Development Host

The devhost role is a flexible way to run multiple development deployments using slim VMs on a single powerful, physical machine on our public infrastructure.


Why not run development deployments on my local machine?

  • Your resources may be too limited for larger projects: RAM, CPU, Disk

  • Multi-machine deployments require extra effort to avoid conflicting resources.

  • External access for colleagues and customers is not easily possible.

Why not run development deployments with a desktop virtualization tool like VirtualBox and Vagrant?

  • Your resources may be even more limited with the overhead of running additional VMs.

  • External access for colleagues and customers is also not possible.

  • Quality of Vagrant boxes proved too hard to ensure properly for a long time - our dev VM-based approach has a much higher coverage with automated testing and a smaller chance to result in unexpected non-working conditions.

  • Using x86-based VMs is not possible on platforms like Apple M-Series.

Why not use persistent development environments using virtual machines in the Flying Circus platform?

  • Resource usage can grow quickly with the number of developers and become increasingly prohibitive.

  • Less flexible and higher turnaround times to set up new environments: every environment requires proper configuration of public DNS and similar resources which only has to happen once for a development host.

What are the benefits of using the devhost role?

  • A single large host (or virtual machine) can provide sufficient power to run parallel tests quickly and exceed RAM usage limits of local machines.

  • Integration with batou supports “single button” deployment options for immediate personal environments without maintaining multiple environment definitions per developer.

  • Allow colleagues to access the same development VMs to assist debugging.

  • Proper pinning of platform releases to ensure repeatability.

  • Fewer dependencies on the work stations – no need to install Virtualbox, Vagrant and additional plugins.

  • Dynamically and quickly create new temporary environments for customer validation.

  • Allow customers and colleagues to access preview environments with automatic Let’s Encrypt integration and instantaneous access using DNS wildcards from any domain.

  • Automatic clean up of unused environments.


  1. Create a new resource group to grant/limit access for your developers. Developers need the login permission to interact with the devhost.

  2. You need a physical machine with sufficient resources (CPU, RAM, SSD) and select the devhost role.

  3. The machine needs to be assigned a public IP address (v4 and v6 are supported).

  4. Choose a public domain name (e.g. and register it as a CNAME for the VM’s public interface, i.e. set CNAME

  5. Add a wildcard DNS for all subdomains within the chosen public domain name, i.e. set * CNAME

  1. Add the publicAddress option to the NixOS configuration on the host, e.g. in /etc/local/nixos/devhost.nix and run fc-manage -b:

    { ... }:
      flyingcircus.roles.devhost.publicAddress = "";

Configuring batou deployments

batou (starting from 2.4) supports a specific provisioning mode that supports defining environments based on VMs organized by the devhost role. A development environment (e.g. named dev) would typically look like this:

$ mkdir -p environments/dev
$ cat >> environments/dev/environment.cfg <<__EOF__
service_user = s-dev
platform = nixos
update_method = rsync

method = fc-nixos-dev-vm
# The release has the format fcrs://channel/release
# release can be empty, than the latest release for the channel is used.
release = fcrs://fc-23.11-production/2024_001
host =

channel =

provision-dynamic-hostname = True
provision-aliases =
components =

$ cat >> environments/dev/provision.nix <<__EOF__
{ lib, pkgs, ... }:
  flyingcircus.roles.webgateway.enable = true;
  flyingcircus.roles.redis.enable = true;
  flyingcircus.roles.percona80.enable = true;
  flyingcircus.roles.postgresql12.enable = true;

$ cat >> environments/dev/ <<__EOF__
COPY ../../../sourcecode /srv/s-dev/

Then, to deploy to your VM simply run:

$ ./batou deploy dev

This will create, start and configure the VM as necessary.

If you want to rebuild your VM from scratch, you can run:

$ ./batou deploy --provision-rebuild dev

The URLs for channels can be looked up in our changelog: each version is listed with a link to the appropriate channel. Only platform releases starting from 23.11 are supported for development VMs, though!

Using the provision-dynamic-hostname switch will result in development VMs receiving a random hostname based on your local batou checkout. This is the core feature that allows using the same environment (e.g. dev) for multiple developers independently. If you leave this off then the VM name will be exactly what is written in the environment.

Using the provision-aliases will create virtual hosts on the dev server that become available as <alias>.<vm> and are protected with Let’s Encrypt certificates automatically. They are intended to pass through access to the UI of your applications and act similar to port forwards for port 443 -> 443. You should use self-signed certificates within the vms. (batou_ext.ssl.Certifiate already allows switching between custom )

As the development VMs are not managed by our inventory you need to place relevant information about roles in a Nix expression file. You can then use a provisioning script to customize the VMs during provisioning. fc-manage will be called automatically for you. In the provision script you can use COPY to copy local files (relative to the environment directory) to the VMs (relative to the root), use RUN to run commands in the VMs (as root) or ECHO to output a local comand (and access environment variables carrying secrets) into a remote file.

Connecting to the VM(s)

batou automatically maintains a number of ssh_config files as well as a specific insecure key pair for each environment so you can easily access the VM via SSH.

For example, to access the VM myvm in the dev environment you can simply run:

$ ssh -F ssh_config_dev myvm

The environment works similar to our regular platform: the user login (developer) represents a human user and the services are installed in the service user (s-dev).

Writing provision scripts

For early changes to the target dev VM that aren’t part of the deployment but are expected by the deployment to be prepared by “the environment” you can write a provision script for which a number of special functions.

COPY sample.txt /tmp/
RUN mkdir /tmp/some/directory

The script will execute on the machine where you started batou and can interact with the VM through the following features:

COPY <local path> <remote path>

Copy a local file to a destination in the VM.

The local path is relative to the environment’s directory (where is placed). The remote path must be absolute.

RUN cmd arg1 arg2

Execute a command as root in the VM.


Using redirections like > will not work here.

ECHO <expression> <remote path>

Execute an expression locally and store its output in a remote path.

This can be used to evaluate a variable from the environment locally and store its result in the VM.

Sometimes it may be necessary to seed data from the environment (like secrets) early to the provisioner in order to set predictable/repeatable passwords for system services. We therefore provide a number of variables to the provision script:


All overrides and secrets for all components in the environment.


The name of the VM being provisioned.


The name of the devhost that the VM is being provisioned onto.


The NixOS channel URL being used.


The list of aliases.


The path to the locally generated SSH config file.


Provision scripts should be kept extremely small. The bulk of the deployment should be handled using batou proper.


batou continues deployment under certain conditions after an error during provisioning. This is explicitly shown and annotated with a corresponding warning. In some situations a partially failed deployment may have created an environment that is broken but needs the deployment to run to be fixed automatically.

Syncing development code into the VM

Often you will be editing code using your local editor or IDE and need it to be synced to the target VM quickly without much repeated effort.

We recommend to integrate this using one or more rsync commands of this form:

rsync -avz --delete --exclude=.git --rsh='ssh -F ${BATOUDIR}/ssh_config_dev' --rsync-path='sudo -u s-dev rsync' ${SOURCE}/ myvm:/srv/s-dev/${TARGET}

You can then use this command either with an on-save hook in your editor or by using a tool that responds to changes in your filesystem (like )

In the future there will be optimized support for this behaviour in batou.

To sync code that is currently being developed on (and assuming you are using an editor / IDE on your local machine)


To manually delete a VM you can use the build script’s destroy action:

$ fc-devhost destroy $vmname

To list all VMs running on a devhost with their users and creation date:

$ fc-devhost ls -l

All other commands can be seen with:

$ fc-devhost --help

If a VM hangs up, it’s possible to see their log in /var/lib/devhost/$vmname/log

Known issues

  • Devhost VMs currently does no (properly) support IPv6 so deployments need to disable IPv6 resolution for internal and public services.