one DNS names to multiple IP Addresses(Round Robin DNS)

[Joseph S D Yao]

On Wed, Sep 09, 2009 at 05:47:34PM +0100, Sam Wilson wrote:
> In article <mailman.450.1252511223.14796.bind-users at lists.isc.org>,
>  Balanagaraju Munukutla <9balan at sg.ibm.com> wrote:
> > Hi
> > 
> > Anybody can help to explain the side effect of configuring the DNS name to 
> > multiple IP addresses(Round Robin DNS).
> 
> If you're planning to use it for load sharing, then the effect is very 
> basic - requests get shared equally among the addresses irrespective of 
> load on the target system or whether the system is offering the service 
> or not.  If one of the target systems goes down then clients which are 
> directed to that system will either get rejected or time out, depending 
> on the type of failure.  You can mitigate this by using watchdog 
> scripts, short TTLs and dynamic DNS updates.
> 
> In short it's cheap and cheerful load balancing.  A large commercial 
> organisation might not want to rely on it, but depending on the 
> application it can work well enough.


There are several problems with using this for load balancing.

The first is, simply, it will not work unless the name server that is
authoritative for this zone is also your resolving name server.  If
there are ANY resolving name servers between the user and the
authoritative name server - as there usually is/are - then it's the
"round robin" policy - or lack thereof - of the last caching name server
before your stub resolver that will dictate how the addresses are
delivered.

Second, if one of the system goes down, then its IP address is still in
the rotation, again, unless some clever dynamic-DNS insertion and
deletion strategy is used.  This means that users will get frustrated
when their Web browser sometimes gets the Web site and sometimes
doesn't; or some automatic process that is trying to get your
information will not fail cleanly.

ISTM, it's better to try and do failover some other way, such as with
high-availability Linux, than to try to get DNS to do load balancing.


-- 
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** Joe Yao				jsdy at tux.org - Joseph S. D. Yao
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