1RANDOM(4)                  Linux Programmer's Manual                 RANDOM(4)


6       random, urandom - kernel random number source devices


9       #include <linux/random.h>
11       int ioctl(fd, RNDrequest, param);


14       The character special files /dev/random and /dev/urandom (present since
15       Linux 1.3.30) provide an interface to the kernel's random number gener‐
16       ator.   The file /dev/random has major device number 1 and minor device
17       number 8.  The file /dev/urandom has major device number  1  and  minor
18       device number 9.
20       The  random  number  generator  gathers environmental noise from device
21       drivers and other sources into an entropy  pool.   The  generator  also
22       keeps  an  estimate of the number of bits of noise in the entropy pool.
23       From this entropy pool, random numbers are created.
25       Linux 3.17 and later provides the simpler and safer getrandom(2) inter‐
26       face  which requires no special files; see the getrandom(2) manual page
27       for details.
29       When read, the /dev/urandom device returns random bytes using a pseudo‐
30       random  number generator seeded from the entropy pool.  Reads from this
31       device do not block (i.e., the CPU is not yielded), but  can  incur  an
32       appreciable delay when requesting large amounts of data.
34       When read during early boot time, /dev/urandom may return data prior to
35       the entropy pool being initialized.  If this  is  of  concern  in  your
36       application, use getrandom(2) or /dev/random instead.
38       The /dev/random device is a legacy interface which dates back to a time
39       where the  cryptographic  primitives  used  in  the  implementation  of
40       /dev/urandom were not widely trusted.  It will return random bytes only
41       within the estimated number of bits of fresh noise in the entropy pool,
42       blocking  if  necessary.  /dev/random is suitable for applications that
43       need high quality randomness, and can afford indeterminate delays.
45       When the entropy pool is empty, reads from /dev/random will block until
46       additional  environmental  noise is gathered.  If open(2) is called for
47       /dev/random with the O_NONBLOCK flag, a  subsequent  read(2)  will  not
48       block  if the requested number of bytes is not available.  Instead, the
49       available bytes are returned.  If no byte is  available,  read(2)  will
50       return -1 and errno will be set to EAGAIN.
52       The  O_NONBLOCK  flag  has  no  effect when opening /dev/urandom.  When
53       calling read(2) for the device /dev/urandom, reads of up to  256  bytes
54       will  return as many bytes as are requested and will not be interrupted
55       by a signal handler.  Reads with a buffer over this  limit  may  return
56       less  than  the requested number of bytes or fail with the error EINTR,
57       if interrupted by a signal handler.
59       Since Linux 3.16, a read(2)  from  /dev/urandom  will  return  at  most
60       32 MB.   A  read(2) from /dev/random will return at most 512 bytes (340
61       bytes on Linux kernels before version 2.6.12).
63       Writing to /dev/random or /dev/urandom will  update  the  entropy  pool
64       with  the  data  written,  but this will not result in a higher entropy
65       count.  This means that it will impact  the  contents  read  from  both
66       files, but it will not make reads from /dev/random faster.
68   Usage
69       The  /dev/random  interface  is  considered  a  legacy  interface,  and
70       /dev/urandom is preferred and sufficient in all  use  cases,  with  the
71       exception  of  applications  which require randomness during early boot
72       time; for  these  applications,  getrandom(2)  must  be  used  instead,
73       because it will block until the entropy pool is initialized.
75       If  a seed file is saved across reboots as recommended below (all major
76       Linux distributions have done this since 2000 at least), the output  is
77       cryptographically secure against attackers without local root access as
78       soon as it is reloaded in the boot sequence, and perfectly adequate for
79       network  encryption  session  keys.   Since  reads from /dev/random may
80       block, users will usually want to open it in nonblocking mode (or  per‐
81       form  a  read with timeout), and provide some sort of user notification
82       if the desired entropy is not immediately available.
84   Configuration
85       If your system does  not  have  /dev/random  and  /dev/urandom  created
86       already, they can be created with the following commands:
88           mknod -m 666 /dev/random c 1 8
89           mknod -m 666 /dev/urandom c 1 9
90           chown root:root /dev/random /dev/urandom
92       When  a  Linux  system starts up without much operator interaction, the
93       entropy pool may be in a fairly predictable state.   This  reduces  the
94       actual  amount  of  noise  in  the entropy pool below the estimate.  In
95       order to counteract this effect, it helps to carry entropy pool  infor‐
96       mation  across  shut-downs and start-ups.  To do this, add the lines to
97       an appropriate script which is run during  the  Linux  system  start-up
98       sequence:
100           echo "Initializing random number generator..."
101           random_seed=/var/run/random-seed
102           # Carry a random seed from start-up to start-up
103           # Load and then save the whole entropy pool
104           if [ -f $random_seed ]; then
105               cat $random_seed >/dev/urandom
106           else
107               touch $random_seed
108           fi
109           chmod 600 $random_seed
110           poolfile=/proc/sys/kernel/random/poolsize
111           [ -r $poolfile ] && bits=$(cat $poolfile) || bits=4096
112           bytes=$(expr $bits / 8)
113           dd if=/dev/urandom of=$random_seed count=1 bs=$bytes
115       Also,  add  the  following  lines in an appropriate script which is run
116       during the Linux system shutdown:
118           # Carry a random seed from shut-down to start-up
119           # Save the whole entropy pool
120           echo "Saving random seed..."
121           random_seed=/var/run/random-seed
122           touch $random_seed
123           chmod 600 $random_seed
124           poolfile=/proc/sys/kernel/random/poolsize
125           [ -r $poolfile ] && bits=$(cat $poolfile) || bits=4096
126           bytes=$(expr $bits / 8)
127           dd if=/dev/urandom of=$random_seed count=1 bs=$bytes
129       In  the  above  examples,  we  assume  Linux  2.6.0  or  later,   where
130       /proc/sys/kernel/random/poolsize  returns  the size of the entropy pool
131       in bits (see below).
133   /proc interfaces
134       The files  in  the  directory  /proc/sys/kernel/random  (present  since
135       2.3.16) provide additional information about the /dev/random device:
137       entropy_avail
138              This  read-only file gives the available entropy, in bits.  This
139              will be a number in the range 0 to 4096.
141       poolsize
142              This file gives the size of the entropy pool.  The semantics  of
143              this file vary across kernel versions:
145              Linux 2.4:
146                     This  file  gives  the size of the entropy pool in bytes.
147                     Normally, this file will have the value 512,  but  it  is
148                     writable,  and  can  be changed to any value for which an
149                     algorithm is available.  The choices  are  32,  64,  128,
150                     256, 512, 1024, or 2048.
152              Linux 2.6 and later:
153                     This file is read-only, and gives the size of the entropy
154                     pool in bits.  It contains the value 4096.
156       read_wakeup_threshold
157              This file contains the number of bits of  entropy  required  for
158              waking   up  processes  that  sleep  waiting  for  entropy  from
159              /dev/random.  The default is 64.
161       write_wakeup_threshold
162              This file contains the number of bits of entropy below which  we
163              wake  up  processes  that  do  a  select(2) or poll(2) for write
164              access to /dev/random.  These values can be changed  by  writing
165              to the files.
167       uuid and boot_id
168              These    read-only    files    contain   random   strings   like
169              6fd5a44b-35f4-4ad4-a9b9-6b9be13e1fe9.  The former  is  generated
170              afresh for each read, the latter was generated once.
172   ioctl(2) interface
173       The  following  ioctl(2)  requests are defined on file descriptors con‐
174       nected to either /dev/random or /dev/urandom.  All  requests  performed
175       will  interact  with  the input entropy pool impacting both /dev/random
176       and /dev/urandom.  The CAP_SYS_ADMIN capability  is  required  for  all
177       requests except RNDGETENTCNT.
180              Retrieve  the entropy count of the input pool, the contents will
181              be the same as the entropy_avail file under  proc.   The  result
182              will be stored in the int pointed to by the argument.
185              Increment  or  decrement  the entropy count of the input pool by
186              the value pointed to by the argument.
188       RNDGETPOOL
189              Removed in Linux 2.6.9.
192              Add some additional entropy to the input pool, incrementing  the
193              entropy  count.   This  differs  from  writing to /dev/random or
194              /dev/urandom, which only adds some data but does  not  increment
195              the entropy count.  The following structure is used:
197                  struct rand_pool_info {
198                      int    entropy_count;
199                      int    buf_size;
200                      __u32  buf[0];
201                  };
203              Here  entropy_count  is  the value added to (or subtracted from)
204              the entropy count, and buf is the buffer of size buf_size  which
205              gets added to the entropy pool.
208              Zero  the  entropy  count  of all pools and add some system data
209              (such as wall clock) to the pools.


212       /dev/random
213       /dev/urandom


216       For an overview and comparison of the various interfaces  that  can  be
217       used to obtain randomness, see random(7).


220       During  early  boot time, reads from /dev/urandom may return data prior
221       to the entropy pool being initialized.


224       mknod(1), getrandom(2), random(7)
226       RFC 1750, "Randomness Recommendations for Security"


229       This page is part of release 4.15 of the Linux  man-pages  project.   A
230       description  of  the project, information about reporting bugs, and the
231       latest    version    of    this    page,    can     be     found     at
232       https://www.kernel.org/doc/man-pages/.
236Linux                             2017-09-15                         RANDOM(4)