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



NAME
       random(3,4,6), urandom - kernel random(3,4,6) number source devices

DESCRIPTION
       The character special files /dev/random(3,4,6) and /dev/urandom (present since
       Linux 1.3.30) provide an interface to the kernel's random(3,4,6) number gener-
       ator.  File /dev/random(3,4,6) 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(3,4,6)  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(1,8) the entropy pool.
       From this entropy pool random(3,4,6) numbers are created.

       When read(2,n,1 builtins), the /dev/random(3,4,6) device will only return random(3,4,6) bytes  within
       the estimated number of bits of noise in(1,8) the entropy pool.  /dev/random(3,4,6)
       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(3,4,6) will block until additional environmental  noise
       is gathered.

       When  read(2,n,1 builtins),  /dev/urandom  device  will  return  as  many  bytes as are
       requested.  As a result, if(3,n) there is  not  sufficient  entropy  in(1,8)  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(1,8) the current non-classified liter-
       ature, but it is theoretically possible that such an attack may  exist.
       If this is a concern in(1,8) your application, use /dev/random(3,4,6) instead.

CONFIGURING
       If  your  system  does  not  have  /dev/random(3,4,6) and /dev/urandom created
       already, they can be created with the following commands:

               mknod(1,2) -m 644 /dev/random(3,4,6) c 1 8
               mknod(1,2) -m 644 /dev/urandom c 1 9
               chown(1,2) root:root /dev/random(3,4,6) /dev/urandom

       When a Linux system starts up without much  operator  interaction,  the
       entropy  pool  may  be in(1,8) a fairly predictable state.  This reduces the
       actual amount of noise in(1,8) 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  following
       lines  to  an  appropriate  script which is run during the Linux system
       start-up sequence:

            echo(1,3x,1 builtins) "Initializing kernel random(3,4,6) number generator..."
            # Initialize kernel random(3,4,6) number generator with random(3,4,6) seed
            # from last shut-down (or start-up) to this start-up.  Load and
            # then save 512 bytes, which is the size of the entropy pool.
            if(3,n) [ -f /var/random-seed ]; then
                 cat /var/random-seed >/dev/urandom
            fi
            dd if(3,n)=/dev/urandom of=/var/random-seed count=1

       Also, add the following lines in(1,8) an appropriate  script  which  is  run
       during the Linux system shutdown:

            # Carry a random(3,4,6) seed from shut-down to start-up for the random(3,4,6)
            # number generator.  Save 512 bytes, which is the size of the
            # random(3,4,6) number generator's entropy pool.
            echo(1,3x,1 builtins) "Saving random(3,4,6) seed..."
            dd if(3,n)=/dev/urandom of=/var/random-seed count=1

PROC INTERFACE
       The  files  in(1,8)  the  directory  /proc(5,n)/sys/kernel/random(3,4,6)  (present since
       2.3.16) provide an additional interface to the /dev/random(3,4,6) device.

       The read-only file(1,n) entropy_avail gives the available entropy. Normally,
       this will be 4096 (bits), a full entropy pool.

       The  file(1,n)  poolsize  gives the size of the entropy pool. Normally, this
       will be 512 (bytes).  It can be changed to any value for which an algo-
       rithm  is  available.  Currently the choices are 32, 64, 128, 256, 512,
       1024, 2048.

       The file(1,n) read_wakeup_threshold contains the number of bits  of  entropy
       required  for  waking  up processes that sleep(1,3) waiting for entropy from
       /dev/random(3,4,6).  The default is 64.  The file(1,n) write_wakeup_threshold  con-
       tains  the  number  of bits of entropy below which we wake up processes
       that do a select(2,7,2 select_tut)() or poll() for write(1,2) access(2,5)  to  /dev/random(3,4,6).   These
       values can be changed by writing to the files.

       The  read-only  files  uuid  and  boot_id  contain  random(3,4,6) strings like
       6fd5a44b-35f4-4ad4-a9b9-6b9be13e1fe9.  The former is  generated  afresh
       for each read(2,n,1 builtins), the latter was generated once.

FILES
       /dev/random(3,4,6)
       /dev/urandom

AUTHOR
       The  kernel's  random(3,4,6)  number  generator  was  written by Theodore Ts'o
       (tytso@athena.mit.edu).

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



Linux                             2003-10-25                         RANDOM(4)

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