Provided by: nbdkit_1.32.5-1ubuntu2_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, although some clients have smaller limits.
"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 -u "$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 -u "$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 -u "$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.
This mode is only supported on some platforms. You can find out if it is supported by
checking if:
$ nbdkit info --dump-plugin
contains:
info_address=yes
[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
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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.
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