[69283] in North American Network Operators' Group
Re: NANOG list reverse DNS handling
daemon@ATHENA.MIT.EDU (Mike Lewinski)
Sat Apr 3 16:03:43 2004
Date: Sat, 03 Apr 2004 14:02:54 -0700
From: Mike Lewinski <mike@rockynet.com>
To: nanog@merit.edu
In-Reply-To: <BB3C8E7B-8587-11D8-AA81-000A95CD987A@muada.com>
Errors-To: owner-nanog-outgoing@merit.edu
Iljitsch van Beijnum wrote:
> There is also a link to a DNS checking tool. However, this tool is
> pretty much useless in situations such as the one in which I found
> myself, as it doesn't answer the real question: what is the TTL for the
> offending DNS information.
You should have the answer to that (more or less- at least the upper
bound) as it is set by you in your zone.
Now, if you want to know how much of the TTL remains wrt to merit.edu
accepting mail, you need to know what resolvers the mail server is
using, and can then query thusly:
$ dig ptr 1.65.149.83.in-addr.arpa @dns.merit.net | grep ^1
1.65.149.83.in-addr.arpa. 86400 IN PTR sequoia.muada.com.
(I see that dns.merit.net is the next IP above mail.merit.net which is
the only MX RR for merit.edu, although that's really still just a guess
as to the resolver it uses)
A second query reveals that the TTL on this record has decreased by a
few seconds. Since your .arpa zone ttl seems to be at one day, it isn't
likely that dns.merit.edu is the resolver for mail.merit.edu (or else it
has since expired from cache):
$ dig ptr 1.65.149.83.in-addr.arpa @dns.merit.net | grep ^1
1.65.149.83.in-addr.arpa. 86398 IN PTR sequoia.muada.com.
Note that this doesn't work if the resolver has an ACL applied that
restricts who can do resolution on it and you don't fall within that
ACL. But the bigger hurdle here is really figuring out what the resolver
mail.merit.edu uses, since it's most likely open. A check of all the
auth DNS servers for merit.edu reveals no evidence of caching for this
particular record.