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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..."     ran-
         dom_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..."      ran-
         dom_seed=/var/run/random-seed  touch  $random_seed  chmod 600 $ran-
         dom_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.