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I use a virtual server (several similar cases) set up and registered by someone else. Its IP is 10.0.0.1 and name a.example.com (both handled by the "someone else" above).

I manage the domain sub.example.com and would like to also register this server as b.sub.example.com.

There are two ways of doing this in my DNS: either

b IN CNAME a.example.com.

or

b IN A 10.0.0.1

Since the IP is not distributed by DHCP (it is therefore hardcoded in the server config) there will be a problem in either case if it changes without me knowing and neither solution above would help. The PTR will point to a.example.com (I do not control the IP reverse mapping)

So what is the proper way (= canonical / best practice) to do the registration on my side?

EDIT: since b is also referenced in the NS I will have to go for the second solution. Please see my answer below for details.

I accepted MadHatter's answer because even though it does not address the best practices / standards for the case above (before the NS update which forces a solution anyway) it gives interesting insights. Thanks!

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1  
You talk to the person in charge and get him do his part of the job? –  TomTom Apr 2 '14 at 7:08
    
@TomTom: everyone did their job. The virtualization group registered the servers in the correct domain (from their perspective) and I want to register them in mine (for various reasons). –  WoJ Apr 2 '14 at 7:12
    
Is this IP address static or not? –  MadHatter Apr 2 '14 at 7:30
    
@MadHatter: it is static (I mentioned in my question that DHCP is not used and the IP is hardcoded in the server - it is actually automatically reserved (and communicated to me) and updated in the example.com DNS) –  WoJ Apr 2 '14 at 7:38
    
So why do you fear it changing without your knowledge? –  MadHatter Apr 2 '14 at 7:39

3 Answers 3

up vote 0 down vote accepted

The DNS isn't well-designed to handle changes in static IP addresses without manual intervention. There are technologies for allowing records to be propagated automatically, such as TSIG, but they require quite a lot of co-operative setup, and even then don't deal well with changes to PTR records.

Manual IP changes happen all the time in the real world. They have to be scheduled well in advance, and handled appropriately. That involves well-timed and thoughtful changes to the DNS, the acceptance that DNS propagation is imperfect and that there will be business effects as a result of the change, and a consequent dislike of such changes.

TomTom's comment above is right; if everyone involved does their jobs, a change will go through with the minimum of effort - but some effort will be needed.

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I completely understand this and it is not my question. I know that a change of the IP will break both of the configurations and that the PTR will point to the example.com entry. Now since both of the configurations work I wanted to understand if one is preferred over the other. –  WoJ Apr 2 '14 at 8:23

I will finally have to use

b IN A 10.0.0.1

because b is also referenced in the NS record (which I did not pay attention to intially)

 IN NS b

It is illegal for b to be a CNAME so there is no choice but to go with the explicit A record.

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The CNAME option will still work, as you say their infrastructure knows of the VPS's IP and they properly update their nameservers.

EDIT: About the PTR record; the only thing you can do is ask them. I myself have a few hosted VMs and all my providers except for one agreed to it. If their policy allows it you are fine.

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That won't fix the PTR issue. –  MadHatter Apr 2 '14 at 7:52

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