centos5.4 kernel random number generator_/dev/urandom及/dev/random

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

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 vulner-
       able 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 $poolfile‘ || bytes=512
            dd if=/dev/urandom f=$random_seed count=1 bs=$bytes

       Also,  add the following lines in an appropriate script. which is run during the Linux system shut-
       down:

            # 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 $poolfile‘ || bytes=512
            dd if=/dev/urandom f=$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  pro-
       cesses  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"