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.  File /dev/random has major device number 1 and
       minor device number 8.  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.

       When read, the /dev/random device will return random bytes only
       within the estimated number of bits of noise in the entropy pool.
       /dev/random should be suitable for uses that need very high quality
       randomness such as one-time pad or key generation.  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 flag O_NONBLOCK, 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.

       A read from the /dev/urandom device will not block waiting for more
       entropy.  If there is not sufficient entropy, a pseudorandom number
       generator is used to create the requested bytes.  As a result, in
       this case the returned values are theoretically vulnerable to a
       cryptographic attack on the algorithms used by the driver.  Knowledge
       of how to do this is not available in the current unclassified
       literature, but it is theoretically possible that such an attack may
       exist.  If this is a concern in your application, use /dev/random
       instead.  O_NONBLOCK has no effect when opening /dev/urandom.  When
       calling read(2) for the device /dev/urandom, signals will not be
       handled until after the requested random bytes have been generated.

       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.

       If you are unsure about whether you should use /dev/random or
       /dev/urandom, then probably you want to use the latter.  As a general
       rule, /dev/urandom should be used for everything except long-lived
       GPG/SSL/SSH keys.

       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

       The kernel random-number generator is designed to produce a small
       amount of high-quality seed material to seed a cryptographic pseudo-
       random number generator (CPRNG).  It is designed for security, not
       speed, and is poorly suited to generating large amounts of random
       data.  Users should be very economical in the amount of seed material
       that they read from /dev/urandom (and /dev/random); unnecessarily
       reading large quantities of data from this device will have a
       negative impact on other users of the device.

       The amount of seed material required to generate a cryptographic key
       equals the effective key size of the key.  For example, a 3072-bit
       RSA or Diffie-Hellman private key has an effective key size of 128
       bits (it requires about 2^128 operations to break) so a key generator
       needs only 128 bits (16 bytes) of seed material from /dev/random.

       While some safety margin above that minimum is reasonable, as a guard
       against flaws in the CPRNG algorithm, no cryptographic primitive
       available today can hope to promise more than 256 bits of security,
       so if any program reads more than 256 bits (32 bytes) from the kernel
       random pool per invocation, or per reasonable reseed interval (not
       less than one minute), that should be taken as a sign that its
       cryptography is not skillfully implemented.

       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..."
           # 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
               touch $random_seed
           chmod 600 $random_seed
           [ -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..."
           touch $random_seed
           chmod 600 $random_seed
           [ -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 Interface
       The files in the directory /proc/sys/kernel/random (present since
       2.3.16) provide an additional interface to the /dev/random device.

       The read-only file entropy_avail gives the available entropy.
       Normally, this will be 4096 (bits), a full entropy pool.

       The file poolsize 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:  This file is read-only, and gives the size of the
                          entropy pool in bits.  It contains the value 4096.

       The file read_wakeup_threshold contains the number of bits of entropy
       required for waking up processes that sleep waiting for entropy from
       /dev/random.  The default is 64.  The file write_wakeup_threshold
       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.

       The read-only files uuid and boot_id 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.

              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.

              Increment or decrement the entropy count of the input pool by
              the value pointed to by the argument.

              Removed in Linux 2.6.9.

              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.

              Zero the entropy count of all pools and add some system data
              (such as wall clock) to the pools.

FILES         top


SEE ALSO         top

       mknod(1), getrandom(2)

       RFC 1750, "Randomness Recommendations for Security"

COLOPHON         top

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Linux                            2016-10-08                        RANDOM(4)