Check name length against the wire form, not the presentation form

[mrideout]

The problem

Dnsruby::Name#initialize compares MaxNameLength (255) against the length of the dotted presentation form:

total_length = labels.length - 1
labels.each { |l| total_length += l.length }

That adds up the label text plus the dots in between. On the wire a length octet replaces each dot, but there is one more label than there are dots, and a zero octet ends the name. So the wire form is always 2 octets longer.

The 255 comes from RFC 1035 Section 2.3.4, and RFC 2181 Section 11 spells out what counts toward it: a name is "limited to 255 octets (including the separators)", the separators being those length octets and the trailing zero. So names 1 or 2 octets over the limit got through and got encoded into real queries and responses. dnspython rejects such a name with NameTooLong, and BIND's dig, host and nslookup reject it with "ran out of space".

The fix

total_length = labels.length + 1

Label#length returns the decoded length, so escaped labels take care of themselves.

Tests

test_name_length did not test what its name said. It built a name with a 253-character first label, which tripped the 63-octet label limit and never reached the name check, so it would have passed with that check deleted. I rewrote it to check that a 255-octet name is accepted and a 256-octet one is not.

test_name_length_in_message is new, covering the Message path the over-long names were reaching the wire through, in both directions: the 256-octet name is refused, and the 255-octet one still round-trips through encode/decode. Both fail without the fix. rake test passes.