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

       random, urandom - kernel random number source devices

       #include <linux/random.h>

       int ioctl(fd, RNDrequest, param);

       The character special files /dev/random and /dev/urandom (present since
       Linux 1.3.30) provide an interface to the kernel's random number gener-
       ator.  File /dev/random has major device number 1 and minor device num-
       ber 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_NON-
       BLOCK, 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

       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  crypto-
       graphic  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  per-
       form  a  read with timeout), and provide some sort of user notification
       if the desired entropy is not immediately 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 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 avail-
       able  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 infor-
       mation 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

           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.   Nor-
       mally, 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 con-
       tains 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  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.

              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.


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

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       description of the project, information about reporting bugs,  and  the
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Linux                             2015-12-28                         RANDOM(4)
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