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I tried doing some research but didn't find what I was looking.

I have server (win2k8fs1). It is a static assigned IP and has an A record 192.168.x.x with the associated PTR record.

Setup:

   a: win2k8fs1 192.168.1.1
   ptr: 192.168.1.1 win2k8fs1.domain.local
   a: filerserver 192.168.1.1

Whenever I added the A Records for fileserver, I was asked if I wanted to create the associate PTR record to match. Is it safe to create the second PTR record for 192.168.1.1?

Thanks.

Jeff
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3 Answers3

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First of all, a computer named win2k8fs1 does not have a PTR record - only IPs have PTR records.

If the current situation is:

wink28fs1.your.domain. IN A 192.168.1.1
192.168.1.1 IN PTR win2kfs1.your.domain.

then adding a second A record for that IP will work just fine.

Distinct A and PTR records only come into play when the connecting host checks the existence and values of both the A and PTR records, for instance on mail servers.

EDITed for better question:

As far as DNS is concerned, sure, every A record can have a corresponding PTR record.
That does not mean you should want to do this, however, as not all services using DNS will utilize the same method of choosing which record to use.

When the same label has multiple RRs for the same resource type, DNS returns all records to the client; which one the client uses is outside the scope of DNS.

This is not an issue (mostly) with A records, as you may want to, for example, host multiple web sites on one IP:

server1 IN A 1.2.3.4
server2 IN A 1.2.3.4

This is perfectly valid and results in the client connecting to the same IP for both hostnames.

However, due to the way DNS usually resolves names to IPs, the same is not true for PTR records:

1.2.3.4 IN PTR server1
1.2.3.4 IN PTR server2

This will, depending on what the client does, not yield the same result for IP -> hostname resolution as it will for hostname -> IP resolution.

As I mentioned earlier, not a problem for services that don't care about this sort of thing.

adaptr
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  • I edited the question. I did not link I could have to PTR records for 192.168.1.1 - doesn't seem logical but I wasn't sure. – Jeff Dec 20 '11 at 14:14
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You fail to mention whether fileserver's A record points to the same IP address as win2k8fs1's. But assuming that it does, you are right to worry: there can be only one PTR record corresponding to a given IP address.

Edit: I stand corrected by adaptr, though I think sandman4 is simply wrong. From Wikipedia's entry on reverse DNS:

While most rDNS entries only have one PTR record, DNS does not restrict the number. However, having multiple PTR records for the same IP address is generally not recommended, unless there is a specific need. [...] Multiple PTR records can cause problems, however, including triggering bugs in programs that only expect single PTR records and, in the case of a large web server, having hundreds of PTR records can cause the DNS packets to be much larger than normal.

So in the end, we find that, as adaptr said (and Connor MacLeod might have), "there should be only one".

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MadHatter
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  • Sorry - yes they point to the same IP. I thought so but was 100% sure. – Jeff Dec 20 '11 at 14:08
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    No, there can be many; however, there SHOULD only be one. – adaptr Dec 20 '11 at 14:08
  • It's Ok to have many – Sandman4 Dec 20 '11 at 14:15
  • Standart is clear - here's an example from the RFC 1035 itself: 10.IN-ADDR.ARPA. PTR MILNET-GW.ISI.EDU. \\ 10.IN-ADDR.ARPA. PTR GW.LCS.MIT.EDU. \\ Any software which uses PTR at all, MUST accept multiple PTR records. – Sandman4 Dec 20 '11 at 19:13
  • My personal questionable opinion: It's actually a good thing to have multiple PTR records. It will help spread standard and software which complies to standards, and whose who do not support - let them break and people will start to avoid it, and eternal peace will come. Amen. – Sandman4 Dec 20 '11 at 19:23
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If you create another record named fileserver with a different IP on the forward lookup tree and a PTR record on the reverse lookup tree, then you'd be doing everything fine.

If you create another record named fileserver with the same IP, then you'd be fine again.

UPDATE: That's a whole other question. Depending on what you want to do there's a very good discussion here.

TL;DR Depends.

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