Document DDoS mitigation (#3090)

docs/admin/deployment.rst

@@ -72,6 +72,10 @@ you create:
    Set to 'gitlab' if you want to use 'GitLab' (corresponding GITLAB_* settings must be set if required).
    Set to 'google' if you want to use 'Google' (corresponding GOOGLE_* settings must be set).
 
+``BLOCKED_IPS``
+   A comma-separated list of IP addresses to be blocked from accessing the app,
+   for example because they are DDoS'ing the server.
+
 ``CELERY_ALWAYS_EAGER``
    Controls whether asynchronous tasks (mainly used during sync) are sent to
    Celery or executed immediately and synchronously. Set this to ``False`` on

docs/admin/maintenance.rst

@@ -40,3 +40,16 @@ Finally, you need to simply access the worker:
 
    # Replace my-app-name with your Heroku app's name.
    celery --broker=`heroku config:get RABBITMQ_URL --app=my-app-name` worker
+
+Mitigating DDoS attacks
+-----------------------
+In a distributed denial-of-service attack (`DDoS`_ attack), the incoming traffic
+flooding the victim originates from many different sources. This stops everyone
+else from accessing the website as there is too much traffic flowing to it.
+
+One way to mitigate DDoS attacks is to identify the IP addresses of the
+attackers and block them. Find the attacking IP addresses in the Log
+Management Add-On (Papertrail) and add them to the BLOCKED_IPs config variable
+in Heroku Settings.
+
+.. _DDoS: https://en.wikipedia.org/wiki/Denial-of-service_attack

pontoon/settings/base.py

@@ -267,6 +267,7 @@ def _default_from_email():
     "django_ace",
 )
 
+# A list of IP addresses to be blocked from accessing the app, because they are DDoS'ing the server
 BLOCKED_IPS = os.environ.get("BLOCKED_IPS", "").split(",")
 
 MIDDLEWARE = (