nixpkgs/nixos/doc/manual/development/running-nixos-tests-interactively.section.md

Running Tests interactively

The test itself can be run interactively. This is particularly useful when developing or debugging a test:

$ nix-build . -A nixosTests.login.driverInteractive
$ ./result/bin/nixos-test-driver
[...]
>>>

::: {.note} By executing the test driver in this way, the VMs executed may gain network & Internet access via their backdoor control interface, typically recognized as eth0. :::

You can then take any Python statement, e.g.

>>> start_all()
>>> test_script()
>>> machine.succeed("touch /tmp/foo")
>>> print(machine.succeed("pwd")) # Show stdout of command

The function test_script executes the entire test script and drops you back into the test driver command line upon its completion. This allows you to inspect the state of the VMs after the test (e.g. to debug the test script).

Shell access in interactive mode

The function <yourmachine>.shell_interact() grants access to a shell running inside a virtual machine. To use it, replace <yourmachine> with the name of a virtual machine defined in the test, for example: machine.shell_interact(). Keep in mind that this shell may not display everything correctly as it is running within an interactive Python REPL, and logging output from the virtual machine may overwrite input and output from the guest shell:

>>> machine.shell_interact()
machine: Terminal is ready (there is no initial prompt):
$ hostname
machine

As an alternative, you can proxy the guest shell to a local TCP server by first starting a TCP server in a terminal using the command:

$ socat 'READLINE,PROMPT=$ ' tcp-listen:4444,reuseaddr

In the terminal where the test driver is running, connect to this server by using:

>>> machine.shell_interact("tcp:127.0.0.1:4444")

Once the connection is established, you can enter commands in the socat terminal where socat is running.

SSH Access for test machines

An SSH-based backdoor to log into machines can be enabled with

{
  name = "…";
  nodes.machines = { /* … */ };
  sshBackdoor.enable = true;
}

This creates a vsock socket for each VM to log in with SSH. This configures root login with an empty password.

When the VMs get started interactively with the test-driver, it's possible to connect to machine with

$ ssh vsock/3 -o User=root

The socket numbers correspond to the node number of the test VM, but start at three instead of one because that's the lowest possible vsock number. The exact SSH commands are also printed out when starting nixos-test-driver.

On non-NixOS systems you'll probably need to enable the SSH config from {manpage}systemd-ssh-proxy(1) yourself.

If starting VM fails with an error like

qemu-system-x86_64: -device vhost-vsock-pci,guest-cid=3: vhost-vsock: unable to set guest cid: Address already in use

it means that the vsock numbers for the VMs are already in use. This can happen if another interactive test with SSH backdoor enabled is running on the machine.

In that case, you need to assign another range of vsock numbers. You can pick another offset with

{
  sshBackdoor = {
    enable = true;
    vsockOffset = 23542;
  };
}

Port forwarding to NixOS test VMs

If your test has only a single VM, you may use e.g.

$ QEMU_NET_OPTS="hostfwd=tcp:127.0.0.1:2222-:22" ./result/bin/nixos-test-driver

to port-forward a port in the VM (here 22) to the host machine (here port 2222).

This naturally does not work when multiple machines are involved, since a single port on the host cannot forward to multiple VMs.

If the test defines multiple machines, you may opt to temporarily set virtualisation.forwardPorts in the test definition for debugging.

Such port forwardings connect via the VM's virtual network interface. Thus they cannot connect to ports that are only bound to the VM's loopback interface (127.0.0.1), and the VM's NixOS firewall must be configured to allow these connections.

Reuse VM state

You can re-use the VM states coming from a previous run by setting the --keep-vm-state flag.

$ ./result/bin/nixos-test-driver --keep-vm-state

The machine state is stored in the $TMPDIR/vm-state-machinename directory.

Interactive-only test configuration

The .driverInteractive attribute combines the regular test configuration with definitions from the interactive submodule. This gives you a more usable, graphical, but slightly different configuration.

You can add your own interactive-only test configuration by adding extra configuration to the interactive submodule.

To interactively run only the regular configuration, build the <test>.driver attribute instead, and call it with the flag result/bin/nixos-test-driver --interactive.