I have a DNS record with three
A records: the main site, and two backups. However, when clients perform a DNS lookup, they reach a backup site.
How can I change the order of the DNS records in
dot.tk's DNS server?
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You're touching on a "feature" of DNS known as DNS Round Robin. It's deliberate. You have no control over the order in which records which satisfy a particular query are provided.
It's crude; but surprisingly effective as a load-balancer.
If, in fact, the "backup" machines should not be touched unless the primary fails, you need to be more sophisticated with your zone files. You could change the TTL (time-to-live period) for the A record to a small number (e.g. 60 seconds) and then set up a script to change that A record via RFC 2136 when you want folks to fail over.
If you have 3 A records coughing up 3 different answers, you are supposed to get rough distribution equally amongst the 3. (one can also assume that M$ DNS will screw this up, but I don't actually know...)
The resource records that exist for a specific combination of name, class and type form what is called a resource record set (RRSet). Just like this term suggests (it being a set) there is no defined order for these records.
Even if you did have the authoritative nameservers for your zone, the only point where you have any actual control, answering with the records in some fixed order of your choosing it's still entirely possible (and actually common) for a resolver server to reorder them before passing the answer on to the client and even if the answer arrives in the desired order to the client host the resolver library can also reorder the result.
Just as an example, look at these queries issued in direct sequence to a caching resolver server:
$ dig @127.0.0.1 www.google.com +noall +answer ; <<>> DiG 9.9.4-P2-RedHat-9.9.4-15.P2.fc20 <<>> @127.0.0.1 www.google.com +noall +answer ; (1 server found) ;; global options: +cmd www.google.com. 275 IN A 184.108.40.206 www.google.com. 275 IN A 220.127.116.11 www.google.com. 275 IN A 18.104.22.168 www.google.com. 275 IN A 22.214.171.124 www.google.com. 275 IN A 126.96.36.199 www.google.com. 275 IN A 188.8.131.52 $ dig @127.0.0.1 www.google.com +noall +answer ; <<>> DiG 9.9.4-P2-RedHat-9.9.4-15.P2.fc20 <<>> @127.0.0.1 www.google.com +noall +answer ; (1 server found) ;; global options: +cmd www.google.com. 274 IN A 184.108.40.206 www.google.com. 274 IN A 220.127.116.11 www.google.com. 274 IN A 18.104.22.168 www.google.com. 274 IN A 22.214.171.124 www.google.com. 274 IN A 126.96.36.199 www.google.com. 274 IN A 188.8.131.52 $
From what it sounds like you are trying to achieve, I don't think adding multiple
A records is part of the solution. It will at best give you a crude form of load balancing but never any kind of reliable failover.
If you run your own name server using BIND this is configured with the rrset-order parameter
rrset-order defines the order in which multiple records of the same type are returned.
There are three options,
fixed - records are returned in the order they are defined in the zone file random - records are returned in a random order cyclic - records are returned in a round-robin fashion
See details here, http://www.zytrax.com/books/dns/ch7/queries.html#rrset-order