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mcw82

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I want to add a second domain to my Kerio server and had some DNS related questions, particularly with the PTR record. I only have one public IP. I have two domains, abc.com and def.com now created in Kerio.

My primary domain, abc.com has a public IP of 1.1.1.1 (theory). I created an A record mail.abc.com to point to 1.1.1.1. There's also a MX record for abc.com pointing to mail.abc.com with a cost of 10. A PTR has also been created with the ISP that owns 1.1.1.1.

I know you should/can only associated one domain to IP for PTR. So what do I need to do with the second domain to prevent problems with sending mail when the recipient server does a reverse dns check?

Can I just change def.com's mx record to point to mail.abc.com with a cost of 10? What would happen if an external mail server does a reverse dns check for def.com's mail but then sees the 1.1.1.1 is associated with mail.abc.com?
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My IT Indy

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99.99% of the time the DNS lookups won't care about a PTR record matching the MX of another domain. I have 7 domains hosted on 1 IP address without issue. I just have the domain mymailserver.com as the MX record for each domain and everything is fine.

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My IT Indy
Kerio Certified Reseller and Hosted Provider
http://www.myitindy.com
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winkelman

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mcw82 wrote on Wed, 14 November 2007 01:49

I know you should/can only associated one domain to IP for PTR.

That's not necessarily true. You can have a Reverse DNS lookup of one IP return several domain names. And servers that do check PTR records will be satisfied if one of the returned domain names matches the mail servers response (and they won't[/shouldn't] care about the others)...
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mcw82

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Thanks for the replies! After a quick chat with our ISP, I guess the PTR question I had depends on how strict an ISP interprets the rules/RFC governing PTR. As Rugby suggested above, I asked about pointing our second domain's mx record to the primary domain's and response was favorable. I can live with 99.99%.


I still don't understand how DNS responds back if an IP has more than one PTR. Is it a round-robin type of thing? This is just for my education.
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winkelman

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mcw82 wrote on Wed, 14 November 2007 17:46

I still don't understand how DNS responds back if an IP has more than one PTR. Is it a round-robin type of thing? This is just for my education.

DNS will simply reply with multiple answers to this singel PTR lookup request.

Try this (on Windows):
command prompt
nslookup
set type=ptr
205.248.106.64
This will return three domain names (maila.microsoft.com, mail4.microsoft.com and smtp.microsoft.com)
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