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[src/trunk]: src/share/man/man4 rnd.4: Explain why libraries should use kern....
details: https://anonhg.NetBSD.org/src/rev/216656022f95
branches: trunk
changeset: 971686:216656022f95
user: nia <nia%NetBSD.org@localhost>
date: Fri May 01 12:30:16 2020 +0000
description:
rnd.4: Explain why libraries should use kern.arandom over /dev/urandom
diffstat:
share/man/man4/rnd.4 | 29 ++++++++++++++++++++++++++---
1 files changed, 26 insertions(+), 3 deletions(-)
diffs (52 lines):
diff -r 53b72b14f9c4 -r 216656022f95 share/man/man4/rnd.4
--- a/share/man/man4/rnd.4 Fri May 01 10:38:59 2020 +0000
+++ b/share/man/man4/rnd.4 Fri May 01 12:30:16 2020 +0000
@@ -1,4 +1,4 @@
-.\" $NetBSD: rnd.4,v 1.29 2020/04/30 03:28:18 riastradh Exp $
+.\" $NetBSD: rnd.4,v 1.30 2020/05/01 12:30:16 nia Exp $
.\"
.\" Copyright (c) 2014-2019 The NetBSD Foundation, Inc.
.\" All rights reserved.
@@ -51,10 +51,32 @@
predictable.
.El
.Pp
-Applications should read from
+Applications can read from
.Pa /dev/urandom
when they need randomly generated data, e.g. key material for
-cryptography or seeds for simulations.
+cryptography or seeds for simulations, and the device is known
+to be available.
+.Pp
+A
+.Xr sysctl 7
+variable,
+.Li kern.arandom ,
+provides equivalent functionality to
+.Pa /dev/urandom
+and will never block. However, it only returns up to 256 bytes per call.
+This is expected to be enough for seeding most cryptographically secure
+random number generators and ciphers, and if more data is required the
+variable can be queried again.
+.Pp
+Applications should read from the sysctl variable when a high level of
+reliability is required, or the runtime environment cannot be predicted,
+e.g. in a library. It is possible that
+.Pa /dev/urandom
+is unavailable due to the application being in a
+.Xr chroot 8
+environment, or when other restrictions such as those enforced by
+.Xr setrlimit 2
+apply.
.Pp
Systems should be engineered to judiciously read at least once from
.Pa /dev/random
@@ -485,6 +507,7 @@
Keys for
.Pa /dev/random ,
.Pa /dev/urandom ,
+.Li kern.arandom ,
and the in-kernel
.Xr cprng 9
subsystem are extracted from the global pool.
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