Updated documentation

docs/markdown/interfaces.md

@@ -212,6 +212,51 @@ specify the target Yggdrasil IPv6 address and port, like so:
     target_port = 4343
 ```
 
+### Automated Blocking
+
+Listener instances of `BackboneInterface` will automatically block fast-flapping clients,
+that repeatedly connect for a short amount of time, and then disconnect. Such behavior usually
+occurs from clients trying to spam the network with announces or path requests, by connecting, dumping
+a massive amount of requests, and then disconnecting in an attempt to bypass rate limits.
+
+While such bypass attempts only have very limited effect on the amount of spam actually dumped (ingress limits
+trigger immediately once a client exceeds rate limits), the behavior is often seen anyways, and
+causes log noise and needless interface rotation. The fast-flapping block ensures such clients
+are never allocated an interface on the transport instance.
+
+Another common cause can simply be clients using implementations that are broken, or automated
+scanning tools attempting to connect to the instance.
+
+```ini
+[[Backbone Listener]]
+  type = BackboneInterface
+  enabled = yes
+  listen_on = 0.0.0.0
+  port = 4242
+
+  # Whether to enable blocking
+  block_fast_flapping = yes
+
+  # How long an IP address stays
+  # blocked, in minutes. Set to
+  # 12 hours by default.
+  fast_flapping_block_time = 720
+
+  # The minimum time, in seconds,
+  # an interface must stay connected
+  # to be considered not fast-flapping.
+  fast_flapping_threshold = 20
+
+  # Amount of fast flaps a remote
+  # IP can perform before having
+  # blocking triggered.
+  fast_flapping_grace = 5
+```
+
+The configuration options listed in the example above are the *default values*, and you do
+not need to add them for automated blocking to work, but you can use them to change the
+behavior from the defaults, if necessary.
+
 ## TCP Server Interface
 
 The TCP Server interface is suitable for allowing other peers to connect over