.\"	$NetBSD: rndctl.8,v 1.29 2021/04/06 22:02:40 riastradh Exp $
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.\" Copyright (c) 1997 Michael Graff
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.Dd April 2, 2021
.Dt RNDCTL 8
.Os
.Sh NAME
.Nm rndctl
.Nd kernel entropy pool management tool
.Sh SYNOPSIS
.Nm
.Op Fl CcEe
.Op Fl d Ar devname | Fl t Ar devtype
.Nm
.Op Fl lsv
.Op Fl d Ar devname | Fl t Ar devtype
.Nm
.Op Fl i
.Fl L Ar save-file
.Nm
.Fl S Ar save-file
.Sh DESCRIPTION
The
.Nm
program displays statistics on the current state of the
.Xr rnd 4
device, and controls which sources are allowed to contribute to the
entropy pool maintained by
.Xr rnd 4 .
.Pp
The following options are available:
.Bl -tag -width 123456
.It Fl C
Disable collection of data for the given device name or device type.
.It Fl c
Enable collection of data for the given device name or device type.
.It Fl d
Only the device named
.Ar devname
is altered or displayed.
This is mutually exclusive with
.Fl t .
.It Fl E
Ignore estimates of entropy from the drivers for the given device name
or type.
.Pp
If collection is still enabled, data is still collected and mixed into
the internal entropy pool, but no entropy is assumed to be present and
data from the selected devices will not unblock
.Pa /dev/random .
.It Fl e
Accept estimates of entropy from the drivers for the given device name
or type.
.Pp
.Cm "rndctl -e"
does not change the estimate provided by the driver; if the driver's
estimate is zero, as it generally is for devices of types other than
.Ic rng ,
it remains zero after
.Cm "rndctl -e" .
.It Fl i
With the
.Fl L
option to load a seed from a file, ignore any estimate in the file of
the entropy of the seed.
This still loads the data into the kernel, but won't unblock
.Pa /dev/random
even if the file claims to have adequate entropy.
This is useful if the file is on a medium, such as an NFS share, that
the operator does not know to be secret.
.It Fl L
Load a seed from
.Ar save-file
generated by
.Cm "rndctl -S" .
Overwrite it with a seed derived by hashing it together with output
from
.Pa /dev/urandom
so that the new seed has at least as much entropy as either the old
seed had or the system already has.
If interrupted, either the old seed or the new seed will be in place.
.It Fl l
List all sources, or, if the
.Fl t
or
.Fl d
flags are specified, only those specified by the
.Ar devtype
or
.Ar devname
specified.
.It Fl S
Generate a seed from the system entropy pool and save it to
.Ar save-file
for later use with
.Cm "rndctl -L" .
.Pp
The file format is specific to
.Nm
and includes an estimate of the amount of saved entropy and a checksum.
The prior internal state of the system entropy pool cannot be recovered
from
.Ar save-file ,
so disclosure of
.Ar save-file
does not compromise past secrets drawn from
.Pa /dev/urandom
or equivalent.
.It Fl s
Display statistics on the current state of the entropy pool.
.It Fl t
All devices of type
.Ar devtype
are altered or displayed.
This is mutually exclusive with
.Fl d .
.Pp
The available types are:
.Bl -tag -width "diskx"
.It Ic disk
Physical hard drives.
.It Ic net
Network interfaces.
.It Ic tape
Tape devices.
.It Ic tty
Terminal, mouse, or other user input devices.
.It Ic rng
Hardware random number generators.
.El
.It Fl v
Verbose output.
.El
.Sh SEE ALSO
.Xr rnd 4 ,
.Xr entropy 7 ,
.Xr rnd 9
.Sh HISTORY
The
.Nm
program was first made available in
.Nx 1.3 .
.Sh AUTHORS
The
.Nm
program was written by
.An Michael Graff
.Aq explorer@flame.org .