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

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  ] [ -nonetlink ] [ -systemd-mark ] [ -readonly ] [ -preinit ] [ -block-size block
       size ] [ -size bytes ] [ -timeout seconds ] [ -name  name  ]  [  -certfile  certfile  ]  [
       -keyfile keyfile ] [ -cacertfile cacertfile ] [ -tlshostname hostname ]

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

       nbd-client nbd-device

       nbd-client -d nbd-device

       nbd-client -c nbd-device

       nbd-client -l host [ port ]

       nbd-client [ -netlink ] -l host

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.

-nonetlink

-L Starting with version 3.17, nbd-client will default to using the netlink interface
to configure an NBD device. This option allows to use the older ioctl() interface
to configure the device.

This option is only available if nbd-client was compiled against libnl-genl. If
that is not the case, nbd-client will only be able to use the ioctl interface (and
the option will not be available).

Note that a future version of nbd-client will require the use of netlink, but it
has not yet been decided when that will be the case.

-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.

-preinit

-P When this option is specified, nbd-client will skip the usual negotiation with the
server, and hand the socket to the kernel immediately after connecting. Only use
this when connecting to specialized NBD servers specifically designed for it. This
requires specifying the size of the device via the -B option, and does not support
TLS.

-readonly

-R When this option is specified, nbd-client will tell the kernel to treat the device
as read-only, even if the server would allow writes.

-size bytes

-B bytes
Force the device size to the specified number of bytes, rather than using the value
from server negotiation. Must be a multiple of the block size. If using preinit
(-P) to skip negotiation, this option is required.

-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.

-no-optgo

-g Disable the use of the NBD_OPT_GO protocol message, and force the use of
NBD_OPT_EXPORT_NAME instead.

The NBD protocol has two phases: the negotiation phase, and the transmission phase.
To move from negotation to transmission, older clients sent the NBD_OPT_EXPORT_NAME
message, for which the server could not produce an error message in case the export
name did not exist (or the client had insufficient permissions to access it). Due
to those limitations, a replacement message NBD_OPT_GO was created instead, which
allows the server to reply with an error in case of any problems.

The protocol allows for a server to discard a message which it does not understand;
however, unfortunately some implementations (including older versions of nbd-
server) did not handle that situation correctly and would get out of sync with the
client when it sent a message which the server did not understand.

When sending NBD_OPT_GO, nbd-client will try to do the right thing and fall back to
NBD_OPT_EXPORT_NAME. However, when the server has the above-described bug, then
this does not work. In such a situation, the client will issue a diagnostic
suggesting the use of this option.

Note that there is a corresponding option for nbdtab, too.

-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.

-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.

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.

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