Displaying PRNG information

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7.1 LPAR mode z/VM guest KVM guest

Read the attributes of the prandom device in sysfs.

About this task

The sysfs representation of a PRNG device is a directory: /sys/devices/virtual/misc/prandom. This sysfs directory contains a number of attributes with information about the device.

Table 1. Attributes with PRNG information
Attribute Explanation
chunksize The size, in bytes, of the random-data bytes buffer that is used to generate new random numbers. The value can be in the range 64 bytes - 64 KB. The default is 256 bytes. It is rounded up to the next 64-byte boundary and can be adjusted as a module parameter when you start the module.
byte_counter The number of random bytes generated since the PRNG device driver was started. You can reset this value only by removing and reloading the kernel module, or rebooting Linux® (if PRNG was compiled into the kernel). This attribute is read-only.
errorflag SHA512 mode only: 0 if the PRNG device driver is instantiated and running well. Any other value indicates a problem. If there is an error indication other than 0:
  • The DRBG does not provide random data bytes to user space
  • The read() function fails
  • The error code errno is set to EPIPE (broken pipe)
This attribute is read-only.
mode SHA512 if the PRNG device driver runs in SHA512 mode, TDES if the PRNG device driver runs in TDES mode. This attribute is read-only.
reseed SHA512 mode only: An integer, writable only by root. Write any integer to this attribute to trigger an immediate reseed of the PRNG. See Reseeding the PRNG.
reseed_limit SHA512 mode only: An integer, writable only by root to query or set the reseed counter limit. Valid values are in the range 10000 - 100000. The default is 100000. See Setting the reseed limit.
strength SHA512 mode only: A read-only integer that shows the security strength according to NIST SP800-57. Returns the integer value of 256.

Procedure

Issue a command of this form to read an attribute: 
# cat /sys/devices/virtual/misc/prandom/<attribute>
where <attribute> is one of the attributes of Table 1.

Example

This example shows a prandom device that is running in SHA512 mode, set to reseed after 2.56 MB: 
# cat /sys/devices/virtual/misc/prandom/chunksize
256
# cat /sys/devices/virtual/misc/prandom/mode
SHA512
# cat /sys/devices/virtual/misc/prandom/reseed_limit
10000