added makeAddressMatcher

Would be nice to add CIDR notation to this, but this is the minimal
thing needed for the security fix.

This commit was sponsored by Ewen McNeill on Patreon.

Utility/IPAddress.hs

@@ -69,3 +69,25 @@ embeddedIpv4 v = case v of
   where
 	toipv4 a b = htonl $ fromIntegral a * (2^halfipv4bits) + fromIntegral b
 	halfipv4bits = 16 :: Word32
+
+{- Given a string containing an IP address, make a function that will
+ - match that address in a SockAddr. Nothing when the address cannot be
+ - parsed.
+ -
+ - This does not involve any DNS lookups.
+ -}
+makeAddressMatcher :: String -> IO (Maybe (SockAddr -> Bool))
+makeAddressMatcher s = go
+	<$> catchDefaultIO [] (getAddrInfo (Just hints) (Just s) Nothing)
+  where
+	hints = defaultHints
+		{ addrSocketType = Stream
+		, addrFlags = [AI_NUMERICHOST]
+		}
+	
+	go [] = Nothing
+	go l = Just $ \sockaddr -> any (match sockaddr) (map addrAddress l)
+
+	match (SockAddrInet _ a) (SockAddrInet _ b) = a == b
+	match (SockAddrInet6 _ _ a _) (SockAddrInet6 _ _ b _) = a == b
+	match _ _ = False