# The Anykernel and Rump Kernels
A driver abstracts an underlying entity. For example, a TCP/IP driver
abstracts the details required to perform networking, the Fast File
System (FFS) driver abstracts how the file system blocks are laid out on
the storage medium, and a PCI network card driver abstracts how to
access device registers to send and receive packets. The kernel
architecture controls how kernel drivers run with respect to other
system components. Some examples of kernel architectures are the
monolithic kernel, microkernel and exokernel. In contrast to the above
architectures, NetBSD is an *anykernel*. This means that some kernel
drivers can be run according to any kernel architecture instead of being
limited to a single one.
When a driver is not run as part of the monolithic kernel, e.g. when it
is run as a microkernel style server, the driver is hosted in a *rump
kernel*. A rump kernel is an ultralightweight virtualized kernel running
on top of high-level hypercall interface. Instead of a low-level
hypercall API typically seen with operating systems with operations such
as "modify page table", the rump kernel hypercall API provides
high-level operations such as "run this code in a thread".
Currently, three implementations of the rump kernel hypercall interface
- The POSIX implementation is included in the NetBSD tree and makes
rump kernels to run as userspace processes on most operating systems
such as NetBSD, Linux and Solaris.
- The Xen implementation allows running rump kernels directly as Xen
DomU's without an intermediate operating system.
- The Linux kernel hypervisor allows rump kernels to run inside the
Rump kernels are radically different from OS virtualization technologies
such as KVM, containers and usermode operating systems. A rump kernel
does not support hosting application processes because a rump kernel is
aimed at virtualizing kernel drivers and application virtualization
would be pure overhead. Instead, existing entities such as processes
from a hosting OS are used as clients for the rump kernel ("application"
in the figure).
As a result of the above design choices, rump kernels are extremely
lightweight. The bootstrap time for rump kernels on POSIX hosts is
measured in milliseconds and memory footprint in 100kB's. This means
that a rump kernel can be bootstrapped for example as part of a command
line tool for virtually no cost or user impact. Rump kernels also
mandate very little from the hypercall implementation meaning that rump
kernels, and by extension NetBSD kernel drivers, can be hosted in
virtually any environment.
Use cases for rump kernels include:
- **Code reuse**: kernel drivers can be reused without having to run a
whole OS. For example, a full-featured TCP/IP stack (IPv6, IPSec,
etc.) can be included in an embedded appliance without having to
write the stack from scratch or waste resources on running an entire
- **Kernel driver virtualization**: every rump kernel has its own
state. Furthermore, the functionality offered by multiple rump
kernels running on the same host does not need to be equal. For
example, multiple different networking stacks optimized for
different purposes are possible.
- **Security**: when hosted on a POSIX system, a rump kernel runs in
its own instance of a userspace process. For example, it is widely
published that file system drivers are vulnerable to untrusted file
system images. Unlike on other general purpose operating systems, on
NetBSD it is possible to mount untrusted file systems, such as those
on a USB stick, in an isolated server with the kernel file system
driver. This isolates attacks and prevents kernel compromises while
not requiring to maintain separate userspace implementations of the
file system drivers or use other resource-intensive approaches such
as virtual machines.
- **Easy prototyping and development**: kernel code can be developed
as a normal userspace application. Once development is finished, the
code can simply be complied into the kernel. This is a much more
convenient and straightforward approach to kernel development than
the use of virtual machines.
- **Safe testing**: kernel code can be tested in userspace on any host
without risk of the test host being affected. Again, virtual
machines are not required.
## Further Reading
The following is the definitive guide to the anykernel and rump kernels
and supercedes all earlier publications and terminology on the subject.
- [Flexible Operating System Internals: The Design and Implementation
of the Anykernel and Rump
### Software using rump kernels
These links are interesting for people who want to use rump kernels in
addition to reading about them.
- [Scripts for building rump kernels for POSIX
- [Rump kernel hypercall implementation for Xen; rump kernels as Xen
- [fs-utils: File system image access
- Fast userspace packet processing: TCP/IP stack for use with
### Articles, Tutorials & Howtos
- [Running rump kernels and applications on Xen without a full
- [PCI device driver support in rump kernels on
- [Experiment with a rump kernel hypervisor for the Linux
(allows rump kernels to run *in* the Linux kernel)
- [Kernel Servers using
- [Tutorial On Rump Kernel Servers and
- [Revolutionizing Kernel Development: Testing With
### Conference publications and talks
- "The Anykernel and Rump Kernels" gives a general overview and
demonstrates rump kernels on Windows and in Firefox. The
are available. Presented at FOSDEM 2013 (Operating Systems track).
- "Rump Device Drivers: Shine On You Kernel Diamond" describes device
driver and USB. The
and [video presentation](http://www.youtube.com/watch?v=3AJNxa33pzk)
are available. Presented at AsiaBSDCon 2010.
- "Fs-utils: File Systems Access Tools for Userland" describes
fs-utils, an mtools-like utility kit which uses rump kernel file
systems as a backend. The
is available. Presented at EuroBSDCon 2009.
- "Rump File Systems: Kernel Code Reborn" describes kernel file system
code and its uses in userspace. The
are available. Presented at the 2009 USENIX Annual Technical
- "Kernel Development in Userspace - The Rump Approach" describes
doing kernel development with rump kernels. The
are available. Presented at BSDCan 2009.
- "Environmental Independence: BSD Kernel TCP/IP in Userspace"
describes networking in rump kernels. The
[video presentation](http://www.youtube.com/watch?v=RxFctq8A0WI) are
available. Presented at AsiaBSDCon 2009.
### Manual pages
The manpages provide the usual type of information. Start from
follow the cross-references in "SEE ALSO".
Any topic related to rump kernels can be discussed on the
Alternatively, you can use a NetBSD mailing which is related to a
The IRC channel for rump kernels is **\#rumpkernel** on
The anykernel and rump kernels were first introduced as a prototype in
NetBSD 5.0. A stable version with numerous new features and improvements
was shipped with NetBSD 6.0.
## Source Code
All of the source code is available from the NetBSD source tree and can
be obtained with the usual methods.
You can also [browse](http://cvsweb.NetBSD.org/bsdweb.cgi/src/) the
source code history online. Code is found from all areas of the source
tree. Some examples of where to look include
CVSweb for NetBSD wikisrc <wikimaster@NetBSD.org> software: FreeBSD-CVSweb