[dns-operations] Trustworthiness of PTR record targets

Lyle lyle at lcrcomputer.net
Tue Mar 4 20:34:47 UTC 2014

On 3/3/2014 11:26 AM, Stephen Malone wrote:
> Hi Folks,
> For PTR records out there that are pointing to domains other than 
> those that you control, I'm looking to understand common practice 
> around their setup. Two questions:
> 1.In general, can I trust PTR records? Is ownership of the target 
> domain validated at setup time by ISPs, and if yes, how is this done?
> 2.If ownership of PTR targets is not routinely validated, is there a 
> risk that the target domain could be blacklisted by anti-spam providers?
> Thanks,
> Stephen Malone
> Microsoft
I am a smaller provider of email services to small businesses in the 
local area.

I send email from a given ip address.  It's up to me to make sure that 
the reverse records point back to the same host name that the mail 
server uses during SMTP session setup.  And further that the A record 
for the host name during SMTP session setup(specifically in the HELO or 
EHLO commands) matches.

All of this is not revelent to the From or To or Reply to addresses, 
either in the envelope or inside the email itself.  More than one domain 
can use a given SMTP server, but it's host name doesn't change because 
of the From or Reply to fields in the HELO or EHLO command lines.

The PTR records 'belong' to the ISP that registers the addresses from 
ARIN, RIPE, etc.  The ISP can delegate the PTR records when a block is 
assigned to a certain customer, but don't have to.  It's up to that 
ISP's policies to police those records.

On the other hand, I don't care what the PTR shows for an ip address I 
used two years ago.  I only care for the PTR records for the ip address 
I am using NOW.  You won't have any knowledge of the ip address used two 
years ago and won't be looking over there.

Now I am not going to talk about SPF or DKIM or DomainKeys as I don't 
use them extensively.

Most blacklisting is done via ip address of the sending mail server.  
Checks against domain names is an entirely different subject and IMHO, 
not in the same league as this message.


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