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Tutorial: Getting Started

Hassan Nadeem edited this page Oct 30, 2017 · 28 revisions

This tutorial is meant as an up-to-date guide for people coming across rump kernels for the first time, and wanting to understand them and/or to use them. There are two parts. First we list a few items of suggested reading. The second, and bulk, part of this tutorial is about playing around with the code. People interested only in playing around with the code can skip the reading.

Notably, this tutorial will only cover basic rump kernel matters. If you are interested in the Rumprun unikernel, it is not absolutely necessary to follow this tutorial (although you will gain a lot of applicable knowledge). If you want to skip straight to Rumprun, see the links at the bottom of this tutorial.

If you find this tutorial lacking in any way, please suggest improvements on the rump kernel mailing list (see the Community page for more info on the list).


The most succinct description of the architecture, motivation and history, including some use cases, is the ;login: magazine article "Rump Kernels: No OS? No Problem!". You can find a free copy here.

For more in-depth knowledge, reading The Design and Implementation of the Anykernel and Rump Kernels is recommended. Notably, read or skim chapters 2 and 3.

After reading those, you should have a solid understanding of what and why a rump kernel is. If you still want to study more, check out the Articles and Publications pages on this wiki.

Hands on!

We will go over the practical part in the comfort of userspace. "Why userspace?", you ask. First, it is the most convenient platform to do experimentation in. Second, most concepts map more or less directly to embedded and cloud platforms, so it is even beneficial to get a hang of debugging and building and testing your projects in userspace before potentially moving them to the final target platform or platforms.

The following should work at least on more or less any Linux system and NetBSD systems.


First, we do a full build of the package. This will give us two things we need for the rest of the tutorial:

  • rump kernel components (i.e. the building blocks of rump kernels)
  • hypercall implementation for POSIX'y userspace (allows rump kernels to run in userspace)

A kernel alone is slightly boring. We will also need the rumpctrl package, which will give us the ability to interact with rump kernel instances:

  • a selection of userland utilities, mostly configuration-oriented (think ifconfig etc.)
  • a transport mechanism for the userland utilities to communicate with remote rump kernels

Notably, the stage is necessary only if you want to run rump kernels in userspace, but rumpctrl is useful no matter which platform you run your production rump kernels on (Xen, KVM, ...).

In more concrete terms, for

git clone

And for rumpctrl:

git clone
cd rumpctrl
git submodule update --init

(Notably, the above process is slightly wasteful in terms of bandwidth usage and will clone the src-netbsd repository twice. Advanced users can clone the repository manually and use the -s argument to the build scripts to specify the location. However, we opted for simplicity in this tutorial.)

Trying it out

So we built things. Now what? Let's assume you've changed back to the top level directory (i.e. cd .. after the last build command). Look into the ./rumpctrl/bin directory. You will see a bunch of potentially familiar userland utilities, e.g. ls, sysctl and ifconfig. What happens if we try to run one?

$ ./rumpctrl/bin/ifconfig 
error: RUMP_SERVER not set
rumpclient init failed

Fail happens. Why? On a regular system, each userland utility communicates implicitly with the host kernel. For example, when you run ls, the utility makes the appropriate system calls to the host kernel. However, there is no implicit rump kernel. That is the reason why we must explicitly tell the rumpctrl utilities which rump kernel we wish for it to communicate with. That information is conveyed by an URL in the RUMP_SERVER environment variable. To know the right URL to set, we must first start a rump kernel server.

If you followed the above build process, you will have a rump kernel server available under ./ As a mandatory argument, the server takes an URL which indicates from where the server listens to for requests. The easiest way is to use local domain sockets, which are identified by unix:// and the remainder of the argument is the pathname. Let's try it out:

$ ./ unix:///tmp/rumpctrlsock

Ok, um, where did it go? By default, a rump_server will background itself, and that is what happened here. You can check that the server is running with ps. You can even kill it with kill. However, we will show a more civilized command for the kill in a while.

You can set $RUMP_SERVER manually to the URL you specified when you ran the server. There is also a bash script, when if sourced, sets the prompt so that the value of $RUMP_SERVER is visible. That will help keep track of which rump kernel server the commands are being sent to. The script also sets $PATH so that the bin directory containing the applications build by rumpctrl is first. Let's give it a spin.

$ . rumpctrl/
rumpctrl (NULL)$ export RUMP_SERVER=unix:///tmp/rumpctrlsock
rumpctrl (unix:///tmp/rumpctrlsock)$ sysctl -w kern.hostname=look_at_them_run
kern.hostname: rump-12224.watou -> look_at_them_run
rumpctrl (unix:///tmp/rumpctrlsock)$ sysctl kern.hostname
kern.hostname = look_at_them_run

Sweet! Let's use ls to see what files the rump kernel is hosting.

rumpctrl (unix:///tmp/rumpctrlsock)$ ls -l
ls: .: Function not implemented

Sour! What's going on? Rump kernels are component-oriented, which means that we must indicate which components we want the rump kernel to support. When we ran rump_server earlier, we did not specify that it should support file systems. No file systems, no ls. So, let's halt the rump kernel and try again.

rumpctrl (unix:///tmp/rumpctrlsock)$ halt
rumpctrl (unix:///tmp/rumpctrlsock)$ ./ -lrumpvfs unix:///tmp/rumpctrlsock
rumpctrl (unix:///tmp/rumpctrlsock)$ ls -l
total 2
drwxr-xr-x  2 0  0  512 Jul 28 23:10 dev
drwxrwxrwt  2 0  0  512 Jul 28 23:10 tmp
rumpctrl (unix:///tmp/rumpctrlsock)$ mount
rumpfs on / type rumpfs (local)

Better. We'll get to how you're supposed to know about -lrumpvfs later in this tutorial.

Now, can we mount tmpfs inside the rump kernel server?

rumpctrl (unix:///tmp/rumpctrlsock)$ mount_tmpfs /swap /tmp
mount_tmpfs: tmpfs on /tmp: Operation not supported by device

Bitter! Looks like you'll have to keep reading this tutorial.

rumpctrl extra tricks

Assuming you have sourced, you can list all of the available commands with rumpctrl_listcmds. Another way would be to list the contents of the bin directory. You can test for an individual command with rumpctrl_hascmd (which may give peace of mind if you e.g. want to execute rm -rf /). Finally, you can "shell out" with rumpctrl_hostcmd. Let's try these out.

rumpctrl (unix:///tmp/rumpctrlsock)$ rumpctrl_listcmds 
arp		ed		mknod		newfs_msdos	route
rumpctrl (unix:///tmp/rumpctrlsock)$ rumpctrl_hascmd ls
rumpctrl (unix:///tmp/rumpctrlsock)$ rumpctrl_hascmd top
rumpctrl (unix:///tmp/rumpctrlsock)$ ls -l /
total 2
drwxr-xr-x  2 0  0  512 Jul 28 23:10 dev
drwxrwxrwt  2 0  0  512 Jul 28 23:10 tmp
rumpctrl (unix:///tmp/rumpctrlsock)$ rumpctrl_hostcmd ls -l /
total 110
drwxr-xr-x   2 root root  4096 Jul 23 23:01 bin

In case you are wondering if we should have used rumpctrl_hostcmd to run rump_server when we restarted it, you are on the right track. We did not do that because rumpctrl_hostcmd had not been introduced yet and because we are sure that there is no rump_server rumpctrl command (well, at least I am sure).

Note: shell redirects or cd will not work with rumpctrl. The behaviour expected -- the shell is running in host userspace, not guest userspace -- but needs to be taken into account.

rumpctrl (unix:///tmp/rumpctrlsock)$ cd /dev
ERROR: cd not available in rumpctrl mode /dev
rumpctrl (unix:///tmp/rumpctrlsock)$ echo messages from earth > /file
bash: /file: Permission denied
rumpctrl (unix:///tmp/rumpctrlsock)$ ls -l /file
ls: /file: No such file or directory
rumpctrl (unix:///tmp/rumpctrlsock)$ echo messages from earth | dd of=/file
0+1 records in
0+1 records out
20 bytes transferred in 0.001 secs (20000 bytes/sec)
rumpctrl (unix:///tmp/rumpctrlsock)$ ls -l /file
-rw-r--r--  1 0  0  20 Jul 29 04:24 /file

Coping with components

Let's assume we want to run some driver in a rump kernel. That driver needs all of its dependencies to work. How to figure out what the dependencies are? For the impatient, there is rump_allserver, which simply loads all components that were available when rump_allserver was built. However, it is better to get into the habit of surgically selecting only the necessary components. This will keep footprint of the rump kernel to a minimum and also allow you to include drivers which were built and installed separate from rump_allserver. We can use the tool rump_wmd (Where's My Dependency) to resolve dependencies. For example, let's assume we want a rump kernel to support the FFS file system driver.

rumpctrl (unix:///tmp/rumpctrlsock)$ rumpctrl_hostcmd ./ -L./ -lrumpfs_ffs
DEBUG0: Searching component combinations. This may take a while ...
DEBUG0: Found a set
-lrumpdev -lrumpdev_disk -lrumpvfs -lrumpfs_ffs

Now, we could start a rump kernel server with FFS, but we'll actually start one which supports tmpfs, to not have to magically come up with an FFS image to mount. Before we can start another server, we have to halt the previously running one. Note, you can halt a server only once, as demonstrated below.

rumpctrl (unix:///tmp/rumpctrlsock)$ halt
rumpctrl (unix:///tmp/rumpctrlsock)$ halt
rumpclient init failed
rumpctrl (unix:///tmp/rumpctrlsock)$ rumpctrl_hostcmd ./ -lrumpfs_tmpfs -lrumpvfs unix:///tmp/rumpctrlsock
rumpctrl (unix:///tmp/rumpctrlsock)$ mount_tmpfs /swap /tmp
rumpctrl (unix:///tmp/rumpctrlsock)$ mount
rumpfs on / type rumpfs (local)
tmpfs on /tmp type tmpfs (local)

If you want, as an exercise create an ISO image and use a rump_server and the rumpfs_cd9660 driver to access the contents. You will need to use the -d argument to rump_server to map the image to the rump kernel namespace. Check out the rump server manpage if you need more information about the -d parameter.

Building applications for the Rumprun unikernel

The process of building applications for Rumprun is detailed in other Tutorials:


Try out at least some of the commands listed by rumpctrl_listcmds. Some of them require rather specially configured rump kernels (e.g. wpa_supplicant), and those are out of the scope of this tutorial. However, you should for example be able to configure a rump server up to a point where ping works.

The manual pages for all of the commands are available at

When you are satisfied with your prowess, move on to for example the kernel development tutorial, which discusses running rump kernels under the debugger.

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