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From: 7thson@slcs.slb.com (Chris Garrigues)
Newsgroups: comp.protocols.tcp-ip.domains
Subject: PTR records of gateways on the Internet
Message-ID: <1991Jan9.195641.17628@slcs.slb.com>
Date: 9 Jan 91 19:56:41 GMT
Sender: news@slcs.slb.com (God of Tripe and Drivel)
Reply-To: 7thson@slcs.slb.com (Chris Garrigues)
Organization: Schlumberger Laboratory for Computer Science
Lines: 33
Nntp-Posting-Host: private-snafu

I was recently trying to map out some of the internet.  I was doing this
by identifying the IP address of gateways with low TTL pings and looking
at the from address of the returning TTL-exceeded packets.  I then tried
to use IN-ADDR.ARPA reverse mappings to map these into names and from
there, I tried to find the other interfaces of the gateways by finding
the A records for those names.

Needless to say, in the process, I discovered that some administrators
appear to have rather bizzare ideas of how to set up these records.

a) I found PTR records that pointed to names that weren't in the DNS.

b) I found PTR records that pointed to names that mapped back to
different addresses.

c) Many of these addresses mapped into names which only had one address.
This either means that no entry was made for the other name or the two
names are entirely unlinked.

Does anybody besides me think that a RFC that clarified what a domain
administrator MAY, SHOULD, and MUST do would be useful to point at when
stumbling over things like these.  Examples might be:

    The name referenced by a PTR record MUST have an A record containing the
    address corresponding to the PTR record.
    
    If a host or gateways has multiple addresses, these addresses SHOULD
    have PTR records pointing either to the same name OR to names with CNAME
    records pointing to a common name.


Chris Garrigues,
Schlumberger Laboratory for Computer Science