Usage¶

The /dev/random interface is considered a legacy interface, and /dev/urandom is preferred and sufficient in all use cases, with the exception of applications which require randomness during early boot time; for these applications, getrandom(2) must be used instead, because it will block until the entropy pool is initialized.

If a seed file is saved across reboots as recommended below, 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. (All major Linux distributions have saved the seed file across reboots since 2000 at least.) 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.

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 interfaces¶

The files in the directory /proc/sys/kernel/random (present since 2.3.16) provide additional information about the /dev/random device:

entropy_avail

This read-only file gives the available entropy, in bits. This will be a number in the range 0 to 4096.

poolsize

This file gives the size of the entropy pool. The semantics of this file vary across kernel versions:

read_wakeup_threshold

This file contains the number of bits of entropy required for waking up processes that sleep waiting for entropy from /dev/random. The default is 64.

write_wakeup_threshold

This file 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.

uuid and boot_id

These read-only files 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.