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NAME

       random, urandom - kernel random number source devices

DESCRIPTION

       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 only return random bytes  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.

       A  read  from  the  /dev/urandom device will not block waiting for more
       entropy.  As a result, if  there  is  not  sufficient  entropy  in  the
       entropy  pool,  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 non-classified
       literature, but it is theoretically possible that such  an  attack  may
       exist.   If  this  is  a  concern  in your application, use /dev/random
       instead.

CONFIGURING

       If your system does  not  have  /dev/random  and  /dev/urandom  created
       already, they can be created with the following commands:

               mknod -m 644 /dev/random c 1 8
               mknod -m 644 /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
       following 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 ] && bytes=‘cat $poolfile‘ || bytes=512
            dd if=/dev/urandom of=$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 ] && bytes=‘cat $poolfile‘ || bytes=512
            dd if=/dev/urandom of=$random_seed count=1 bs=$bytes

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.  Normally,  this
       will  be  512  (bytes).   It  can  be changed to any value for which an
       algorithm is available.  Currently the choices are 32,  64,  128,  256,
       512, 1024, 2048.

       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() or poll() 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.

FILES

       /dev/random
       /dev/urandom

AUTHOR

       The kernel’s random number  generator  was  written  by  Theodore  Ts’o
       (tytso@athena.mit.edu).

SEE ALSO

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