mirror of https://github.com/mkerrisk/man-pages
148 lines
5.0 KiB
Groff
148 lines
5.0 KiB
Groff
.\" Copyright (c) 1997 John S. Kallal (kallal@voicenet.com)
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.\"
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.\" This is free documentation; you can redistribute it and/or
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.\" modify it under the terms of the GNU General Public License as
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.\" published by the Free Software Foundation; either version 2 of
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.\" the License, or (at your option) any later version.
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.\"
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.\" Some changes by tytso and aeb.
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.\"
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.\" 2004-12-16, John V. Belmonte/mtk, Updated init and quit scripts
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.\" 2004-04-08, AEB, Improved description of read from /dev/urandom
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.\"
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.TH RANDOM 4 2003-10-25 "Linux" "Linux Programmer's Manual"
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.SH NAME
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random, urandom \- kernel random number source devices
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.SH DESCRIPTION
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The character special files \fB/dev/random\fP and
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\fB/dev/urandom\fP (present since Linux 1.3.30)
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provide an interface to the kernel's random number generator.
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File \fB/dev/random\fP has major device number 1
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and minor device number 8. File \fB/dev/urandom\fP
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has major device number 1 and minor device number 9.
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.LP
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The random number generator gathers environmental noise
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from device drivers and other sources into an entropy pool.
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The generator also keeps an estimate of the
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number of bits of noise in the entropy pool.
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From this entropy pool random numbers are created.
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.LP
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When read, the \fB/dev/random\fP device will only return random bytes
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within the estimated number of bits of noise in the entropy
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pool. \fB/dev/random\fP should be suitable for uses that need very
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high quality randomness such as one-time pad or key generation.
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When the entropy pool is empty, reads from \fB/dev/random\fP will block
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until additional environmental noise is gathered.
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.LP
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A read from the \fB/dev/urandom\fP device will not block
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waiting for more entropy.
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As a result, if there is not sufficient entropy in the
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entropy pool, the returned values are theoretically vulnerable to a
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cryptographic attack on the algorithms used by the driver. Knowledge of
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how to do this is not available in the current non-classified
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literature, but it is theoretically possible that such an attack may
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exist. If this is a concern in your application, use \fB/dev/random\fP
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instead.
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.SH CONFIGURING
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If your system does not have
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\fB/dev/random\fP and \fB/dev/urandom\fP created already, they
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can be created with the following commands:
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.nf
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mknod \-m 644 /dev/random c 1 8
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mknod \-m 644 /dev/urandom c 1 9
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chown root:root /dev/random /dev/urandom
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.fi
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When a Linux system starts up without much operator interaction,
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the entropy pool may be in a fairly predictable state.
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This reduces the actual amount of noise in the entropy pool
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below the estimate. In order to counteract this effect, it helps to carry
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entropy pool information across shut-downs and start-ups.
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To do this, add the following lines to an appropriate script
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which is run during the Linux system start-up sequence:
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.nf
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echo "Initializing random number generator..."
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random_seed=/var/run/random-seed
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# Carry a random seed from start-up to start-up
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# Load and then save the whole entropy pool
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if [ \-f $random_seed ]; then
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cat $random_seed >/dev/urandom
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else
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touch $random_seed
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fi
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chmod 600 $random_seed
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poolfile=/proc/sys/kernel/random/poolsize
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[ \-r $poolfile ] && bytes=`cat $poolfile` || bytes=512
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dd if=/dev/urandom of=$random_seed count=1 bs=$bytes
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.fi
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Also, add the following lines in an appropriate script which is
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run during the Linux system shutdown:
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.nf
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# Carry a random seed from shut-down to start-up
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# Save the whole entropy pool
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echo "Saving random seed..."
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random_seed=/var/run/random-seed
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touch $random_seed
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chmod 600 $random_seed
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poolfile=/proc/sys/kernel/random/poolsize
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[ \-r $poolfile ] && bytes=`cat $poolfile` || bytes=512
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dd if=/dev/urandom of=$random_seed count=1 bs=$bytes
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.fi
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.SH "PROC INTERFACE"
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The files in the directory
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.I /proc/sys/kernel/random
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(present since 2.3.16) provide an additional interface to the
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.B /dev/random
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device.
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.LP
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The read-only file
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.I entropy_avail
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gives the available entropy. Normally, this will be 4096 (bits),
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a full entropy pool.
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.LP
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The file
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.I poolsize
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gives the size of the entropy pool. Normally, this will be 512 (bytes).
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It can be changed to any value for which an algorithm is available.
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Currently the choices are 32, 64, 128, 256, 512, 1024, 2048.
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.LP
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The file
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.I read_wakeup_threshold
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contains the number of bits of entropy required for waking up processes
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that sleep waiting for entropy from
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.BR /dev/random .
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The default is 64.
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The file
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.I write_wakeup_threshold
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contains the number of bits of entropy below which we wake up
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processes that do a
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.I select()
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or
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.I poll()
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for write access to
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.BR /dev/random .
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These values can be changed by writing to the files.
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.LP
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The read-only files
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.I uuid
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and
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.I boot_id
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contain random strings like 6fd5a44b-35f4-4ad4-a9b9-6b9be13e1fe9.
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The former is generated afresh for each read, the latter was
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generated once.
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.SH FILES
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/dev/random
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.br
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/dev/urandom
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.SH AUTHOR
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The kernel's random number generator was written by
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Theodore Ts'o (tytso@athena.mit.edu).
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.SH "SEE ALSO"
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mknod (1)
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.br
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RFC\ 1750, "Randomness Recommendations for Security"
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