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I am in the process of setting up a new server for my web application (the site will be moved, it is not for load balancing or the like), which has a different IP address from my existing server. My current server has a reverse DNS PTR record set up pointing its IP to mydomain.com. Is it bad to set up a reverse DNS PTR record for the new IP pointing to mydomain.com as well? Or should I wait until I do my migration to set up the record?

Update: I forgot to mention, the A record for the mydomain.com points to the old server's IP address, not the new one, if it matters.

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It isn't clear to me what particular service your system is running. You say domain, are you talking about a web server? The PTR record is barely used at all for HTTP, so it probably doesn't matter at all. OTOH incorrect PTR config can seriously break email. –  Zoredache Apr 21 '10 at 17:12
    
Both servers are webservers that also send and receive email (which is why I asked) –  Daniel Vandersluis Apr 21 '10 at 17:43
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4 Answers

up vote 6 down vote accepted

If it is convenient for you as a temporary solution, it should be perfectly acceptable. I cannot think of many scenarios where having multiple PTR records with the same hostname will introduce any technical issues.

One potential scenario would be mail delivery on the new server. At least, if the forward lookup resolves to the old server. Fickle mail servers will bounce mail without hostnames/IPs being able to resolve both ways and match.

Outside of that, and I'm really trying, I can't think of any. If there's more, it's likely to be of limited scope like above.

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If you have two IPs resolving to the same domain name, then you cannot have Forward Confirmed Reverse DNS (FCrDNS) for both, which is the check that many authentication schemes use (such as email servers when deciding whether to deliver your mail).

In order to get forward confirmed reverse DNS, an IP address must resolve to a hostname that resolves back that that IP address and only that IP address.

You could, however, have one IP resolving to sub01.example.com and another resolving to sub02.example.com and still have FCrDNS for both.

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As long as you keep your A record pointing to one specific IP address (no round robin) this should not cause any problems.

Of course, the best practice is to always have 1 <-> 1 resolution to close the circle.

There's some thorough explanation at digitalpoint.com. The point is, it's RFC design goal, but the practical approach is - at times you don't even have access to some reverse entries (f.e. former ISP having stale records), and it shouldn't be a problem (assuming you only use 1 "live" address).

So in brief:

  • If you want your reverse DNS entry to "wait" for you when you migrate - it seems absolutely OK.
  • If you're using both servers at the same time for production - I'm not sure. Theoretically it's bad practice (see RFC 1912), but I don't think anything but mail would complain about it.
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This should be fine as a temporary solution, however some programs may only recognize the first PTR record and may drop anything else. Other clients may drop the request if your request contains more data then expected. Hopefully both of these cases will be rare.

References:

http://en.wikipedia.org/wiki/Reverse_DNS_lookup#Multiple_pointer_records

http://www.ietf.org/mail-archive/web/dnsop/current/msg01011.html

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I think you misunderstood his question. He's talking about different IPs. –  Warner Apr 21 '10 at 17:02
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