urandom(4)



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, 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.  (All  major  Linux  distributions
       have  saved  the  seed file across reboots since 2000 at least.)  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.

   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 5.07 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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