random(4), urandom(4) - kernel random number source devices



  • 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  ap‐
           plication, 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 ex‐
           ception  of  applications  which  require  randomness during early boot
           time; for these applications, getrandom(2) must be  used  instead,  be‐
           cause 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  al‐
           ready, 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 or‐
           der to counteract this effect, it helps to carry entropy pool  informa‐
           tion  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  se‐
           quence:
    
               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 ac‐
                  cess 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 re‐
           quests 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 5.05 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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