SeedRNG — seedrng(8)

by Jason A. Donenfeld

SeedRNG is a simple program made for seeding the Linux kernel random number generator from seed files. The program takes no arguments, must be run as root, and always attempts to do something useful.

This program is useful in light of the fact that the Linux kernel RNG cannot be initialized from shell scripts, and new seeds cannot be safely generated from boot time shell scripts either.

It should be run once at init time and once at shutdown time. It can be run at other times without detriment as well. Whenever it is run, it writes existing seed files into the RNG pool, and then creates a new seed file. If the RNG is initialized at the time of creating a new seed file, then that new seed file is marked as "creditable", which means it can be used to initialize the RNG. Otherwise, it is marked as "non-creditable", in which case it is still used to seed the RNG's pool, but will not initialize the RNG.

In order to ensure that entropy only ever stays the same or increases from one seed file to the next, old seed values are hashed together with new seed values when writing new seed files:

new_seed = new_seed[:-32] || HASH(fixed_prefix || real_time || boot_time || old_seed_len || old_seed || new_seed_len || new_seed)

The seed is stored in LOCALSTATEDIR/seedrng/, which can be adjusted at compile time. If the SEEDRNG_SKIP_CREDIT environment variable is set to 1, true, yes, or y, then seeds never credit the RNG, even if the seed file is creditable.

Being a single C file, seedrng.c, SeedRNG is meant to be copy and pasted verbatim into various minimal init system projects and tweaked as needed. Please do not package this repo as a standalone program: it is intended as utility code meant to be imported into existing projects instead.

Building & Installing

$ make
$ sudo make install

In addition to the usual compiler environment variables (CFLAGS, etc), the following environment variable is respected during compilation:

• LOCALSTATEDIR default: /var/lib

The following environment variables are respected during installation:

• PREFIX default: /usr
• DESTDIR default:
• SBINDIR default: $(PREFIX)/sbin

Usage

# seedrng

However, this invocation should generally come from init and shutdown scripts.

License

This program is licensed under any one of the the following licenses, so that it can be incorporated into other software projects as needed:

- GPL-2.0
- Apache-2.0
- MIT
- BSD-1-Clause
- CC0-1.0