Why is DNS returning two different results at random

By 'DNS failover' I take it you mean DNS Round Robin combined with some monitoring, i.e. publishing multiple IP addresses for a DNS hostname, and removing a dead address when monitoring detects that a server is down. This can be workable for small, less trafficked websites.

By design, when you answer a DNS request you also provide a Time To Live (TTL) for the response you hand out. In other words, you're telling other DNS servers and caches "you may store this answer and use it for x minutes before checking back with me". The drawbacks come from this:

• With DNS failover, a unknown percentage of your users will have your DNS data cached with varying amounts of TTL left. Until the TTL expires these may connect to the dead server. There are faster ways of completing failover than this.
• Because of the above, you're inclined to set the TTL quite low, say 5-10 minutes. But setting it higher gives a (very small) performance benefit, and may help your DNS propagation work reliably even if there is a short glitch in network traffic. So using DNS based failover goes against high TTLs, but high TTLs are a part of DNS and can be useful.

The more common methods of getting good uptime involve:

• Placing servers together on the same LAN.
• Place the LAN in a datacenter with highly available power and network planes.
• Use a HTTP load balancer to spread load and fail over on individual server failures.
• Get the level of redundancy / expected uptime you require for your firewalls, load balancers and switches.
• Have a communication strategy in place for full-datacenter failures, and the occasional failure of a switch / database server / other resource that cannot easily be mirrored.

A very small minority of web sites use multi-datacenter setups, with 'geo-balancing' between datacenters.

In DNS, propagation is based almost entirely on the serial number in the SOA record. If you don't change it, it will break. Some hosting companies let you make changes all over your domain, but then you have to click on a "publish" link of some sort to indicate that you are ready to commit those changes upon the world.

Also, by "subdomain" I assume you mean you are creating a record, not an actual subdomain. That is, you are adding an A record for testing within your domain, not a subdomain. The difference is only really important when you ask questions that DNS types like myself have to answer. A "subdomain" means NS records to delegate to another set of name servers. I am assuming you mean the misused term to mean "an A record."

For the people who can resolve the name, what do they see? The proper A record? If so, then I would think it is a propagation error; try seeing if you can bump up the serial number or add another record (foo?) and see if that also fails to propagate. If it does, well, call your DNS host.

Note that one of the three name servers serving your zone is down: ns1.compuone.com seems to not be up from where I am at least.