urandom


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 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 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 unclassified litera-
       ture, but it is theoretically possible that such an attack  may  exist.
       If this is a concern in your application, use /dev/random instead.

   Usage
       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

   Configuration
       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 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.  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

       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.

FILES
       /dev/random
       /dev/urandom

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

COLOPHON
       This  page  is  part of release 3.35 of the Linux man-pages project.  A
       description of the project, and information about reporting  bugs,  can
       be found at http://man7.org/linux/man-pages/.



Linux                             2010-08-29                         RANDOM(4)
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