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

NAME
       random, urandom - kernel random number source devices

SYNOPSIS
       #include 

       int ioctl(fd, RNDrequest, param);

DESCRIPTION
       The character special files /dev/random and /dev/urandom (present since Linux 1.3.30) pro‐
       vide 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  num‐
       ber  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  sub‐
       sequent  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.

   Usage
       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 immedi‐
       ately available.

       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 ran‐
       dom 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 gen‐
       erator 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 ker‐
       nel 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 imple‐
       mented.

   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  appro‐
       priate 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 sys‐
       tem 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/pool‐
       size 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 addi‐
       tional 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.

       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

SEE ALSO
       getrandom(2), mknod(1)
       RFC 1750, "Randomness Recommendations for Security"

COLOPHON
       This page is part of release 4.04 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 http://www.kernel.org/doc/man-pages/.

Linux                                       2015-12-28                                  RANDOM(4)

Go-to-top  




Designed by SanjuD(@ngineerbabu)