From: Yves-Eric <yemartin@...>
Date: 2009-03-02T13:09:01+09:00
Subject: Re: DNS MX lookup issue: local machine returned as MX for an 	non-existant domain!

Problem solved. Executive summary: it is a documentation bug. DNS
name queries need a final dot.

Do not query resources for "domain.com" as the Resolv documentation
[1] suggests. One should instead query for "domain.com.", with an
extra dot at the end. The first form, without the final dot, will
also be looked up in the local domain (for example, you could look
for just "www", and you would get the result for
"www.localdomain.com"). The problem is when you have a wildcard DNS
entry in the local domain: then any non-existent domain would match
that entry.

Because the current documentation uses "www.ruby-lang.org" as an
example, a lookup could occur for something like
"www.ruby-lang.org.localdomain.com", which is **not** what users
unfamiliar with DNS would expect. More importantly, even people
familiar with DNS could be mislead by the documentation and forget
the final dot. So, to follow the principle of least surprise, the
documentation should be updated to include the final dot in example
DNS name strings.

Where should I fill a bug report?


PS: thanks again for your suggestions Brian. While it sounded quite
daunting at first, I took the plunge into Ruby source code, and it
was really not that bad. With a few strategically placed STDERR.puts,
I could quickly get to the root of the problem. For the record, I
also learned that "[|proto]" means that the packet was truncated at
the "proto" protocol level, in our case "domain", and we needed to
capture more data. So here is a new cleaned-up dump (thanks also for
the Socket.do_not_reverse_lookup for noise reduction, it was indeed
Ruby doing the PTR queries), where the second query shows the problem
clearly:


# dump of Resolv::DNS activity:
host > dns:  0+ MX? i-dont-exist.com. (49)
dns > host:  0 NXDomain 0/1/0 (122)
host > dns:  1+ MX? i-dont-exist.com.mydomain.com. (61)
dns > host:  1 2/13/11 CNAME mydomain.com., MX mydomain.com. 10 (491)


[1] http://www.ruby-doc.org/core-1.9/classes/Resolv.html

Best regards,

--
Yves-Eric