Implements user script for ip reassociation alerts

Addresses #4614

scripts/plugins/alerts/network/ip_reassignment/locales/en.lua

@@ -0,0 +1,8 @@
+--
+-- (C) 2020 - ntop.org
+--
+
+return {
+   description = "Trigger alerts when an IP address, previously seen with a MAC address, is now seen with another MAC address. This alert might indicate an ARP spoof attempt. Only works for the builtin alert recipient.", -- 
+   title = "IP Reassignment",
+}

scripts/plugins/alerts/network/ip_reassignment/manifest.lua

@@ -0,0 +1,10 @@
+--
+-- (C) 2019-20 - ntop.org
+--
+
+return {
+  title = "Remote to Remote",
+  description = "Detects remote to remote flows and triggers alerts",
+  author = "ntop",
+  dependencies = {},
+}

scripts/plugins/alerts/network/ip_reassignment/user_scripts/system/ip_reassignment.lua

@@ -0,0 +1,69 @@
+--
+-- (C) 2019-20 - ntop.org
+--
+
+local flow_consts = require("flow_consts")
+local user_scripts = require("user_scripts")
+
+-- #################################################################
+
+local IP_REASSIGNMENT_KEY = "ntopng.prefs.ip_reassignment_alerts"
+
+-- #################################################################
+
+local function dummy()
+   -- Nothing to do here, the plugin is only meant to set a preference which is then
+   -- read from C.
+   return
+end
+
+-- #################################################################
+
+local script = {
+   -- Script category
+   category = user_scripts.script_categories.network,
+
+   -- Off by default
+   default_enabled = false,
+
+   -- NOTE: hooks defined below
+   hooks = {
+      min = dummy
+   },
+
+   gui = {
+      i18n_title = "ip_reassignment.title",
+      i18n_description = "ip_reassignment.description",
+   }
+}
+
+-- #################################################################
+
+function script.onLoad(hook, hook_config)
+   if hook_config and hook_config.enabled then
+      ntop.setPref(IP_REASSIGNMENT_KEY, "1")
+   end
+
+end
+
+-- #################################################################
+
+function script.onUnload(hook, hook_config)
+   ntop.delCache(IP_REASSIGNMENT_KEY)
+end
+
+-- #################################################################
+
+function script.onEnable(hook, hook_config)
+   ntop.setPref(IP_REASSIGNMENT_KEY, "1")
+end
+
+-- #################################################################
+
+function script.onDisable(hook, hook_config)
+   ntop.delCache(IP_REASSIGNMENT_KEY)
+end
+
+-- #################################################################
+
+return script