NAME | SYNOPSIS | DESCRIPTION | FILES | NOTES | BUGS | SEE ALSO | COLOPHON

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

NAME         top

       random, urandom - kernel random number source devices

SYNOPSIS         top

       #include <linux/random.h>

       int ioctl(fd, RNDrequest, param);

DESCRIPTION         top

       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
       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 (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 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 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
       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 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
       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 incre‐
              ment 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         top

       /dev/random
       /dev/urandom

NOTES         top

       For an overview and comparison of the various interfaces that can be
       used to obtain randomness, see random(7).

BUGS         top

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

SEE ALSO         top

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

       RFC 1750, "Randomness Recommendations for Security"

COLOPHON         top

       This page is part of release 4.13 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)

Pages that refer to this page: getrandom(2)getentropy(3)sd_id128_get_machine(3)sd_id128_randomize(3)proc(5)systemd.unit(5)tmpfiles.d(5)capabilities(7)random(7)systemd-random-seed.service(8)