Advanced Round Robin Implementation

[Barry Margolin]

In article <37E2D28C.ABF0173F at gbl.com.br>,
Rodrigo Meirelles  <rodrigo at gbl.com.br> wrote:
>
>    Hi friends!
>
>    I have a bad feeling about something i'm trying to implement.
>
>    I have to do a kind of backup system using DNS.
>    I have a domain "eclipse.com.br" and its www is configured that way
>below :
>
>    www    IN    A    x.y.z.w
>    www    IN    A    x.y.z.w2
>
>I want that when somebody try to access www just the first
>entry(x.y.z.w) answer, and the second entry(x.y.z.w2) just answer the
>query when the first were down.
>  Is it possible or not ?

Not with DNS.  Round robin simply reorders the responses each time, so some
clients will get them in the order x.y.z.w,x.y.z.w2, while others will get
them in the order x.y.z.w2,x.y.z.w.  Some applications try all the
addresses they get until they succeed in connecting, but many just try the
first one and give up.

>  Because i think that yahoo, microsoft and others, for example, must
>have many
>servers to answer the dns queryes, but when one of them is down the
>address must
>not be the answer from a query, because the request won't get responde
>and will return a "host down" status.

Are you talking about DNS servers, or web servers?  Multiple DNS servers
are usually implemented by having multiple NS records pointing to different
hostnames.  DNS servers try all the NS records until succeess.

If you're talking about web servers, the most common solution is to use a
multiplexer like Cisco Local Director.  This is a NAT-like device that has
one IP address and forwards connections to several real server addresses.
It monitors the servers, and if one of them goes down it stops forwarding
connections to it.

-- 
Barry Margolin, barmar at bbnplanet.com
GTE Internetworking, Powered by BBN, Burlington, MA
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