random, urandom - kernel random number source devices


   #include <linux/random.h>

   int ioctl(fd, RNDrequest, param);


   The character special files /dev/random and /dev/urandom (present since
   Linux 1.3.30) provide  an  interface  to  the  kernel's  random  number
   generator.   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)
   interface  which requires no special files; see the getrandom(2) manual
   page for details.

   When read,  the  /dev/urandom  device  returns  random  bytes  using  a
   pseudorandom 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.

   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
   perform   a   read  with  timeout),  and  provide  some  sort  of  user
   notification if the desired entropy is not immediately available.

   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
   information 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

       echo "Initializing random number generator..."
       # 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
           touch $random_seed
       chmod 600 $random_seed
       [ -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..."
       touch $random_seed
       chmod 600 $random_seed
       [ -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:

          This read-only file gives the available entropy.  Normally, this
          will be 4096 (bits), a full entropy pool.

          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.

          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.

          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
   connected  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.

          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.

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

          Removed in Linux 2.6.9.

          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.

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


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


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




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

   RFC 1750, "Randomness Recommendations for Security"


   This page is part of release 4.09 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


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