If you've found and fixed a bug, we better write a test for it. nrk uses several test-frameworks and methodologies to ensure everything works as expected:

  • Regular unit tests: Those can be executed running cargo test in the project folder. Sometimes adding RUST_TEST_THREADS=1 is necessary due to the structure of the runner/frameworks used. This should be indicated in the individual READMEs.
  • A slightly more exhaustive variant of unit tests is property based testing. We use proptest to make sure that the implementation of kernel sub-systems corresponds to a reference model implementation.
  • Integration tests are found in the kernel, they typically launch a qemu instance and use rexpect to interact with the guest.
  • Fuzz testing: TBD.

Running tests

To run the unit tests of the kernel:

  1. cd kernel
  2. RUST_BACKTRACE=1 RUST_TEST_THREADS=1 cargo test --bin nrk

To run the integration tests of the kernel:

  1. cd kernel
  2. RUST_TEST_THREADS=1 cargo test --test integration-test

If you would like to run a specific integration test you can pass it with --:

  1. RUST_TEST_THREADS=1 cargo test --test integration-test -- userspace_smoke

In case an integration test fails, adding --nocapture at the end (needs to come after the --) will make sure that the underlying run.py invocations are printed to the stdout. This can be helpful to figure out the exact run.py invocation that a test is doing so you can invoke it yourself manually for debugging.

Parallel testing for he kernel is not possible at the moment due to reliance on build flags for testing.

Writing a unit-test for the kernel

Typically these can just be declared in the code using #[test]. Note that tests by default will run under the unix platform. A small hack is necessary to allow tests in the x86_64 to compile and run under unix too: When run on a x86-64 unix platform, the platform specific code of the kernel in arch/x86_64/ will be included as a module named x86_64_arch whereas normally it would be arch. This is a double-edged sword: we can now write tests that test the actual bare-metal code (great), but we can also easily crash the test process by calling an API that writes an MSR for example (e.g, things that would require ring 0 priviledge level).

Writing an integration test for the kernel

Integration tests typically spawns a QEMU instance and beforehand compiles the kernel/user-space with a custom set of Cargo feature flags. Then it parses the qemu output to see if it gave the expected output. Part of those custom compile flags will also choose a different main() function than the one you're seeing (which will go off to load and schedule user-space programs for example).

There is two parts to the integration test.

  • The host side (that will go off and spawn a qemu instance) for running the integration tests. It is found in kernel/tests/integration-test.rs.
  • The corresponding main functions in the kernel that gets executed for a particular example are located at kernel/src/integration_main.rs

To add a new integration test the following tests may be necessary:

  1. Modify kernel/Cargo.toml to add a feature (under [features]) for the test name.
  2. Optional: Add a new xmain function and test implementation in it to kernel/src/integration_main.rs with the used feature name as an annotation. It may also be possible to re-use an existing xmain function, in that case make not of the feature name used to include it.
  3. Add a runner function to kernel/tests/integration-test.rs that builds the kernel with the cargo feature runs it and checks the output.


nrk has support for three network interfaces at the moment: virtio, e1000 and vmxnet3. virtio and e1000 are available by using the respective rumpkernel drivers (and it's network stack). vmxnet3 is a standalone implementation that uses smoltcp for the network stack and is also capable of running in ring 0.


A simple check is to use ping (on the host) to test the network stack functionality and latency. Adaptive ping -A, flooding ping -f are good modes to see that the low-level parts of the stack work and can handle an "infinite" amount of packets.

Some expected output if it's working:

$ ping
64 bytes from icmp_seq=1 ttl=64 time=0.259 ms
64 bytes from icmp_seq=2 ttl=64 time=0.245 ms
64 bytes from icmp_seq=3 ttl=64 time=0.267 ms
64 bytes from icmp_seq=4 ttl=64 time=0.200 ms

For network tests, it's easiest to start a DHCP server for the tap interface so the VM receives an IP by communicating with the server:

# Stop apparmor from blocking a custom dhcp instance
service apparmor stop
# Terminate any (old) existing dhcp instance
sudo  killall dhcpd
# Spawn a dhcp server, in the kernel/ directory do:
sudo dhcpd -f -d tap0 --no-pid -cf ./tests/dhcpd.conf

A fully automated CI test that checks the network using ping is available as well, it can be invoked with the following command:

RUST_TEST_THREADS=1 cargo test --test integration-test -- s04_userspace_rumprt_net

socat and netcat

socat is a helpful utility on the host to interface with the network, for example to open a UDP port and print on incoming packets on the command line, the following command can be used:

socat UDP-LISTEN:8889,fork stdout

Similarly we can use netcat to connect to a port and send a payload:

nc 6337

The integration tests s05_redis_smoke and s04_userspace_rumprt_net make use of those tool to verify that networking is working as expected.


tcpdump is another handy tool to see all packets that are exchanged on a given interface etc. For debugging nrk network issues, this command is useful as it displays all packets on tap0:

tcpdump -i tap0 -vvv -XX