Lua: Fix root zone queries in DNS example

Queries for the root zone (e.g. `dig -t NS`) produce a name of only `\x00`,
which would cause the query parser to fail.

1 files changed, 23 insertions, 20 deletions

diff --git a/test/lua/dissector.lua b/test/lua/dissector.lua
index 9ea14b8dc8..95ad5b2157 100644
--- a/test/lua/dissector.lua
+++ b/test/lua/dissector.lua
@@ -317,8 +317,8 @@ dprint2("MyDNS Prefs registered")
 local DNS_HDR_LEN = 12
 
 -- the smallest possible DNS query field size
--- has to be at least a label length octet, label character, label null terminator, 2-bytes type and 2-bytes class
-local MIN_QUERY_LEN = 7
+-- has to be at least a label null terminator, 2-bytes type and 2-bytes class
+local MIN_QUERY_LEN = 5
 
 ----------------------------------------
 -- some forward "declarations" of helper functions we use in the dissector
@@ -452,13 +452,13 @@ function dns.dissector(tvbuf,pktinfo,root)
             -- adding it to the tree, because we want to identify the correct bytes it covers
             local label_count, name, name_len = getQueryName(tvbuf:range(pos,pktlen_remaining))
             if not label_count then
-                q_tree:add_expert_info(PI_MALFORMED, PI_ERROR, name)
+                queries_tree:add_expert_info(PI_MALFORMED, PI_ERROR, name)
                 return
             end
 
             -- now add the first query to the 'Queries' child tree we just created
             -- we're going to change the string generated by this later, after we figure out the subsequent fields.
-            -- the whole query field is the query name field length we just got, plus the 20 byte type and 2-byte class
+            -- the whole query field is the query name field length we just got, plus 2-byte type and 2-byte class.
             local q_tree = queries_tree:add(pf_query, tvbuf:range(pos, name_len + 4))
 
             q_tree:add(pf_query_name, tvbuf:range(pos, name_len), name)
@@ -608,11 +608,12 @@ end
 -- This function is given the TvbRange object from the dissector() function, and needs to
 -- parse it.
 -- On success, it returns three things: the number of labels, the name string, and how
--- many bytes it covered of the buffer (which is always 2 more than the name length in this case).
+-- many bytes it covered of the buffer.
 -- On failure, it returns nil and the error message.
 getQueryName = function (tvbr)
     local label_count = 0
     local name = ""
+    local name_len = 0
 
     local len_remaining = tvbr:len()
     if len_remaining < 2 then
@@ -623,28 +624,30 @@ getQueryName = function (tvbr)
     local barray = tvbr:bytes() -- gets a ByteArray of the TvbRange
     local pos = 0 -- unlike Lua, ByteArray uses 0-based indexing
 
-    -- get the first octet/label-length
-    local label_len = barray:get_index(pos)
-    if label_len == 0 then
-        return nil, "invalid initial label length of 0"
-    end
-
-    while label_len > 0 do
+    repeat
+        local label_len = barray:get_index(pos)
         if label_len >= len_remaining then
             return nil, "invalid label length of "..label_len
         end
         pos = pos + 1  -- move past label length octet
-        -- append the label and a dot to name string
-        -- note: this uses the new method of ByteArray:raw(), added in 1.11.3
-        name = name .. barray:raw(pos, label_len) .. "."
+        if label_len > 0 then
+            -- append the label and a dot to name string
+            -- note: this uses the new method of ByteArray:raw(), added in 1.11.3
+            name = name .. barray:raw(pos, label_len) .. "."
+            label_count = label_count + 1
+            pos = pos + label_len -- move past label
+        end
+        name_len = name_len + label_len + 1
         len_remaining = len_remaining - (label_len + 1) -- subtract label and its length octet
-        label_count = label_count + 1
-        pos = pos + label_len -- move past label
-        label_len = barray:get_index(pos)
-    end
+    until label_len == 0
 
     -- we appended an extra dot, so get rid of it
     name = name:sub(1, -2)
 
-    return label_count, name, name:len() + 2
+    if name == "" then
+        -- this is the root zone (.)
+        name = "<Root>"
+    end
+
+    return label_count, name, name_len
 end