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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.   Since  Linux  5.6, the O_NONBLOCK flag is ignored as
       /dev/random will no longer block except during early boot process.  In  earlier  versions,
       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 before Linux 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  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,   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  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 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 Linux 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"