load-balancing in DNS using two A records

[Mark Andrews]

In message <20111220182222.GA3085 at fantomas.sk>, Matus UHLAR - fantomas writes:
> 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)
> 
> When one of those ip fails, you can expect half of your connections to 
> such host fail, and it's up to the client how to handle this situation. 
> 
> Long time ago when we were trying to have multiple web servers for 
> redundancy and balancing, we have found that multiple IP's is not a 
> good solution (parts of web pages didn't load). We selected L3 
> switches then...

Which is really the result of badly designed clients.  Clients are getting
better with address affinity and fast failover on unreachable servers.
 
> Different situation is when you have multiple providers and want to use 
> multiple uplinks with different IPs for the same servers. While this 
> can work with some NAT playing, it should be better to ger your 
> provider-independent address space (if possible) and use separate 
> uplinks. That gives you much better line saturation.
> -- 
> Matus UHLAR - fantomas, uhlar at fantomas.sk ; http://www.fantomas.sk/
> Warning: I wish NOT to receive e-mail advertising to this address.
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-- 
Mark Andrews, ISC
1 Seymour St., Dundas Valley, NSW 2117, Australia
PHONE: +61 2 9871 4742                 INTERNET: marka at isc.org