* Installs ``ferm`` (for iptables management), configuring a basic firewall

  which allows ICMP echo requests (PING), incoming connection on TCP port 22

  (SSH), and also introduces rate-limitting for incoming ICMP echo request

  pacakges and (new) TCP connections. The rate-limitting is based on the source

  IP address, using the ``iptables hashlimit`` module.

Role dependencies

~~~~~~~~~~~~~~~~~

Depends on the following roles:

* **backup_client**

Backups

~~~~~~~

If the backup for this role has been enabled, the following paths are backed-up:

**/var/log**

  Log files from the system.

**/home**

**/root**

**/etc/shadow**

  Operating system user passwords.

**/var/mail**

  Local user's mails.

**/var/spool/cron**

  Local user's cronjobs.

Parameters

~~~~~~~~~~

**apt_proxy** (string, optional, ``None``)

  URI of a caching proxy that should be used when retrieving the packages via

  apt.

**os_users** (list, optional, ``[]``)

  A list of operating system users that should be set-up on a server. Each item

  is a dictionary with the following options describing the user parameters:

  **name** (string, mandatory)

    Name of the operating system user that should be created. User's default

  **password** (string, optional, ``!`` - no password)

    Encrypted password that should be set for the user.

**os_groups** (list, optional, ``[]``)

  A list of operating system groups that should be set-up on a server. Each item

  is a dictionary with the following options describing the group parameters:

  **name** (string, mandatory)

    Name of the operating system group that should be created.

  **gid** (number, optional, ``whatever OS picks``)

    GID for the operating system group.

**common_packages** (list, optional, ``[]``)

  List of additional operating system packages that should be installed on the

  server. Each element of the list should be a simple string denoting the name

  of the package.

**ca_certificates** (list, optional, ``{}``)

  Dictionary containing the CA certificates to deploy. Keys are base filenames

  (**without extension**) to be used when placing a certificate file in

  directory ``/usr/local/share/ca-certificates/``, while values are

  corresponding content to be placed in the file.

**incoming_connection_limit** (string, optional, ``3/second``)

  Rate at which the incoming ICMP echo-request packages and new TCP connections

  will be accepted at. The value should be specified in the same format as value

  for the ``iptables hashlimit`` option ``--hashlimit-upto``.

**incoming_connection_limit_burst** (string, optional, ``9``)

  Initial burst of packages that should be accepted when the client with

  distinct source IP address connects to the server for the first time (usually

  higher than ``incoming_connection_limit``), even if it would go above the

  specified connection limit.

**prompt_colour** (string, optional, ``none``)

  Colour for showing the Bash prompt. Supported values are:

  ``black``, ``red``, ``green``, ``brown``, ``blue``, ``purple``, ``cyan``,

  ``light_gray``, ``dark_gray``, ``light_red``, ``light_green``, ``yellow``,

  ``light_blue``, ``light_purple``, ``light_cyan``, ``white``, ``none``.

  You should probably *not* use the ``black`` colour. Setting affects Bash

  shells *only*. Setting the value to ``none`` uses default terminal colour.

**prompt_id** (string, optional, ``NONE``)

  Optional identifier appended to regular Bash prompt, useful for visually

  identifying distinct environments. For example, if set to ``test``, resulting