<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, ... }:
      { services.postgresql.enable = true;
        services.postgresql.package = pkgs.postgresql92;
      };
  };
</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 =
  { privateNetwork = true;
    hostAddress = "192.168.100.10";
    localAddress = "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>
