DNS – Can Multiple CNAMES Exist for a Single Name?

There are two alternate views of this question, and it's one that is ultimately going to be debated forever. I'm not going to give my opinion (because I'm torn myself), but the general arguments each way typically are:

• You should define A records for your physical machines, and then CNAME services onto those machines. This does make it rather clear as to what is what, and in the event that you need to renumber there's not a lot of records to change -- just the machine records. On the other hand, it does increase your DNS lookup load somewhat, and "auxiliary" IPs (think SSL vhosts) don't fit neatly into this model.

• The literal meaning of "canonical name" (CNAME) is to define strict aliases of the same name (think mail and smtp), and if you have multiple services running on the same machine they should all have A records, because it reduces load on DNS and some services (NS records and, to a lesser extent, MX records) really aren't impressed with dealing with CNAMEs, so if you have to handle those services differently anyway, we may as well do it for everything.

Jeff, I disagree, load balancing does not imply redundancy, it's quite the opposite in fact. The more servers you have, the more likely you'll have a failure at a given instant. That's why redundancy IS mandatory when doing load balancing, but unfortunately there are a lot of solutions which only provide load balancing without performing any health check, resulting in a less reliable service.

DNS roundrobin is excellent to increase capacity, by distributing the load across multiple points (potentially geographically distributed). But it does not provide fail-over. You must first describe what type of failure you are trying to cover. A server failure must be covered locally using a standard IP address takeover mechanism (VRRP, CARP, ...). A switch failure is covered by resilient links on the server to two switches. A WAN link failure can be covered by a multi-link setup between you and your provider, using either a routing protocol or a layer2 solution (eg: multi-link PPP). A site failure should be covered by BGP : your IP addresses are replicated over multiple sites and you announce them to the net only where they are available.

From your question, it seems that you only need to provide a server fail-over solution, which is the easiest solution since it does not involve any hardware nor contract with any ISP. You just have to setup the appropriate software on your server for that, and it's by far the cheapest and most reliable solution.

You asked "what if an haproxy machine fails ?". It's the same. All people I know who use haproxy for load balancing and high availability have two machines and run either ucarp, keepalived or heartbeat on them to ensure that one of them is always available.

Hoping this helps!