urandom

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/ran-
       dom 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  environmen-
       tal noise is gathered.

       When  read,  /dev/urandom  device  will	return	as  many bytes as are
       requested.  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 lit-
       erature, 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 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 "Initializing kernel random number generator..."
	    # Initialize kernel random number generator with random 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 [ -f /var/random-seed ]; then
		 cat /var/random-seed >/dev/urandom
	    fi
	    dd if=/dev/urandom of=/var/random-seed count=1

       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 for the random
	    # number generator.	 Save 512 bytes, which is the size of the
	    # random number generator’s entropy pool.
	    echo "Saving random seed..."
	    dd if=/dev/urandom of=/var/random-seed count=1

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.  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 con-
       tains 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"




Linux				  2003-10-25			    RANDOM(4)