypted before being sent over the wire; however, doing so is not
safe in combination with swapping over an NBD device:

In order to free memory by swapping, the kernel needs to be sure that the write to the nbd
device has finalized. For this, it needs to be able to receive an NBD_CMD_WRITE reply
which informs it that the write has completed successfully and that the memory may be
released. Receiving data over the network, however, requires that the kernel allocate
memory first, which is impossible if we're low on memory (a likely situation when trying
to swap). This is likely to cause a deadlock when we're low on memory and there are high
amounts of network traffic.

To remedy this situation, the kernel sets the PF_MEMALLOC option on the nbd socket; when
low on memory, it will throw away all packets except for those destined to a socket with
that option set, relying on the normal TCP retransmit system to ensure that data is not
lost. This avoids the deadlock described above.

However, the PF_MEMALLOC option is set on the socket that is connected to the nbd device,
not the encrypted socket connected to the encrypting/decrypting proxy. As such, when using
TLS, the PF_MEMALLOC option is not set on the socket that actually receives data from the
network, which means that the deadlock reappears.

For this reason, if the -swap option is used when TLS is in use, nbd-client will issue an
appropriate warning.

-certfile file

-F Use the specified file as the client certificate for TLS authentication to the
server.

-keyfile file

-K Use the specified file as the private key for the client cerificate.

-cacertfile file

-A Use the specified file as the CA certificate for TLS authentication to the server.

-tlshostname hostname

-H Use the specified hostname for the TLS context. If not specified, the hostname used
to connect to the server will be used.

EXAMPLES

       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 "/dev/nbd0":

         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

AUTHOR