From e8ff5a36c776c6808aacaa6ba488fe5fd19db497 Mon Sep 17 00:00:00 2001
From: Michael Orlitzky <michael@orlitzky.com>
Date: Wed, 8 Jul 2015 23:21:07 -0400
Subject: [PATCH 1/1] Add the no-equal-neighbors restriction for labels.

---
 doc/TODO      |   1 -
 src/Domain.hs | 114 +++++++++++++++++++++++++++++++++++++++++++++++---
 2 files changed, 108 insertions(+), 7 deletions(-)
 delete mode 100644 doc/TODO

diff --git a/doc/TODO b/doc/TODO
deleted file mode 100644
index 0e56c5f..0000000
--- a/doc/TODO
+++ /dev/null
@@ -1 +0,0 @@
-1. Add no-equal-brothers restriction for subdomains (RFC1034).
diff --git a/src/Domain.hs b/src/Domain.hs
index f19b111..46c9306 100644
--- a/src/Domain.hs
+++ b/src/Domain.hs
@@ -74,7 +74,11 @@ letter = fmap Letter Parsec.letter
 --     manner...
 --
 --   Since each part of DNS name is composed of our custom types, it
---   suffices to munge the equality for 'Letter'.
+--   suffices to munge the equality for 'Letter'. RFC4343
+--   <https://tools.ietf.org/html/rfc4343> clarifies the
+--   case-insensitivity rules, but the fact that we're treating DNS
+--   names as strings makes most of those problems go away (in
+--   exchange for new ones).
 --
 instance Eq Letter where
   (Letter l1) == (Letter l2) = (toLower l1) == (toLower l2)
@@ -384,6 +388,14 @@ length_ldh_str_let_dig (LdhStrLetDig (Just ldhstring) _) =
 --
 --     <label> ::= <letter> [ [ <ldh-str> ] <let-dig> ]
 --
+--   We allow the slightly more general syntax from RFC1123, Section 2.1:
+--
+--     The syntax of a legal Internet host name was specified in RFC-952
+--     [DNS:4].  One aspect of host name syntax is hereby changed: the
+--     restriction on the first character is relaxed to allow either a
+--     letter or a digit.  Host software MUST support this more liberal
+--     syntax.
+--
 data Label = Label Letter (Maybe LdhStrLetDig)
   deriving (Eq, Show)
 
@@ -468,6 +480,8 @@ data Subdomain =
   SubdomainMultipleLabel Label Subdomain
   deriving (Eq, Show)
 
+
+
 instance Pretty Subdomain where
   pretty_show (SubdomainSingleLabel l) = pretty_show l
   pretty_show (SubdomainMultipleLabel l s) =
@@ -486,6 +500,18 @@ instance Pretty Subdomain where
 --   from left to right, so we pick off the leading label and then
 --   recurse into the definition of subdomain.
 --
+--   According to RFC1034, Section 3.1, two neighboring labels in a
+--   DNS name cannot be equal:
+--
+--     Each node has a label, which is zero to 63 octets in length.  Brother
+--     nodes may not have the same label, although the same label can be used
+--     for nodes which are not brothers.  One label is reserved, and that is
+--     the null (i.e., zero length) label used for the root.
+--
+--   We enforce this restriction, but the result is usually that we
+--   only parse the part of the subdomain leading up to the repeated
+--   label.
+--
 --   ==== _Examples_
 --
 --   >>> import Text.Parsec ( parseTest )
@@ -501,6 +527,17 @@ instance Pretty Subdomain where
 --   >>> pretty_print $ parse subdomain "" "www.example.com"
 --   www.example.com
 --
+--   We reject a subdomain with equal neighbors, but this leads to
+--   only the single first label being parsed instead:
+--
+--   >>> pretty_print $ parse subdomain "" "www.www.example.com"
+--   www
+--
+--   But not one with a repeated but non-neighboring label:
+--
+--   >>> pretty_print $ parse subdomain "" "www.example.www.com"
+--   www.example.www.com
+--
 subdomain :: Parser Subdomain
 subdomain = try both <|> just_one
   where
@@ -509,13 +546,73 @@ subdomain = try both <|> just_one
       l <- label
       char '.'
       s <- subdomain
-      return (SubdomainMultipleLabel l s)
+      let result = SubdomainMultipleLabel l s
+      if (subdomain_has_equal_neighbors result)
+      then fail "subdomain cannot have equal neighboring labels"
+      else return result
 
     just_one :: Parser Subdomain
     just_one = fmap SubdomainSingleLabel label
 
 
 
+-- | Retrieve a list of labels contained in a 'Subdomain'.
+--
+--   ==== _Examples_
+--
+--   >>> let (Right r) = parse subdomain "" "a"
+--   >>> pretty_print $ subdomain_labels r
+--   ["a"]
+--
+--   >>> let (Right r) = parse subdomain "" "example.com"
+--   >>> pretty_print $ subdomain_labels r
+--   ["example","com"]
+--
+--   >>> let (Right r) = parse subdomain "" "www.example.com"
+--   >>> pretty_print $ subdomain_labels r
+--   ["www","example","com"]
+--
+subdomain_labels :: Subdomain -> [Label]
+subdomain_labels (SubdomainSingleLabel l) = [l]
+subdomain_labels (SubdomainMultipleLabel l s) = l : (subdomain_labels s)
+
+
+-- | Return a list of pairs of neighboring labels in a subdomain.
+--
+--   ==== _Examples_
+--
+--   >>> let (Right r) = parse subdomain "" "www.example.com"
+--   >>> pretty_print $ subdomain_label_neighbors r
+--   ["(\"www\",\"example\")","(\"example\",\"com\")"]
+--
+subdomain_label_neighbors :: Subdomain -> [(Label,Label)]
+subdomain_label_neighbors s =
+  zip ls (tail ls)
+  where
+    ls = subdomain_labels s
+
+
+-- | Return @True@ if the subdomain has any two equal neighboring
+--   labels, and @False@ otherwise.
+--
+--   ==== _Examples_
+--
+--   >>> let (Right r) = parse subdomain "" "www.example.com"
+--   >>> subdomain_has_equal_neighbors r
+--   False
+--
+--   >>> let (Right l) = parse label "" "www"
+--   >>> let (Right s) = parse subdomain "" "www.example.com"
+--   >>> let bad_subdomain = SubdomainMultipleLabel l s
+--   >>> subdomain_has_equal_neighbors bad_subdomain
+--   True
+--
+subdomain_has_equal_neighbors :: Subdomain -> Bool
+subdomain_has_equal_neighbors s =
+  or [ x == y | (x,y) <- subdomain_label_neighbors s ]
+
+
+
 -- * Domains
 
 -- | An RFC1035 domain. According to RFC1035 a domain can be either a
@@ -577,16 +674,21 @@ instance Pretty Domain where
 --   Anything over 255 characters is an error, so the root will be
 --   parsed:
 --
---   >>> let big_label = replicate 63 'x'
---   >>> let big_subdomain = concat $ replicate 5 (big_label ++ ".")
+--   >>> let big_l1 = replicate 63 'x'
+--   >>> let big_l2 = replicate 63 'y' -- Avoid equal neighboring labels!
+--   >>> let big_labels = big_l1 ++ "." ++ big_l2 ++ "."
+--   >>> let big_subdomain = concat $ replicate 3 big_labels
 --   >>> parseTest domain big_subdomain
 --   DomainRoot
 --
 --   But exactly 255 is allowed:
 --
 --   >>> import Data.List ( intercalate )
---   >>> let big_label = replicate 63 'x'
---   >>> let big_subdomain = intercalate "." (replicate 4 big_label)
+--   >>> let l1 = replicate 63 'w'
+--   >>> let l2 = replicate 63 'x'
+--   >>> let l3 = replicate 63 'y'
+--   >>> let l4 = replicate 63 'z'
+--   >>> let big_subdomain = intercalate "." [l1,l2,l3,l4]
 --   >>> let (Right r) = parse domain "" big_subdomain
 --   >>> length (pretty_show r)
 --   255
-- 
2.43.2