[Docs] Firewall guide

Information about:
- What we do about firewalling (i.e.: NOTHING)
- How could deployer know what to do (i.e.: RTfancyManual)
- How is the haproxy configuration generated in case someone
wants to use a similar things for its firewalling.

Change-Id: Ied5c1baa54f257639537942fabcc2f367c1651d8

doc/source/draft-operations-guide/maintenance-tasks/firewalls.rst

@@ -2,13 +2,54 @@
 Firewalls
 =========
 
-This is a draft backups page for the proposed OpenStack-Ansible
-operations guide.
 
-.. TODO Describe general approaches to adding firewalls to OSA infrastructure.
+OpenStack-Ansible does not configure firewalling for its
+infrastructure. It is up to the deployer to define the perimeter
+and its firewalling configuration.
 
-Finding ports used by an external IP address
-~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+By default, OpenStack-Ansible relies on Ansible SSH connections,
+and needs the TCP port 22 to be opened on all hosts
+internally.
+
+For more information on generic OpenStack firewalling, see the
+`OpenStack default port documentation <https://docs.openstack.org/newton/config-reference/firewalls-default-ports.html>`_
+
+You can find in each of the role's respective documentatione, the
+default variables for the ports used within the scope of the role.
+Reviewing the documentation allow you to find the variable names
+if you want to use a different port.
+
+.. note:: OpenStack-Ansible's group vars conveniently expose the vars outside of the
+   `role scope <https://github.com/openstack/openstack-ansible/blob/master/playbooks/inventory/group_vars/all.yml>`_
+   in case you are relying on the OpenStack-Ansible groups to
+   configure your firewall.
+
+Finding ports for your external load balancer
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+As explained in the previous section, you can find (in each role
+documentation) the default variables used for the public
+interface endpoint ports.
+
+For example, the
+`os_glance documentation <https://docs.openstack.org/developer/openstack-ansible-os_glance/#default-variables>`_
+lists the variable ``glance_service_publicuri``. This contains
+the port used for the reaching the service externally. In
+this example, it is equal to to ``glance_service_port``, whose
+value is 9292.
+
+As a hint, you could find the whole list of public URI defaults
+by executing the following:
+
+.. code::
+
+   cd /etc/ansible/roles
+   grep -R -e publicuri -e port *
+
+.. note::
+
+   `Haproxy <https://github.com/openstack/openstack-ansible/blob/master/playbooks/vars/configs/haproxy_config.yml>`_
+   can be configured with OpenStack-Ansible.
+   The automatically generated ``/etc/haproxy/haproxy.cfg`` file have
+   enough information on the ports to open for your environment.
 
-.. TODO  explain how to find the ports used by the external IP
-   (whether you deploy haproxy or not), and what are the default ports