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    Command:

    urandom

    
    
    

    SYNOPSIS

           #include <linux/random.h>
    
           int ioctl(fd, RNDrequest, param);
    
    
    

    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.
    
           Writing  to  /dev/random  or  /dev/urandom will update the entropy pool
           with the data written, but this will not result  in  a  higher  entropy
           count.   This  means  that  it  will impact the contents read from both
           files, but it will not make reads from /dev/random faster.
    
       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.
    
    
           While  some  safety margin above that minimum is reasonable, as a guard
           against flaws in the CPRNG algorithm, no cryptographic primitive avail-
           able  today  can  hope to promise more than 256 bits of security, so if
           any program reads more than 256 bits (32 bytes) from the kernel  random
           pool  per  invocation, or per reasonable reseed interval (not less than
           one minute), that should be taken as a sign that  its  cryptography  is
           not skillfully implemented.
    
       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
    
                              are 32, 64, 128, 256, 512, 1024, or 2048.
    
                  Linux 2.6:  This file is read-only, and gives the  size  of  the
                              entropy pool in bits.  It contains the value 4096.
    
           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(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.
    
       ioctl(2) interface
           The following ioctl(2) requests are defined on  file  descriptors  con-
           nected  to  either /dev/random or /dev/urandom.  All requests performed
           will interact with the input entropy pool  impacting  both  /dev/random
           and  /dev/urandom.   The  CAP_SYS_ADMIN  capability is required for all
           requests except RNDGETENTCNT.
    
           RNDGETENTCNT
                  Retrieve the entropy count of the input pool, the contents  will
                  be  the  same  as the entropy_avail file under proc.  The result
                  will be stored in the int pointed to by the argument.
    
           RNDADDTOENTCNT
                  Increment or decrement the entropy count of the  input  pool  by
                  the value pointed to by the argument.
    
           RNDGETPOOL
                  Removed in Linux 2.6.9.
    
           RNDADDENTROPY
                  Add  some additional entropy to the input pool, incrementing the
                  entropy count.  This differs  from  writing  to  /dev/random  or
                  /dev/urandom,  which  only adds some data but does not increment
                  the entropy count.  The following structure is used:
    
                      struct rand_pool_info {
                          int    entropy_count;
                          int    buf_size;
                          __u32  buf[0];
                      };
    
                  Here entropy_count is the value added to  (or  subtracted  from)
                  the  entropy count, and buf is the buffer of size buf_size which
                  gets added to the entropy pool.
    
           RNDZAPENTCNT, RNDCLEARPOOL
    
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