Implement alerts via email

Closes #272

scripts/lua/modules/email_utils.lua

@@ -0,0 +1,44 @@
+--
+-- (C) 2018 - ntop.org
+--
+
+local email = {}
+
+local function getEmailMessage(now_ts, from, to, subject, body)
+  local now = os.date("%a, %d %b %Y %X", now_ts) -- E.g. "Tue, 3 Apr 2018 14:58:00"
+  local msg_id = "<" .. now_ts .. "." .. os.clock() .. "@ntopng>"
+
+  local lines = {
+    "From: " .. from,
+    "To: " .. to,
+    "Subject: " .. subject,
+    "Date: " ..  now,
+    "Message-ID: " .. msg_id,
+  }
+
+  return table.concat(lines, "\r\n") .. "\r\n\r\n" .. body .. "\r\n"
+end
+
+function email.sendNotification(notif)
+  if notif.action == "release" then
+    -- Avoid sending release notifications
+    return
+  end
+
+  local email_address = ntop.getPref("ntopng.prefs.alerts.email_address")
+  local smtp_server = ntop.getPref("ntopng.prefs.alerts.smtp_server")
+
+  if isEmptyString(email_address) or isEmptyString(smtp_server) then
+    return false
+  end
+
+  local product = ntop.getInfo(false).product
+  local subject = product .. " [" .. string.upper(notif.severity) .. "]: " .. alertTypeLabel(alertType(notif.type), true)
+  local message_body = unescapeHtmlEntities(noHtml(notif.message))
+  local from_to = email_address
+  local message = getEmailMessage(os.time(), from_to, from_to, subject, message_body)
+
+  return ntop.sendMail(from_to, from_to, message, smtp_server)
+end
+
+return email