Reverse DNS Question

Jim Mercer jim at
Wed Apr 21 01:31:44 CDT 2010

On Tue, Apr 20, 2010 at 10:26:17AM -1000, Antonio Querubin wrote:
> On Tue, 20 Apr 2010, James Martin wrote:
> >What is the purpose for this besides resolving name-based reverse lookups?
> >Are there any definitive guides out there on how this works (besides the
> >ARIN site)?
> It's for resolving address-based lookups.  When ARIN allocates address 
> space to you, you now become responsible for the reverse-lookups for that 
> allocated address range.

with forward DNS, anyone can map a domain to any arbitrary IP address, such
as mapping to the same IP address as

there is nothing to prevent this, and in some cases it is acceptable, and in
some cases, possibly nefarious.

when the registeries (ARIN/RIPE/APNIC/etc) require the "owner" of an ip block
to define name servers for reverse maps, it provides a mechanism to double
check if a domain/ip-addr map is valid.

it isn't 100%, for sure, but, it is substantially better than nothing.

in this sense, can have an A record of

and, through the reverse map, will have a PTR record
of ""

in fact, there can be multiple PTR records, in case you have multiple
domains pointing at the same IP address.

on many unix(-ish) systems, the "host" command will show you the reverse PTR
record, if you run: host , it might show:

user at hostname% host domain name pointer

keep in mind, this will only work if the name servers registered for the ip
block actually contain data.

check out:

and, go to "Guide to reverse zones" in:

hope this is helpful

Jim Mercer        jim at        +92 336 520-4504
"I'm Prime Minister of Canada, I live here and I'm going to take a leak."
   - Lester Pearson in 1967, during a meeting between himself and
    President Lyndon Johnson, whose Secret Service detail had taken over
    Pearson's cottage retreat.  At one point, a Johnson guard asked
    Pearson, "Who are you and where are you going?"

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