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I host multiple domains on a single IP address:

cats.com 
A @ --> 10.20.30.40
A mail --> 10.20.30.40
MX @ --> mail

dogs.com
A @ --> 10.20.30.40
A mail --> 10.20.30.40
MX @ --> mail

I need to set up a PTR record for the IP address. This is so that forward-confirmed reverse DNS will succeed for other mail servers.

DNS query type PTR on 10.20.30.40 --> returns PTR-record="cats.com" (1 result)
DNS query type A on "cats.com" --> returns A-record=10.20.30.40 (1 result)

If I can only have one PTR record for my IP, that points to cats.com, how will FCrDNS ever succeed for dogs.com?

Can I just use the domain name they both share (80.70.60.50.static.host.net) in the PTR ?

Thanks!

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2 Answers 2

up vote 2 down vote accepted

Reverse DNS can only really have one name -- if you list more than one, DNS will just round-robin between them. So you just have to pick one "official" name for the server and always use that. All the other domains are, in essence, just aliases to the original.

Most services don't care what the name is that your server uses, they just care that the reverse lookup returns a domain name that resolve correctly going the other way as well.

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If your reverse lookup does not match the forward lookup for your MX record, you will have your e-Mail systematically tagged as spam by many e-Mail servers. –  Warner Jul 9 '10 at 13:21
    
So, the following will NOT pass an MX record check? from:info@dog.com, PTR 10.20.30.40 -> cats.com, MX dogs.com -> 10.20.30.40 –  dbasch Jul 9 '10 at 15:04
    
@Warner: the solution would be to make the MX record match the hostname, then. The MX record (target) doesn't need to relate at all to the email destination address. –  tylerl Jul 10 '10 at 8:01
    
That would be one of the methods used to lessen spam, dbasch. You understand me correctly. I don't think you understand me, tyler. –  Warner Jul 14 '10 at 3:17
    
You can put in rDNS any name (server's hostname or one of domain names, that hosted on it), it's up to you. What about "match for MX record", and prevent e-mails tagged as spam, you'll need to set up SPF-records correctly, that's all. –  Denis Dec 4 '10 at 13:54

I've only ever came across single reverse DNS entries aswell. I think your customers should be relaying through your SMTP host (with your rDNS) or their own host with their rDNS.

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