X-Git-Url: http://gitweb.michael.orlitzky.com/?p=email-validator.git;a=blobdiff_plain;f=src%2FMain.hs;h=1cc4c73aa47d963e56f30a7188f05781f010b76c;hp=c220aaadc186d1be01d2432c7b22694323ee756d;hb=HEAD;hpb=62e8d2df29b616854b97eb703dcddc796e2c88ca

diff --git a/src/Main.hs b/src/Main.hs
index c220aaa..ab5f93a 100644
--- a/src/Main.hs
+++ b/src/Main.hs
@@ -65,16 +65,39 @@ common_domains = map BS.pack [ "aol.com",
 
 
 -- | 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
@@ -88,26 +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 ()