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

       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) pro-
       vide 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 gener-
       ator 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 crypto-
       graphic 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 envi-
       ronmental 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 avail-
       able.  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  inter-
       rupted 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

       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  random-
       ness  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 at-
       tackers without local root access as soon as it is reloaded in the boot sequence, and per-
       fectly  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 immedi-
       ately 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  appro-
       priate script which is run during the Linux system start-up sequence:

           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 sys-
       tem 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/pool-
       size 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 addi-
       tional information about the /dev/random device:

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

              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 re-
       quired 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 ran-
       domness, 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 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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