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

Provided by: nbd-client_3.23-3ubuntu1.22.04.1_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 ] [
       -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