random

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

NAME
       random, urandom - kernel random number source devices

SYNOPSIS
       #include <linux/random.h>

       int ioctl(fd, RNDrequest, param);

DESCRIPTION
       The character special files /dev/random and /dev/urandom (present since
       Linux 1.3.30) provide an interface to the kernel's random number gener-
       ator.   The file /dev/random has major device number 1 and minor device
       number 8.  The file /dev/urandom has major device number  1  and  minor
       device number 9.

       The  random  number  generator  gathers environmental noise from device
       drivers and other sources into an entropy  pool.   The  generator  also
       keeps  an  estimate of the number of bits of noise in the entropy pool.
       From this entropy pool, random numbers are created.

       Linux 3.17 and later provides the simpler and safer getrandom(2) inter-
       face  which requires no special files; see the getrandom(2) manual page
       for details.

       When read, the /dev/urandom device returns random bytes using a pseudo-
       random  number generator seeded from the entropy pool.  Reads from this
       device do not block (i.e., the CPU is not yielded), but  can  incur  an
       appreciable delay when requesting large amounts of data.

       When read during early boot time, /dev/urandom may return data prior to
       the entropy pool being initialized.  If this  is  of  concern  in  your
       application, use getrandom(2) or /dev/random instead.

       The /dev/random device is a legacy interface which dates back to a time
       where the  cryptographic  primitives  used  in  the  implementation  of
       /dev/urandom were not widely trusted.  It will return random bytes only
       within the estimated number of bits of fresh noise in the entropy pool,
       blocking  if  necessary.  /dev/random is suitable for applications that
       need high quality randomness, and can afford indeterminate delays.

       When the entropy pool is empty, reads from /dev/random will block until
       additional  environmental  noise is gathered.  If open(2) is called for
       /dev/random with the O_NONBLOCK flag, a  subsequent  read(2)  will  not
       block  if the requested number of bytes is not available.  Instead, the
       available bytes are returned.  If no byte is  available,  read(2)  will
       return -1 and errno will be set to EAGAIN.

       The  O_NONBLOCK  flag  has  no  effect when opening /dev/urandom.  When
       calling read(2) for the device /dev/urandom, reads of up to  256  bytes
       will  return as many bytes as are requested and will not be interrupted
       by a signal handler.  Reads with a buffer over this  limit  may  return
       less  than  the requested number of bytes or fail with the error EINTR,
       if interrupted by a signal handler.

       Since Linux 3.16, a read(2)  from  /dev/urandom  will  return  at  most
       32 MB.   A  read(2) from /dev/random will return at most 512 bytes (340
       bytes on Linux kernels before version 2.6.12).

       Writing to /dev/random or /dev/urandom will  update  the  entropy  pool
       with  the  data  written,  but this will not result in a higher entropy
       count.  This means that it will impact  the  contents  read  from  both
       files, but it will not make reads from /dev/random faster.

   Usage
       The  /dev/random  interface  is  considered  a  legacy  interface,  and
       /dev/urandom is preferred and sufficient in all  use  cases,  with  the
       exception  of  applications  which require randomness during early boot
       time; for  these  applications,  getrandom(2)  must  be  used  instead,
       because it will block until the entropy pool is initialized.

       If  a seed file is saved across reboots as recommended below (all major
       Linux distributions have done this since 2000 at least), the output  is
       cryptographically secure against attackers without local root access as
       soon as it is reloaded in the boot sequence, and perfectly adequate for
       network  encryption  session  keys.   Since  reads from /dev/random may
       block, users will usually want to open it in nonblocking mode (or  per-
       form  a  read with timeout), and provide some sort of user notification
       if the desired entropy is not immediately available.

   Configuration
       If your system does  not  have  /dev/random  and  /dev/urandom  created
       already, they can be created with the following commands:

           mknod -m 666 /dev/random c 1 8
           mknod -m 666 /dev/urandom c 1 9
           chown root:root /dev/random /dev/urandom

       When  a  Linux  system starts up without much operator interaction, the
       entropy pool may be in a fairly predictable state.   This  reduces  the
       actual  amount  of  noise  in  the entropy pool below the estimate.  In
       order to counteract this effect, it helps to carry entropy pool  infor-
       mation  across  shut-downs and start-ups.  To do this, add the lines to
       an appropriate script which is run during  the  Linux  system  start-up
       sequence:

           echo "Initializing random number generator..."
           random_seed=/var/run/random-seed
           # Carry a random seed from start-up to start-up
           # Load and then save the whole entropy pool
           if [ -f $random_seed ]; then
               cat $random_seed >/dev/urandom
           else
               touch $random_seed
           fi
           chmod 600 $random_seed
           poolfile=/proc/sys/kernel/random/poolsize
           [ -r $poolfile ] && bits=$(cat $poolfile) || bits=4096
           bytes=$(expr $bits / 8)
           dd if=/dev/urandom of=$random_seed count=1 bs=$bytes

       Also,  add  the  following  lines in an appropriate script which is run
       during the Linux system shutdown:

           # Carry a random seed from shut-down to start-up
           # Save the whole entropy pool
           echo "Saving random seed..."
           random_seed=/var/run/random-seed
           touch $random_seed
           chmod 600 $random_seed
           poolfile=/proc/sys/kernel/random/poolsize
           [ -r $poolfile ] && bits=$(cat $poolfile) || bits=4096
           bytes=$(expr $bits / 8)
           dd if=/dev/urandom of=$random_seed count=1 bs=$bytes

       In  the  above  examples,  we  assume  Linux  2.6.0  or  later,   where
       /proc/sys/kernel/random/poolsize  returns  the size of the entropy pool
       in bits (see below).

   /proc interfaces
       The files  in  the  directory  /proc/sys/kernel/random  (present  since
       2.3.16) provide additional information about the /dev/random device:

       entropy_avail
              This  read-only file gives the available entropy, in bits.  This
              will be a number in the range 0 to 4096.

       poolsize
              This file gives the size of the entropy pool.  The semantics  of
              this file vary across kernel versions:

              Linux 2.4:
                     This  file  gives  the size of the entropy pool in bytes.
                     Normally, this file will have the value 512,  but  it  is
                     writable,  and  can  be changed to any value for which an
                     algorithm is available.  The choices  are  32,  64,  128,
                     256, 512, 1024, or 2048.

              Linux 2.6 and later:
                     This file is read-only, and gives the size of the entropy
                     pool in bits.  It contains the value 4096.

       read_wakeup_threshold
              This file contains the number of bits of  entropy  required  for
              waking   up  processes  that  sleep  waiting  for  entropy  from
              /dev/random.  The default is 64.

       write_wakeup_threshold
              This file contains the number of bits of entropy below which  we
              wake  up  processes  that  do  a  select(2) or poll(2) for write
              access to /dev/random.  These values can be changed  by  writing
              to the files.

       uuid and boot_id
              These    read-only    files    contain   random   strings   like
              6fd5a44b-35f4-4ad4-a9b9-6b9be13e1fe9.  The former  is  generated
              afresh for each read, the latter was generated once.

   ioctl(2) interface
       The  following  ioctl(2)  requests are defined on file descriptors con-
       nected to either /dev/random or /dev/urandom.  All  requests  performed
       will  interact  with  the input entropy pool impacting both /dev/random
       and /dev/urandom.  The CAP_SYS_ADMIN capability  is  required  for  all
       requests except RNDGETENTCNT.

       RNDGETENTCNT
              Retrieve  the entropy count of the input pool, the contents will
              be the same as the entropy_avail file under  proc.   The  result
              will be stored in the int pointed to by the argument.

       RNDADDTOENTCNT
              Increment  or  decrement  the entropy count of the input pool by
              the value pointed to by the argument.

       RNDGETPOOL
              Removed in Linux 2.6.9.

       RNDADDENTROPY
              Add some additional entropy to the input pool, incrementing  the
              entropy  count.   This  differs  from  writing to /dev/random or
              /dev/urandom, which only adds some data but does  not  increment
              the entropy count.  The following structure is used:

                  struct rand_pool_info {
                      int    entropy_count;
                      int    buf_size;
                      __u32  buf[0];

                  };

              Here  entropy_count  is  the value added to (or subtracted from)
              the entropy count, and buf is the buffer of size buf_size  which
              gets added to the entropy pool.

       RNDZAPENTCNT, RNDCLEARPOOL
              Zero  the  entropy  count  of all pools and add some system data
              (such as wall clock) to the pools.

FILES
       /dev/random
       /dev/urandom

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

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

SEE ALSO
       mknod(1), getrandom(2), random(7)

       RFC 1750, "Randomness Recommendations for Security"

COLOPHON
       This page is part of release 4.15 of the Linux  man-pages  project.   A
       description  of  the project, information about reporting bugs, and the
       latest    version    of    this    page,    can     be     found     at
       https://www.kernel.org/doc/man-pages/.

Linux                             2017-09-15                         RANDOM(4)
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