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urandom(4) [bsd man page]

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
       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.

   Usage
       The /dev/random interface is considered a legacy interface, and /dev/urandom is preferred and sufficient in all use cases, with the  excep-
       tion 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 ade-
       quate 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.

   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 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
       sequence:

	   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 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.

       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 4.15 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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