# Declarative Container Specification

You can also specify containers and their configuration in the host’s `configuration.nix`. For example, the following specifies that there shall be a container named `database` running PostgreSQL:

    containers.database =
      { config =
          { config, pkgs, ... }:
          {  = true;
           = pkgs.postgresql_9_6;
          };
      };

If you run `nixos-rebuild switch`, the container will be built. If the container was already running, it will be updated in place, without rebooting. The container can be configured to start automatically by setting `containers.database.autoStart = true` in its configuration.

By default, declarative containers share the network namespace of the host, meaning that they can listen on (privileged) ports. However, they cannot change the network configuration. You can give a container its own network as follows:

    containers.database = {
      privateNetwork = true;
      hostAddress = "192.168.100.10";
      localAddress = "192.168.100.11";
    };

This gives the container a private virtual Ethernet interface with IP address `192.168.100.11`, which is hooked up to a virtual Ethernet interface on the host with IP address `192.168.100.10`. (See the next section for details on container networking.)

To disable the container, just remove it from `configuration.nix` and run `nixos-rebuild switch`. Note that this will not delete the root directory of the container in `/var/lib/containers`. Containers can be destroyed using the imperative method: `nixos-container destroy foo`.

Declarative containers can be started and stopped using the corresponding systemd service, e.g. `systemctl start container@database`.