Provided by: nbdkit_1.16.2-1ubuntu3_amd64 
NAME
nbdkit-info-plugin - serve client and server information
SYNOPSIS
nbdkit info [mode=]exportname|base64exportname|address|
time|uptime|conntime
DESCRIPTION
"nbdkit-info-plugin" is a test plugin which serves information about the client and server in a disk
image back to the client.
In its default mode ("mode=exportname") it converts the export name passed from the client into a disk
image. "mode=base64exportname" is similar except the client must base64-encode the data in the export
name, allowing arbitrary binary data to be sent (see "EXAMPLES" below to make this clearer). Export
names are limited to 4096 bytes by the NBD protocol, and nbdkit limits them to a few bytes smaller than
this.
"mode=address" creates a disk which contains the client's IP address and port number as a string.
"mode=time", "mode=uptime" and "mode=conntime" report server wallclock time, nbdkit uptime, or time since
the connection was opened respectively and may be used to measure latency.
The plugin only supports read-only access. To make the disk writable, add nbdkit-cow-filter(1) on top.
EXAMPLES
Create a “reflection disk”. By setting the export name to "hello" when we open it, a virtual disk of
only 5 bytes containing these characters is created. We then display the contents:
$ nbdkit --exit-with-parent info mode=exportname &
$ nbdsh -u 'nbd://localhost/hello' -c - <<'EOF'
size = h.get_size()
print("size = %d" % size)
buf = h.pread(size, 0)
print("buf = %r" % buf)
EOF
size = 5
buf = b"hello"
By running the info plugin, you can pass whole bootable VMs on the qemu command line:
$ nbdkit info mode=base64exportname
$ qemu-system-x86_64 \
-drive 'snapshot=on,file.driver=nbd,file.host=localhost,file.port=10809,file.export=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'
Another use for the info plugin is to send back the client's IP address:
$ nbdkit info mode=address
$ nbdsh -u 'nbd://localhost' -c 'print(h.pread(h.get_size(), 0))'
which will print something like:
b'[::1]:58912'
This plugin can also return the wallclock time:
$ nbdkit info time --run 'nbdsh --connect $uri -c "sys.stdout.buffer.write(h.pread(12,0))" | hexdump -C'
00000000 00 00 00 00 5d 8f 24 c7 00 04 24 01
└─────┬─────┘
┌─┘
│
$ date --date="@$(( 0x5d8f24c7 ))"
Sat 28 Sep 10:15:51 BST 2019
or the nbdkit server uptime:
$ nbdkit info uptime --run 'nbdsh --connect $uri -c "sys.stdout.buffer.write(h.pread(12,0))" | hexdump -C'
00000000 00 00 00 00 00 00 00 00 00 00 60 4b
└──┬──┘
0x604b is about 25ms
or the time since the client opened the connection:
$ nbdkit info conntime --run 'nbdsh --connect $uri -c "sys.stdout.buffer.write(h.pread(12,0))" | hexdump -C'
00000000 00 00 00 00 00 00 00 00 00 00 00 e0
└─┬─┘
0xe0 is about 200μs
PARAMETERS
[mode=]address
Send the client's IP address and client port number as a string in the usual format. For Unix
sockets this sets the disk to the string "unix" to avoid leaking host paths.
[mode=]base64exportname
Send the export name passed by the client, assuming the client string is base64 encoded.
This mode is only supported if nbdkit was compiled with GnuTLS ≥ 3.6.0. You can find out by checking
if:
$ nbdkit info --dump-plugin
contains:
info_base64=yes
[mode=]exportname
Send the raw export name passed by the client. Note the export name cannot contain ASCII NUL
characters.
This is the default mode.
[mode=]time
Reflect server wallclock time as seconds and microseconds since the Epoch (see gettimeofday(2)):
┌────────┬────────┬────────────┬──────────────────────┐
│ offset │ length │ format │ field │
╞════════╪════════╪════════════╪══════════════════════╡
│ 0 │ 8 │ 64 bit int │ seconds │
│ │ │ big endian │ │
├────────┼────────┼────────────┼──────────────────────┤
│ 8 │ 4 │ 32 bit int │ microseconds │
│ │ │ big endian │ │
└────────┴────────┴────────────┴──────────────────────┘
To be able to read this atomically you must read the whole 12 bytes in a single request.
Note that exposing server time may be insecure. It is safer to use "mode=uptime" or "mode=conntime"
instead.
[mode=]uptime
Reflect nbdkit uptime in seconds and microseconds (ie. both fields are 0 immediately after nbdkit
starts, although a client would never be able to observe this). The format is exactly the same as
for "mode=time" above.
In the current implementation this can jump forwards or backwards discontinuously if the server time
is adjusted. In future we may fix this bug.
[mode=]conntime
Reflect time since the NBD client connection was opened in seconds and microseconds. The format is
exactly the same as for "mode=time" above.
In the current implementation this can jump forwards or backwards discontinuously if the server time
is adjusted. In future we may fix this bug.
"mode=" is a magic config key and may be omitted in most cases. See "Magic parameters" in nbdkit(1).
FILES
$plugindir/nbdkit-info-plugin.so
The plugin.
Use "nbdkit --dump-config" to find the location of $plugindir.
VERSION
"nbdkit-info-plugin" first appeared in nbdkit 1.16.
SEE ALSO
nbdkit(1), nbdkit-plugin(3), nbdkit-cow-filter(1), nbdkit-data-plugin(1).
AUTHORS
Richard W.M. Jones
COPYRIGHT
Copyright (C) 2019 Red Hat Inc.
LICENSE
Redistribution and use in source and binary forms, with or without modification, are permitted provided
that the following conditions are met:
• Redistributions of source code must retain the above copyright notice, this list of conditions and
the following disclaimer.
• Redistributions in binary form must reproduce the above copyright notice, this list of conditions and
the following disclaimer in the documentation and/or other materials provided with the distribution.
• Neither the name of Red Hat nor the names of its contributors may be used to endorse or promote
products derived from this software without specific prior written permission.
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INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
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