NixOS Containers

Setup native systemd-nspawn containers, which are running NixOS and are configured and managed by NixOS using the containers directive.

See Docker page for OCI container (Docker, Podman) configuration.

Configuration

The following example creates a container called webserver running a httpd web server. It will start automatically at boot and has its private network subnet.

❄︎ /etc/nixos/configuration.nix

networking.nat = {
  enable = true;
  internalInterfaces = ["ve-+"];
  externalInterface = "ens3";
  # Lazy IPv6 connectivity for the container
  enableIPv6 = true;
};

containers.webserver = {
  autoStart = true;
  privateNetwork = true;
  hostAddress = "192.168.100.10";
  localAddress = "192.168.100.11";
  hostAddress6 = "fc00::1";
  localAddress6 = "fc00::2";
  config = { config, pkgs, lib, ... }: {

    services.httpd = {
      enable = true;
      adminAddr = "admin@example.org";
    };

    networking = {
      firewall.allowedTCPPorts = [ 80 ];

      # Use systemd-resolved inside the container
      # Workaround for bug https://github.com/NixOS/nixpkgs/issues/162686
      useHostResolvConf = lib.mkForce false;
    };
    
    services.resolved.enable = true;

    system.stateVersion = "24.11";
  };
};

In order to reach the web application on the host system, we have to open Firewall port 80 and also configure NAT through networking.nat. The web service of the container will be available at http://192.168.100.11

Networking

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By default, if privateNetwork is not set, the container shares the network with the host, enabling it to bind any port on any interface. However, when privateNetwork is set to true, the container gains its private virtual eth0 and ve-<container_name> on the host. This isolation is beneficial when you want the container to have its dedicated networking stack.

NAT (Network Address Translation)

Bridge

networking = {
  bridges.br0.interfaces = [ "eth0s31f6" ]; # Adjust interface accordingly
  
  # Get bridge-ip with DHCP
  useDHCP = false;
  interfaces."br0".useDHCP = true;

  # Set bridge-ip static
  interfaces."br0".ipv4.addresses = [{
    address = "192.168.100.3";
    prefixLength = 24;
  }];
  defaultGateway = "192.168.100.1";
  nameservers = [ "192.168.100.1" ];
};

containers.<name> = {
  privateNetwork = true;
  hostBridge = "br0"; # Specify the bridge name
  localAddress = "192.168.100.5/24";
  config = { };
};

Usage

List containers

# machinectl list

Checking the status of the container

# systemctl status container@webserver

Login into the container

# nixos-container root-login webserver

Start or stop a container

# nixos-container start webserver
# nixos-container stop webserver

Destroy a container including its file system

# nixos-container destroy webserver

View log for container

# journalctl -M webserver

Further informations are available in the NixOS Manual, NixOS manual.

Tips and tricks

Define and create nixos-container from a Flake file

We can define and create a custom container called container from a file stored as flake.nix. In this case we use the unstable branch of the nixpkgs repository as a source.

{
  inputs.nixpkgs.url = "nixpkgs/nixos-unstable";

  outputs = { self, nixpkgs }: {

    nixosConfigurations.container = nixpkgs.lib.nixosSystem {
      system = "x86_64-linux";
      modules =
        [ ({ pkgs, ... }: {
            boot.isContainer = true;

            networking.firewall.allowedTCPPorts = [ 80 ];

            services.httpd = {
              enable = true;
              adminAddr = "morty@example.org";
            };
          })
        ];
    };

  };
}

To create and run that container, enter following commands. In this example the flake.nix file is in the same directory.

# nixos-container create flake-test --flake .
host IP is 10.233.4.1, container IP is 10.233.4.2

# nixos-container start flake-test

Use agenix secrets in container

To add agenix secrets to a container bind mount the ssh-host.key and import the agenix.nixosModule and set age.identityPaths Source

{ agenix, ... }:
{

  containers."withSecret" = {

    # pass the private key to the container for agenix to decrypt the secret
    bindMounts."/etc/ssh/ssh_host_ed25519_key".isReadOnly = true;

    config =
      {
        config,
        lib,
        pkgs,
        ...
      }:
      {
        imports = [ agenix.nixosModules.default ]; # import agenix-module into the nixos-container

        age.identityPaths = [ "/etc/ssh/ssh_host_ed25519_key" ]; # isn't set automatically when openssh is not setup
        # import the secret
        age.secrets."secret-name" = {
          file = ../secrets/secret.age;
        };
      };
  };
}

Troubleshooting

I have changed the host's channel and some services are no longer functional

Symptoms:

• Lost data in PostgreSQL database
• MySQL has changed its path, where it creates the database

Solution

If you did not have a system.stateVersion option set inside your declarative container configuration, it will use the default one for the channel. Your data might be safe, if you did nothing meanwhile. Add the missing system.stateVersion to your container, rebuild, and possibly stop/start the container.

See also

• NixOS Manual, Chapter on Container Management
• Blog Article - Declarative NixOS Containers
• NixOS Discourse - Extra-container: Run declarative containers without full system rebuilds
• Nixpkgs - nixos-container.pl
• Nixpkgs - nixos-containers.nix
• nixos-nspawn
• tfc/nspawn-nixos
• MicroVMs as a more isolated alternative, e.g. with https://github.com/astro/microvm.nix