<section  xmlns="http://docbook.org/ns/docbook"
          xmlns:xlink="http://www.w3.org/1999/xlink"
          xmlns:xi="http://www.w3.org/2001/XInclude"
          version="5.0"
          xml:id="sec-declarative-containers">
 <title>Declarative Container Specification</title>

 <para>
  You can also specify containers and their configuration in the host’s
  <filename>configuration.nix</filename>. For example, the following specifies
  that there shall be a container named <literal>database</literal> running
  PostgreSQL:
<programlisting>
containers.database =
  { config =
      { config, pkgs, ... }:
      { <xref linkend="opt-services.postgresql.enable"/> = true;
      <xref linkend="opt-services.postgresql.package"/> = pkgs.postgresql_9_6;
      };
  };
</programlisting>
  If you run <literal>nixos-rebuild switch</literal>, 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 <literal>containers.database.autoStart = true</literal> in its
  configuration.
 </para>

 <para>
  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:
<programlisting>
containers.database = {
  <link linkend="opt-containers._name_.privateNetwork">privateNetwork</link> = true;
  <link linkend="opt-containers._name_.hostAddress">hostAddress</link> = "192.168.100.10";
  <link linkend="opt-containers._name_.localAddress">localAddress</link> = "192.168.100.11";
};
</programlisting>
  This gives the container a private virtual Ethernet interface with IP address
  <literal>192.168.100.11</literal>, which is hooked up to a virtual Ethernet
  interface on the host with IP address <literal>192.168.100.10</literal>. (See
  the next section for details on container networking.)
 </para>

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

 <para>
  Declarative containers can be started and stopped using the corresponding
  systemd service, e.g. <literal>systemctl start container@database</literal>.
 </para>
</section>
