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Re: Question for this IP's PTR

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On Fri, 24 Mar 2023 20:32:31 -0400
Greg Wooledge <greg@wooledge.org> wrote:

> On Sat, Mar 25, 2023 at 08:28:03AM +0800, fh@dnsbed.com wrote:
> > Greetings,
> > 
> > as you see this PTR,
> > 
> > $ dig -x 1.1.1.1 +short
> > one.one.one.one.
> > 
> > so 2.2.2.2 can have the PTR two.two.two.two? and 3.3.3.3 can have
> > three.three.three.three?  
> 
> Any IP address can have any PTR value.  You just have to petition the
> owner of the IP address range to set it.
> 
> I didn't know .one was a valid TLD.  It looks like .two is not, so if
> someone were to assign "two.two.two.two" as the PTR value of an IP
> address, that PTR would not resolve back to any IP address.  (An IP
> address block owner might reject such a petition.)
> 

In general, at this time, a mail server will look at the IP address of
a potential sender, check the PTR, then check for an A record matching
the PTR, pointing back to the IP address. The PTR does not (currently)
need to be related to an email domain using the address.

A competent ISP will have set up its IP addresses with complementary
PTR-A record pairs. Unfortunately, many use PTRs in the form
xxxxx-11-22-33-44 which is perfectly valid, but may be rejected by mail
servers as likely spammers (mine does). If you already have a PTR-A
pair that doesn't look like this (e.g. is some form of your user name
or account reference) you're probably OK.

The relevant RFC allows (or did when I last looked) multiple PTR
records for one IP address, but I don't think there's much software
which can deal with that, or will return more than one. On the other
hand, it's quite common for a single mail server to deal with many
domains, so it's not reasonable to expect a sender or HELO/EHLO to
match the PTR. My email server checks for a complementary PTR-A pair
that can both be found in public DNS, and goes no further. I believe
that is a typical setting.

-- 
Joe

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Thread

Question for this IP's PTR fh@dnsbed.com - 2023-03-25 01:30 +0100
  Re: Question for this IP's PTR Greg Wooledge <greg@wooledge.org> - 2023-03-25 01:40 +0100
    Re: Question for this IP's PTR fh@dnsbed.com - 2023-03-25 02:30 +0100
    Re: Question for this IP's PTR Jeremy Ardley <jeremy@ardley.org> - 2023-03-25 02:30 +0100
    Re: Question for this IP's PTR Joe <joe@jretrading.com> - 2023-03-25 10:30 +0100
  Re: Question for this IP's PTR debian-user@howorth.org.uk - 2023-03-25 19:20 +0100

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