1RANDOM(4) Linux Programmer's Manual RANDOM(4)
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6 random, urandom - kernel random number source devices
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9 The character special files /dev/random and /dev/urandom (present since
10 Linux 1.3.30) provide an interface to the kernel's random number gener‐
11 ator. File /dev/random has major device number 1 and minor device num‐
12 ber 8. File /dev/urandom has major device number 1 and minor device
13 number 9.
14 The random number generator gathers environmental noise from device
16 drivers and other sources into an entropy pool. The generator also
17 keeps an estimate of the number of bits of noise in the entropy pool.
18 From this entropy pool random numbers are created.
19 When read, the /dev/random device will only return random bytes within
21 the estimated number of bits of noise in the entropy pool. /dev/random
22 should be suitable for uses that need very high quality randomness such
23 as one-time pad or key generation. When the entropy pool is empty,
24 reads from /dev/random will block until additional environmental noise
25 is gathered.
26 A read from the /dev/urandom device will not block waiting for more
28 entropy. As a result, if there is not sufficient entropy in the
29 entropy pool, the returned values are theoretically vulnerable to a
30 cryptographic attack on the algorithms used by the driver. Knowledge
31 of how to do this is not available in the current non-classified liter‐
32 ature, but it is theoretically possible that such an attack may exist.
33 If this is a concern in your application, use /dev/random instead.
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36 If your system does not have /dev/random and /dev/urandom created
37 already, they can be created with the following commands:
38 mknod -m 644 /dev/random c 1 8
40 mknod -m 644 /dev/urandom c 1 9
41 chown root:root /dev/random /dev/urandom
42 When a Linux system starts up without much operator interaction, the
44 entropy pool may be in a fairly predictable state. This reduces the
45 actual amount of noise in the entropy pool below the estimate. In
46 order to counteract this effect, it helps to carry entropy pool infor‐
47 mation across shut-downs and start-ups. To do this, add the following
48 lines to an appropriate script which is run during the Linux system
49 start-up sequence:
50 echo "Initializing random number generator..."
52 random_seed=/var/run/random-seed
53 # Carry a random seed from start-up to start-up
54 # Load and then save the whole entropy pool
55 if [ -f $random_seed ]; then
56 cat $random_seed >/dev/urandom
57 else
58 touch $random_seed
59 fi
60 chmod 600 $random_seed
61 poolfile=/proc/sys/kernel/random/poolsize
62 [ -r $poolfile ] && bytes=`cat $poolfile` || bytes=512
63 dd if=/dev/urandom of=$random_seed count=1 bs=$bytes
64 Also, add the following lines in an appropriate script which is run
66 during the Linux system shutdown:
67 # Carry a random seed from shut-down to start-up
69 # Save the whole entropy pool
70 echo "Saving random seed..."
71 random_seed=/var/run/random-seed
72 touch $random_seed
73 chmod 600 $random_seed
74 poolfile=/proc/sys/kernel/random/poolsize
75 [ -r $poolfile ] && bytes=`cat $poolfile` || bytes=512
76 dd if=/dev/urandom of=$random_seed count=1 bs=$bytes
77
79 The files in the directory /proc/sys/kernel/random (present since
80 2.3.16) provide an additional interface to the /dev/random device.
81 The read-only file entropy_avail gives the available entropy. Normally,
83 this will be 4096 (bits), a full entropy pool.
84 The file poolsize gives the size of the entropy pool. Normally, this
86 will be 512 (bytes). It can be changed to any value for which an algo‐
87 rithm is available. Currently the choices are 32, 64, 128, 256, 512,
88 1024, 2048.
89 The file read_wakeup_threshold contains the number of bits of entropy
91 required for waking up processes that sleep waiting for entropy from
92 /dev/random. The default is 64. The file write_wakeup_threshold con‐
93 tains the number of bits of entropy below which we wake up processes
94 that do a select() or poll() for write access to /dev/random. These
95 values can be changed by writing to the files.
96 The read-only files uuid and boot_id contain random strings like
98 6fd5a44b-35f4-4ad4-a9b9-6b9be13e1fe9. The former is generated afresh
99 for each read, the latter was generated once.
100
102 /dev/random
103 /dev/urandom
104
106 The kernel's random number generator was written by Theodore Ts'o
107 (tytso@athena.mit.edu).
108
110 mknod (1)
111 RFC 1750, "Randomness Recommendations for Security"
112Linux 2003-10-25 RANDOM(4)