RANDOM(4) - man page online | special files

Kernel random number source devices.

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


random, urandom - kernel random number source devices


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


The character special files /dev/random and /dev/urandom (present since Linux 1.3.30) pro‐ vide 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 return random bytes only within the estimated num‐ ber 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. If open(2) is called for /dev/random with the flag O_NONBLOCK, a sub‐ sequent read(2) will not block if the requested number of bytes is not available. Instead, the available bytes are returned. If no byte is available, read(2) will return -1 and errno will be set to EAGAIN. A read from the /dev/urandom device will not block waiting for more entropy. If there is not sufficient entropy, a pseudorandom number generator is used to create the requested bytes. As a result, in this case 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 literature, but it is theoretically possible that such an attack may exist. If this is a concern in your application, use /dev/random instead. O_NONBLOCK has no effect when opening /dev/urandom. When calling read(2) for the device /dev/urandom, signals will not be handled until after the requested random bytes have been generated. Since Linux 3.16, a read(2) from /dev/urandom will return at most 32 MB. A read(2) from /dev/random will return at most 512 bytes (340 bytes on Linux kernels before version 2.6.12). 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 perform a read with timeout), and provide some sort of user notification if the desired entropy is not immedi‐ ately 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 ran‐ dom 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 or Diffie-Hellman private key has an effective key size of 128 bits (it requires about 2^128 operations to break) so a key gen‐ erator needs only 128 bits (16 bytes) of seed material from /dev/random. While some safety margin above that minimum is reasonable, as a guard against flaws in the CPRNG algorithm, no cryptographic primitive available today can hope to promise more than 256 bits of security, so if any program reads more than 256 bits (32 bytes) from the ker‐ nel 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 imple‐ mented. Configuration If your system does not have /dev/random and /dev/urandom created already, they can be created with the following commands: mknod -m 666 /dev/random c 1 8 mknod -m 666 /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 information across shut-downs and start-ups. To do this, add the lines to an appro‐ priate 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 $poolfile ] && bits=$(cat $poolfile) || bits=4096 bytes=$(expr $bits / 8) dd if=/dev/urandom of=$random_seed count=1 bs=$bytes Also, add the following lines in an appropriate script which is run during the Linux sys‐ tem 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 $poolfile ] && bits=$(cat $poolfile) || bits=4096 bytes=$(expr $bits / 8) dd if=/dev/urandom of=$random_seed count=1 bs=$bytes In the above examples, we assume Linux 2.6.0 or later, where /proc/sys/kernel/random/pool‐ size returns the size of the entropy pool in bits (see below). /proc Interface The files in the directory /proc/sys/kernel/random (present since 2.3.16) provide an addi‐ tional interface to the /dev/random device. The read-only file entropy_avail gives the available entropy. Normally, 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 bytes. Normally, this file will have the value 512, but it is writable, and can be changed to any value for which an algorithm is available. The choices 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 contains 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 connected 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 Zero the entropy count of all pools and add some system data (such as wall clock) to the pools.


/dev/random /dev/urandom


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


This page is part of release 4.04 of the Linux man-pages project. A description of the project, information about reporting bugs, and the latest version of this page, can be found at
Linux 2015-12-28 RANDOM(4)
This manual Reference Other manuals
random(4) referred by aesni(4freebsd) | BSD::arc4random(3pm) | BUS_SETUP_INTR(9freebsd) | bus_setup_intr(9freebsd) | BUS_TEARDOWN_INTR(9freebsd) | bus_teardown_intr(9freebsd) | cryptsetup(8) | getrandom(2) | glxsb(4freebsd) | hifn(4freebsd) | padlock(4freebsd) | PDL::IO::Misc(3pm) | PDL::LinearAlgebra(3pm) | PDL::LinearAlgebra::Real(3pm) | PDL::Primitive(3pm) | proc(5) | pwqgen(1) | random(4freebsd) | random_harvest(9freebsd) | rndtest(4freebsd)
refer to getrandom(2) | ioctl(2) | mknod(1) | open(2) | poll(2) | read(2) | select(2)