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I manage multiple websites that currently have the following DNS configuration:

example.com      - A Record - Production Server IP
test.example.com - A Record - Test Server IP
www.example.com  - CNAME    - example.com
beta.example.com - CNAME    - test.example.com
dev.example.com  - CNAME    - test.example.com

Is this an appropriate use of CNAME records? I've looked online and have not found a clear answer. Some people claim that CNAME records are bad (they are not, however, clear on why this is) and propose the following setup:

example.com      - A Record - Production Server IP
test.example.com - A Record - Test Server IP
www.example.com  - A Record - Production Server IP
beta.example.com - A Record - Test Server IP
dev.example.com  - A Record - Test Server IP

Which one of these is the better approach (and why)?

Note: The subdomains do not require their own MX records, so that is not an issue here.

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3 Answers

up vote 29 down vote accepted

Yes, that's an appropriate use of CNAMEs. In the discussions I've been part of, the arguments tend to go like this:

Against CNAMEs:

  • There is a (tiny) performance penalty, as the downstream DNS caches need to perform 2 DNS lookups, one for the CNAME and one for the A-Record the CNAME points to.
  • Vague, bogus arguments about CNAMEs having less "authority" or compatibility issues.

In favor of CNAMEs:

  • They provide a clean abstraction between hardware (physical servers) and services.
  • They simplify DNS management -- when a server moves, you only need to change one record.

After trying a couple of different ways to do this, I now have a personal favorite style. It is:

  • One A Record for each physical server; with a fairly low TTL (perhaps 30 minutes); giving the server a human-friendly name.
  • One CNAME for each service; with a high TTL (perhaps 24 hours); pointing to the above server names.
  • As the sole exeption to the rules above, the domain root is an A-Record, pointing to the webserver / web load balancer. (The @ is required to be an A-record.)

I find that this setup works well. It keeps extra DNS lookups for the CNAMES down; and if a server crashes I can still change public DNS around fairly fast.

Here's a (improvised) example in BIND syntax:

;name     ttl   class rr     value 
server01  30m   IN    A
server02  30m   IN    A

webmail   24h   IN    CNAME  server01
extranet  24h   IN    CNAME  server02
ftp       24h   IN    CNAME  server02
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Thank you, finally a reasonable opinion on CNAMEs thats laid out clearly and concisely. –  Tyler Aug 12 '12 at 4:08
@Jesper Mortensen: could you please update a bit the answer with a small example, particularly I did not understand your 3rd point when you say "As the sole exeption to the rules above, the domain root is an A-Record," you already said in 1st point that you use one A Record for each physical layer server. (BTW the links has gone) –  Marco Demaio Feb 21 '13 at 20:07
@Marco Demaio: About the "domain root A-Record": A second level domain like company.comis a zone apex. It needs a SOA record. Thus it must be an A Record and not a CNAME -- see serverfault.com/questions/170194/… –  Jesper Mortensen Feb 21 '13 at 21:01
@Jesper Mortensen: thanks now I undertsood what you meant. FYI I just opened a new question asking about alternative ways alternatives to avoid writing same IP in DNS for mutiple domains: serverfault.com/questions/481500/… –  Marco Demaio Feb 22 '13 at 13:19
@ is not required to have an A record; rather, a CNAME is prohibited. –  Michael Hampton May 11 '13 at 16:00
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Yes it's appropriate.

My Best Practices, which many people share, are to create 1 A record for each server IP; and use CNAMES for anything else.

A common example would be:

server1.example.com.      IN A
server2.example.com.      IN A
www                       IN CNAME  server1
ftp                       IN CNAME  server1
beta                      IN CNAME  server2
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I know in this question they said mail is not an issue here, but let's suppose that you use also mail how would you go with MX records? Thanks! –  Marco Demaio Feb 21 '13 at 19:59
The MX record would point to the server name as well. IN MX server1 and for convenience I would recommend also setting up imap or pop and smtp CNAMEs, possibly also mail, as many e-mail programs guess this. Setting up the correct SRV records is also a good idea, but since this is a relatively basic question SRV records might be a bit much for a simple configuration. –  Chris S Feb 21 '13 at 20:07
A quick comment, MXrecords must not be CNAMEs, see serverfault.com/a/232243/2874 It probably works fine in practice -- but still, better not to do so. –  Jesper Mortensen Feb 21 '13 at 22:45
BIND will refuse to load the zone if you point a MX or SRV record at a CNAME... I probably should have made that clear that the MX record needs to point to the A record. Thank you. –  Chris S Feb 22 '13 at 0:40
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I agree with Chris.. If you move a site or site(s) to a new server with a new IP you are just changing one line.. it makes for less screw ups in DNS

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