man > random

random(4)                             Kernel Interfaces 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  mi‐
       nor 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 be‐
       ing 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 en‐
       tropy 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 environ‐
       mental 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  con‐
       tents 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 dur‐
       ing  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 se‐
       quence, and perfectly adequate for network encryption session keys.  (All major Linux distri‐
       butions 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 cre‐
       ated 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 addi‐
       tional 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/ran‐
       dom  or  /dev/urandom.   All requests performed will interact with the input entropy pool im‐
       pacting 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 en‐
              tropy_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 random‐
       ness, see random(7).

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

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

       RFC 1750, "Randomness Recommendations for Security"

Linux man-pages 6.7                          2023-10-31                                    random(4)