systemd can help in a number of ways with providing reliable, high quality random numbers from early boot on.
Today’s computer systems require random number generators for numerous cryptographic and other purposes. On Linux systems, the kernel’s entropy pool is typically used as high-quality source of random numbers. The kernel’s entropy pool combines various entropy inputs together, mixes them and provides an API to userspace as well as to internal kernel subsystems to retrieve it. This entropy pool needs to be initialized with a minimal level of entropy before it can provide high quality, cryptographic random numbers to applications. Until the entropy pool is fully initialized application requests for high-quality random numbers cannot be fulfilled.
The Linux kernel provides three relevant userspace APIs to request random data from the kernel’s entropy pool:
system call with its
flags parameter set to 0. If invoked the calling
program will synchronously block until the random pool is fully initialized
and the requested bytes can be provided.
getrandom() system call with its
flags parameter set to
GRND_NONBLOCK. If invoked the request for random bytes will fail if the
pool is not initialized yet.
Reading from the
pseudo-device will always return random bytes immediately, even if the pool
is not initialized. The provided random bytes will be of low quality in this
case however. Moreover the kernel will log about all programs using this
interface in this state, and which thus potentially rely on an uninitialized
(Strictly speaking there are more APIs, for example
/dev/random, but these
should not be used by almost any application and hence aren’t mentioned here.)
Note that the time it takes to initialize the random pool may differ between systems. If local hardware random number generators are available, initialization is likely quick, but particularly in embedded and virtualized environments available entropy is small and thus random pool initialization might take a long time (up to tens of minutes!).
Modern hardware tends to come with a number of hardware random number generators (hwrng), that may be used to relatively quickly fill up the entropy pool. Specifically:
All recent Intel and AMD CPUs provide the CPU opcode
RDRAND to acquire random bytes. Linux
includes random bytes generated this way in its entropy pool, but didn’t use
to credit entropy for it (i.e. data from this source wasn’t considered good
enough to consider the entropy pool properly filled even though it was
used). This has changed recently however, and most big distributions have
turned on the
CONFIG_RANDOM_TRUST_CPU=y kernel compile time option. This
means systems with CPUs supporting this opcode will be able to very quickly
reach the “pool filled” state.
The TPM security chip that is available on all modern desktop systems has a
hwrng. It is also fed into the entropy pool, but generally not credited
entropy. You may use
rng_core.default_quality=1000 on the kernel command
line to change that, but note that this is a global setting affect all
hwrngs. (Yeah, that’s weird.)
Many Intel and AMD chipsets have hwrng chips. Their Linux drivers usually
don’t credit entropy. (But there’s
Various embedded boards have hwrng chips. Some drivers automatically credit entropy, others do not. Some WiFi chips appear to have hwrng sources too, and they usually do not credit entropy for them.
virtio-rng is used in virtualized environments and retrieves random data
from the VM host. It credits full entropy.
The EFI firmware typically provides a RNG API. When transitioning from UEFI to kernel mode Linux will query some random data through it, and feed it into the pool, but not credit entropy to it. What kind of random source is behind the EFI RNG API is often not entirely clear, but it hopefully is some kind of hardware source.
If neither of these are available (in fact, even if they are), Linux generates entropy from various non-hwrng sources in various subsystems, all of which ultimately are rooted in IRQ noise, a very “slow” source of entropy, in particular in virtualized environments.
systemd’s Use of Random Numbers
systemd is responsible for bringing up the OS. It generally runs as the first userspace process the kernel invokes. Because of that it runs at a time where the entropy pool is typically not yet initialized, and thus requests to acquire random bytes will either be delayed, will fail or result in a noisy kernel log message (see above).
Various other components run during early boot that require random bytes. For example, initial RAM disks nowadays communicate with encrypted networks or access encrypted storage which might need random numbers. systemd itself requires random numbers as well, including for the following uses:
systemd assigns ‘invocation’ UUIDs to all services it invokes that uniquely
identify each invocation. This is useful retain a global handle on a specific
service invocation and relate it to other data. For example, log data
collected by the journal usually includes the invocation UUID and thus the
runtime context the service manager maintains can be neatly matched up with
the log data a specific service invocation generated. systemd also
/etc/machine-id with a randomized UUID. (systemd also makes use
of the randomized “boot id” the kernel exposes in
/proc/sys/kernel/random/boot_id). These UUIDs are exclusively Type 4 UUIDs,
i.e. randomly generated ones.
systemd maintains various hash tables internally. In order to harden them against collision attacks they are seeded with random numbers.
At various places systemd needs random bytes for temporary file name generation, UID allocation randomization, and similar.
systemd-resolved and systemd-networkd use random number generators to harden the protocols they implement against packet forgery.
systemd-udevd and systemd-nspawn can generate randomized MAC addresses for network devices.
Note that these cases generally do not require a cryptographic-grade random number generator, as most of these utilize random numbers to minimize risk of collision and not to generate secret key material. However, they usually do require “medium-grade” random data. For example: systemd’s hash-maps are reseeded if they grow beyond certain thresholds (and thus collisions are more likely). This means they are generally fine with low-quality (even constant) random numbers initially as long as they get better with time, so that collision attacks are eventually thwarted as better, non-guessable seeds are acquired.
systemd's Demand on the Kernel Entropy Pool Minimal
Since most of systemd’s own use of random numbers do not require
cryptographic-grade RNGs, it tries to avoid reading entropy from the kernel
entropy pool if possible. If it succeeds this has the benefit that there’s no
need to delay the early boot process until entropy is available, and noisy
kernel log messages about early reading from
/dev/urandom are avoided
When generating Type 4
systemd tries to use Intel’s and AMD’s RDRAND CPU opcode directly, if
available. While some doubt the quality and trustworthiness of the entropy
provided by these opcodes, they should be good enough for generating UUIDs,
if not key material (though, as mentioned, today’s big distributions opted
to trust it for that too, now, see above — but we are not going to make that
decision for you, and for anything key material related will only use the
kernel’s entropy pool). If RDRAND is not available or doesn’t work, it will
getrandom() as fallback, and
/dev/urandom on old kernels
where that system call doesn’t exist yet. This means on non-Intel/AMD
systems UUID generation will block on kernel entropy initialization.
For seeding hash tables, and all the other similar purposes systemd first
tries RDRAND, and if that’s not available will try to use asynchronous
getrandom() (if the kernel doesn’t support this system call,
/dev/urandom is used). This may fail too in case the pool is not
initialized yet, in which case it will fall back to glibc’s internal rand()
calls, i.e. weak pseudo-random numbers. This should make sure we use good
random bytes if we can, but neither delay boot nor trigger noisy kernel log
messages during early boot for these use-cases.
systemd’s Support for Filling the Kernel Entropy Pool
systemd has various provisions to ensure the kernel entropy is filled during boot, in order to ensure the entropy pool is filled up quickly.
When systemd’s PID 1 detects it runs in a virtualized environment providing
virtio-rng interface it will load the necessary kernel modules to make
use of it during earliest boot, if possible — much earlier than regular
kernel module loading done by
systemd-udevd.service. This should ensure
that in VM environments the entropy pool is quickly filled, even before
systemd invokes the first service process — as long as the VM environment
provides virtualized RNG hardware (and VM environments really should!).
system service will load a random seed from
into the kernel entropy pool. By default it does not credit entropy for it
though, since the seed is — more often than not — not reset when ‘golden’
master images of an OS are created, and thus replicated into every
installation. If OS image builders carefully reset the random seed file
before generating the image it should be safe to credit entropy, which can
be enabled by setting the
$SYSTEMD_RANDOM_SEED_CREDIT environment variable
for the service to
1 (or even
force, see man page). Note however, that
this service typically runs relatively late during early boot: long after
the initial RAM disk (
initrd) completed, and after the
/var/ file system
became writable. This is usually too late for many applications, it is hence
not advised to rely exclusively on this functionality to seed the kernel’s
entropy pool. Also note that this service synchronously waits until the
kernel’s entropy pool is initialized before completing start-up. It may thus
be used by other services as synchronization point to order against, if they
require an initialized entropy pool to operate correctly.
EFI boot loader included in systemd is able to maintain and provide a random
seed stored in the EFI System Partition (ESP) to the booted OS, which allows
booting up with a fully initialized entropy pool from earliest boot
on. During installation of the boot loader (or when invoking
a seed file with an initial seed is placed in a file
in the ESP. In addition, an identically sized randomized EFI variable called
the the ‘system token’ is set, which is written to the machine’s firmware
NVRAM. During boot, when
systemd-boot finds both the random seed file and
the system token they are combined and hashed with SHA256 (in counter mode,
to generate sufficient data), to generate a new random seed file to store in
the ESP as well as a random seed to pass to the OS kernel. The new random
seed file for the ESP is then written to the ESP, ensuring this is completed
before the OS is invoked. Very early during initialization PID 1 will read
the random seed provided in the EFI variable and credit it fully to the
kernel’s entropy pool.
This mechanism is able to safely provide an initialized entropy pool already
initrd and guarantees that different seeds are passed from the boot
loader to the OS on every boot (in a way that does not allow regeneration of
an old seed file from a new seed file). Moreover, when an OS image is
replicated between multiple images and the random seed is not reset, this
will still result in different random seeds being passed to the OS, as the
per-machine ‘system token’ is specific to the physical host, and not
included in OS disk images. If the ‘system token’ is properly initialized
and kept sufficiently secret it should not be possible to regenerate the
entropy pool of different machines, even if this seed is the only source of
Note that the writes to the ESP needed to maintain the random seed should be minimal. The size of the random seed file is directly derived from the Linux kernel’s entropy pool size, which defaults to 512 bytes. This means updating the random seed in the ESP should be doable safely with a single sector write (since hard-disk sectors typically happen to be 512 bytes long, too), which should be safe even with FAT file system drivers built into low-quality EFI firmwares.
As a special restriction: in virtualized environments PID 1 will refrain
from using this mechanism, for safety reasons. This is because on VM
environments the EFI variable space and the disk space is generally not
maintained physically separate (for example,
qemu in EFI mode stores the
variables in the ESP itself). The robustness towards sloppy OS image
generation is the main purpose of maintaining the ‘system token’ however,
and if the EFI variable storage is not kept physically separate from the OS
image there’s no point in it. That said, OS builders that know that they are
not going to replicate the built image on multiple systems may opt to turn
off the ‘system token’ concept by setting
random-seed-mode always in the
file. If done,
systemd-boot will use the random seed file even if no
system token is found in EFI variables.
With the three mechanisms described above it should be possible to provide early-boot entropy in most cases. Specifically:
On EFI systems,
systemd-boot’s random seed logic should make sure good
entropy is available during earliest boot — as long as
used as boot loader, and outside of virtualized environments.
On virtualized systems, the early
virtio-rng hookup should ensure entropy
is available early on — as long as the VM environment provides virtualized
RNG devices, which they really should all do in 2019. Complain to your
hosting provider if they don’t.
On Intel/AMD systems systemd’s own reliance on the kernel entropy pool is minimal (as RDRAND is used on those for UUID generation). This only works if the CPU has RDRAND of course, which most physical CPUs do (but I hear many virtualized CPUs do not. Pity.)
In all other cases,
systemd-random-seed.service will help a bit, but — as
mentioned — is too late to help with early boot.
This primarily leaves two kind of systems in the cold:
Some embedded systems. Many embedded chipsets have hwrng functionality these
days. Consider using them while crediting
rng_core.default_quality=1000 on the kernel command line is
your friend). Or accept that the system might take a bit longer to
boot. Alternatively, consider implementing a solution similar to
systemd-boot’s random seed concept in your platform’s boot loader.
Virtualized environments that lack both virtio-rng and RDRAND. Tough luck. Talk to your hosting provider, and ask them to fix this.
Also note: if you deploy an image without any random seed and/or without
installing any ‘system token’ in an EFI variable, as described above, this
means that on the first boot no seed can be passed to the OS
either. However, as the boot completes (with entropy acquired elsewhere),
systemd will automatically install both a random seed in the GPT and a
‘system token’ in the EFI variable space, so that any future boots will have
entropy from earliest boot on — all provided
systemd-boot is used.
Why don’t you just use getrandom()? That’s all you need!
Did you read any of the above? getrandom() is hooked to the kernel entropy pool, and during early boot it’s not going to be filled yet, very likely. We do use it in many cases, but not in all. Please read the above again!
Why don’t you use getentropy()? That’s all you need!
Same story. That call is just a different name for
flags set to zero, and some additional limitations, and thus it also needs
the kernel’s entropy pool to be initialized, which is the whole problem we
are trying to address here.
Why don’t you generate your UUIDs with
uuidd? That’s all you
First of all, that’s a system service, i.e. something that runs as “payload”
of systemd, long after systemd is already up and hence can’t provide us
UUIDs during earliest boot yet. Don’t forget: to assign the invocation UUID
uuidd.service start we already need a UUID that the service is
supposed to provide us. More importantly though,
uuidd needs state/a random
seed/a MAC address/host ID to operate, all of which are not available during
Why don’t you generate your UUIDs with
That’s all you need!
This is just a different, more limited interface to
/dev/urandom. It gains
uuidd above these are system services, hence come too late for our
use-case. In addition much of what
rngd provides appears to be equivalent
being more complex and involving userspace. These services partly measure
system behavior (such as scheduling effects) which the kernel either
already feeds into its pool anyway (and thus shouldn’t be fed into it a
second time, crediting entropy for it a second time) or is at least
something the kernel could much better do on its own. Hence, if what these
daemons do is still desirable today, this would be much better implemented
in kernel (which would be very welcome of course, but wouldn’t really help
us here in our specific problem, see above).
Why don’t you use
That’s all you need!
This doesn’t solve the issue, since it requires a nonce to start from, and
it gets that from
getrandom(), and thus we have to wait for random pool
initialization the same way as calling
arc4random() is nothing more than optimization, in fact it
implements similar algorithms that the kernel entropy pool implements
anyway, hence besides being able to provide random bytes with higher
throughput there’s little it gets us over just using
it’s not supported by glibc. And as long as that’s the case we are not keen
on using it, as we’d have to maintain that on our own, and we don’t want to
maintain our own cryptographic primitives if we don’t have to. Since
systemd’s uses are not performance relevant (besides the pool initialization
delay, which this doesn’t solve), there’s hence little benefit for us to
call these functions. That said, if glibc learns these APIs one day, we’ll
certainly make use of them where appropriate.
This is boring: NetBSD had boot loader entropy seed support since ages!
Yes, NetBSD has that, and the above is inspired by that (note though: this article is about a lot more than that). NetBSD’s support is not really safe, since it neither updates the random seed before using it, nor has any safeguards against replicating the same disk image with its random seed on multiple machines (which the ‘system token’ mentioned above is supposed to address). This means reuse of the same random seed by the boot loader is much more likely.
Why does PID 1 upload the boot loader provided random seed into kernel instead of kernel doing that on its own?
That’s a good question. Ideally the kernel would do that on its own, and we wouldn’t have to involve userspace in this.
What about non-EFI?
The boot loader random seed logic described above uses EFI variables to pass
the seed from the boot loader to the OS. Other systems might have similar
functionality though, and it shouldn’t be too hard to implement something
similar for them. Ideally, we’d have an official way to pass such a seed as
part of the
struct boot_params from the boot loader to the kernel, but
this is currently not available.
I use a different boot loader than
systemd-boot, I’d like to use boot
loader random seeds too!
Well, consider just switching to
systemd-boot, it’s worth it. See
for an introduction why. That said, any boot loader can re-implement the
logic described above, and can pass a random seed that systemd as PID 1
will then upload into the kernel’s entropy pool. For details see the Boot
Loader Interface documentation.
Why not pass the boot loader random seed via kernel command line instead of as EFI variable?
The kernel command line is accessible to unprivileged processes via
/proc/cmdline. It’s not desirable if unprivileged processes can use this
information to possibly gain too much information about the current state
of the kernel’s entropy pool.
systemd-boot rewrite the ‘system token’ too each time
when updating the random seed file stored in the ESP?
The system token is stored as persistent EFI variable, i.e. in some form of NVRAM. These memory chips tend be of low quality in many machines, and hence we shouldn’t write them too often. Writing them once during installation should generally be OK, but rewriting them on every single boot would probably wear the chip out too much, and we shouldn’t risk that.