load-balancing in DNS using two A records

Kevin Darcy kcd at chrysler.com
Thu Dec 22 18:52:03 UTC 2011

On 12/20/2011 1:22 PM, Matus UHLAR - fantomas wrote:
> On 20.12.11 19:37, Martin T wrote:
>> I have seen setups where one domain name has two address records.
>> First IP address is in the ISP-A network and the other one is in the
>> ISP-B network. In case I execute "host www.<domainname>.com", I always
>> get two IP addresses as a reply and they always appear by turns. Am I
>> correct, that setup like this provides redundancy as well as
>> load-balancing?
> Kind of. It's much better to have real load-balancing and vailover by 
> multiple links or L3 load balancers.
>> Is there some common method in BIND to give out IP
>> addresses by turns? Last but not least, how do application layer(for
>> example www, ssh) handle such setup?
> bind usually gives all possible addresses for a name in random order. 
> You can affect this a bit by using "sortlist" statement, where you can 
> tell BIND which address to prefer for which client (and, intermediate 
> server may re-sort according to its knowledge)
Just be aware, Wintel clients often choose addresses 
out-of-received-sequence according to their notion of "subnet 
prioritization" (older OSes) and/or RFC 3484 logic (newer ones), thus 
effectively overriding any sortlisting you do on the BIND side.

     - Kevin

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