X-Git-Url: http://gitweb.michael.orlitzky.com/?a=blobdiff_plain;f=src%2FMain.hs;h=ab5f93ac4cdf8a40125ac34ba6b014e91bc60df1;hb=HEAD;hp=1cc4c73aa47d963e56f30a7188f05781f010b76c;hpb=2e8aab2a69a8b9887e4f037115b80a2063d3a6b7;p=email-validator.git

diff --git a/src/Main.hs b/src/Main.hs
index 1cc4c73..ab5f93a 100644
--- a/src/Main.hs
+++ b/src/Main.hs
@@ -41,28 +41,63 @@ import EmailAddress(
 resolv_conf ::  ResolvConf
 resolv_conf = defaultResolvConf { resolvTimeout = 10 * 1000 * 1000 }
 
--- | A list of common domains, there's no need to waste MX lookups
---   on these.
+-- | A list of common domains, there's no need to waste MX lookups on
+--   these. This is a very limited list; I don't want to be in the
+--   business of monitoring a million domains for MX record updates.
 common_domains :: [Domain]
 common_domains = map BS.pack [ "aol.com",
                                "comcast.net",
+                               "cox.net",
                                "gmail.com",
+                               "gmx.de",
+                               "googlemail.com",
+                               "hotmail.com",
+                               "icloud.com",
+                               "live.com",
+                               "me.com",
                                "msn.com",
+                               "outlook.com",
+                               "proton.me",
+                               "protonmail.ch",
+                               "protonmail.com",
                                "yahoo.com",
                                "verizon.net" ]
 
 
 -- | Check whether the given domain has a valid MX record.
-validate_mx :: Resolver -> Domain -> IO Bool
+--
+--   NULLMX (RFC7505) records consisting of a single period must not
+--   be accepted. Moreover, the existence of a NULLMX must be reported
+--   back to the caller because the whole point of a NULLMX is that
+--   its existence should preempt an @A@ record check. We abuse the
+--   return type for this, and return @Nothing@ in the event of a
+--   NULLMX. Otherwise we return @Just True@ or @Just False@ to
+--   indicate the existence (or not) of MX records.
+--
+--   RFC7505 states that a domain MUST NOT have any other MX records
+--   if it has a NULLMX record. We enforce this. If you have a NULLMX
+--   record and some other MX record, we consider the set invalid.
+--
+validate_mx :: Resolver -> Domain -> IO (Maybe Bool)
 validate_mx resolver domain
-  | domain `elem` common_domains = return True
+  | domain `elem` common_domains = return $ Just True
   | otherwise = do
       result <- lookupMX resolver domain
       case result of
-        -- A list of one or more elements?
-        Right (_:_) -> return True
-        _           -> return False
-
+        Left _ ->
+            return $ Just False
+        Right mxs ->
+            case mxs of
+              [] -> return $ Just False
+              _  -> if any (is_null) mxs
+                   then return Nothing
+                   else return $ Just True
+  where
+    nullmx :: Domain
+    nullmx = BS.pack "."
+
+    is_null :: (Domain,Int) -> Bool
+    is_null (mx,prio) = mx == nullmx && prio == 0
 
 -- | Check whether the given domain has a valid A record.
 validate_a :: Resolver -> Domain -> IO Bool
@@ -76,26 +111,33 @@ validate_a resolver domain
 
 
 -- | Validate an email address by doing some simple syntax checks and
---   (if those fail) an MX lookup. We don't count an A record as a mail
---   exchanger.
+--   (if those fail) an MX lookup. We don't count an @A@ record as a mail
+--   exchanger unless @accept_a@ is True. And even then, the existence
+--   of a NULLMX record will preclude the acceptance of an @A@ record.
+--   The domain @example.com@ is a great test case for this behavior.
 validate :: Resolver -> Bool -> Bool -> Address -> IO (Address, Bool)
 validate resolver accept_a rfc5322 address = do
   let valid_syntax = validate_syntax rfc5322 address
   if valid_syntax then do
     let (_,domain) = parts address
     mx_result <- validate_mx resolver domain
-    if mx_result
-    then return (address, True)
-    else
-      if accept_a
-      then do
-        a_result <- validate_a resolver domain
-        return (address, a_result)
-      else
-        return (address, False)
-  else
-    return (address, False)
-
+    case mx_result of
+      Nothing  ->
+          -- NULLMX, don't fall back to 'A' records under any
+          -- circumstances.
+          return (address, False)
+      Just mxr ->
+          if mxr
+          then return (address, True)
+          else
+            if accept_a
+            then do
+              a_result <- validate_a resolver domain
+              return (address, a_result)
+            else
+              return (address, False)
+          else
+            return (address, False)
 
 
 main :: IO ()