urandom (4)

Leading comments

Copyright (c) 1997 John S. Kallal (kallal@voicenet.com)

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Some changes by tytso and aeb.

2004-12-16, John V. Belmonte/mtk, Updated init and quit scripts
2004-04-08, AEB, Improved description of read from /de...

(The comments found at the beginning of the groff file "man4/urandom.4".)

NAME

random, urandom - kernel random number source devices

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 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 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. If open(2) is called for /dev/random with the flag O_NONBLOCK, a subsequent 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 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 or Diffie-Hellman private key has an effective key size of 128 bits (it requires about 2^128 operations to break) so a key generator 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 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 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 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 $poolfile ] && bits=$(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 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 $poolfile ] && bits=$(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/poolsize 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 additional 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.

FILES

/dev/random
/dev/urandom

SEE ALSO

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

COLOPHON

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 www.kernel.org/doc/man-pages