patch - for resolv.rb to support mDNS "cache flush" bit

[Sam Roberts]

mDNS uses the high-bit of the CLASS of a resource record to store a
"cache flush" bit.

This is DNS-incompatible, of course, but note only 2 bits of the 16
CLASS bits are used so far, and classes other than "1" (internet) are
rarely seen. It is unlikely that DNS will be needing the next thousands
of values anytime soon.

Functional requirements for mDNS are:

1 - have the cache flush bit not be considered when choosing a Class to
decode the rdata

2 - be able to retrieve the saved cache flush bit later


A way of doing this with a small set of changes to resolv.rb is:

Index: resolv.rb
===================================================================
RCS file: /src/ruby/lib/resolv.rb,v
retrieving revision 1.17.2.9
diff -u -r1.17.2.9 resolv.rb
--- resolv.rb 5 Feb 2005 18:31:20 -0000 1.17.2.9
+++ resolv.rb 6 Feb 2005 22:32:59 -0000
@@ -1322,8 +1322,10 @@
def get_rr
name = self.get_name
type, klass, ttl = self.get_unpack('nnN')
- typeclass = Resource.get_class(type, klass)
- return name, ttl, self.get_length16 {typeclass.decode_rdata(self)}
+ typeclass = Resource.get_class(type, klass % 0x8000)
+ data = self.get_length16 {typeclass.decode_rdata(self)}
+ data.instance_variable_set@cacheflush, true) if (klass >> 15) == 1
+ return name, ttl, data
end
end
end


Another way would be to add


class Resource
attr_accessor :cacheflush
...


and use the attribute rather than #instance_variable_set. This is less
"weird", but instance_variable_set is more backwards compatible (i.e.,
if the bit isn't set (DNS' case) then no extra attributes exist in the
resource data class).


Thoughts please?


Thank you,
Sam

Example packet:


require 'resolv.rb'
require 'pp'

class Resolv
class DNS
class Name

# NOTE - could #inspect be added to Name? An array of labels every
# time I do a 'pp' is making output much harder to read than
# necessary.
def inspect
to_s + (absolute? ? '.' : '')
end
end
end
end

data1 = "\000\000\204\000\000\000\000\002\000\000\000\001\010ensemble\005_http\004_tcp\005local\000\000!\200\001\000\000\000<\000\021\000\000\000\000\000P\010ensemble\300 \300\f\000\020\200\001\000\000\000<\000\001\000\3007\000\001\200\001\000\000\000<\000\004\300\250{\232"

msg = Resolv:NS::Message.decode(data1)

pp msg