nixpkgs/nixos/modules/security/rngd.nix
nicoo e64d3f60fb nixos/modules/security/rngd: Disable by default
`rngd` seems to be the root cause for slow boot issues, and its functionality is
redundant since kernel v3.17 (2014), which introduced a `krngd` task (in kernel
space) that takes care of pulling in data from hardware RNGs:

> commit be4000bc4644d027c519b6361f5ae3bbfc52c347
> Author: Torsten Duwe <duwe@lst.de>
> Date:   Sat Jun 14 23:46:03 2014 -0400
>
>     hwrng: create filler thread
>
>     This can be viewed as the in-kernel equivalent of hwrngd;
>     like FUSE it is a good thing to have a mechanism in user land,
>     but for some reasons (simplicity, secrecy, integrity, speed)
>     it may be better to have it in kernel space.
>
>     This patch creates a thread once a hwrng registers, and uses
>     the previously established add_hwgenerator_randomness() to feed
>     its data to the input pool as long as needed. A derating factor
>     is used to bias the entropy estimation and to disable this
>     mechanism entirely when set to zero.

Closes: #96067
2020-09-09 21:51:25 -04:00

57 lines
1.5 KiB
Nix

{ config, lib, pkgs, ... }:
with lib;
let
cfg = config.security.rngd;
in
{
options = {
security.rngd = {
enable = mkOption {
type = types.bool;
default = false;
description = ''
Whether to enable the rng daemon. Devices that the kernel recognises
as entropy sources are handled automatically by krngd.
'';
};
debug = mkOption {
type = types.bool;
default = false;
description = "Whether to enable debug output (-d).";
};
};
};
config = mkIf cfg.enable {
systemd.services.rngd = {
bindsTo = [ "dev-random.device" ];
after = [ "dev-random.device" ];
# Clean shutdown without DefaultDependencies
conflicts = [ "shutdown.target" ];
before = [
"sysinit.target"
"shutdown.target"
];
description = "Hardware RNG Entropy Gatherer Daemon";
# rngd may have to start early to avoid entropy starvation during boot with encrypted swap
unitConfig.DefaultDependencies = false;
serviceConfig = {
ExecStart = "${pkgs.rng-tools}/sbin/rngd -f"
+ optionalString cfg.debug " -d";
# PrivateTmp would introduce a circular dependency if /tmp is on tmpfs and swap is encrypted,
# thus depending on rngd before swap, while swap depends on rngd to avoid entropy starvation.
NoNewPrivileges = true;
PrivateNetwork = true;
ProtectSystem = "full";
ProtectHome = true;
};
};
};
}