random(4) — Linux manual page

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, 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
       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         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 5.13 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                          2021-03-22                      RANDOM(4)

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