add notification for unexpected plugins (#4659)

scripts/plugins/alerts/security/unexpected_dhcp/user_scripts/flow/unexpected_dhcp.lua

@@ -5,18 +5,20 @@
 local user_scripts = require("user_scripts")
 local flow_consts = require("flow_consts")
 
+local UNEXPECTED_PLUGINS_ENABLED_CACHE_KEY = "ntopng.cache.user_scripts.unexpected_plugins_enabled"
+
 -- #################################################################
 
 local script = {
    -- Script category
-   category = user_scripts.script_categories.security, 
+   category = user_scripts.script_categories.security,
 
    -- Priority
    prio = -20, -- Lower priority (executed after) than default 0 priority
 
    -- This module is disabled by default
    default_enabled = false,
-   
+
    -- NOTE: hooks defined below
    hooks = {},
 
@@ -41,20 +43,30 @@ local script = {
 
 -- #################################################################
 
+function script.onEnable(hook, hook_config)
+   -- Set a flag to indicate to the notifications system that an unexpected plugin
+   -- has been enabled
+   if isEmptyString(ntop.getCache(UNEXPECTED_PLUGINS_ENABLED_CACHE_KEY)) then
+      ntop.setCache(UNEXPECTED_PLUGINS_ENABLED_CACHE_KEY, "1")
+   end
+end
+
+-- #################################################################
+
 function script.hooks.protocolDetected(now, conf)
    if flow.isServerUnicast() then
       if(table.len(conf.items) > 0) then
          local ok = 0
          local flow_info = flow.getInfo()
          local server_ip = flow_info["srv.ip"]
-         
+
          for _, dns_ip in pairs(conf.items) do
             if server_ip == dns_ip then
                ok = 1
                break
             end
          end
-         
+
          if ok == 0 then
             flow.triggerStatus(
                flow_consts.status_types.status_unexpected_dhcp.create(