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Accepted
Joseph Sturtevant
Asked: 2010-09-17 09:55:51 +0800 CST

Should CNAME Be Used For Subdomains?

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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.

domain-name-system cname-record

2 Answers

  1. Best Answer

    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      192.168.0.3
server02  30m   IN    A      192.168.0.4

webmail   24h   IN    CNAME  server01
extranet  24h   IN    CNAME  server02
ftp       24h   IN    CNAME  server02
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  2. 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      192.168.0.1
server2.example.com.      IN A      192.168.5.2
www                       IN CNAME  server1
ftp                       IN CNAME  server1
beta                      IN CNAME  server2
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