dns question

On Fri, Mar 31, 2000 at 12:15:58AM +0000, arson smith wrote:
> I have a linux server running dns and would like
> to have it switch from one server to another if
> one goes down. basicly have www.x.com point
> to x.x.x.21 normaly but if x.x.x.21 goes down then
> dns reasings www.x.com to x.x.x.20. Do I need
> to do this in DNS or is there something else I
> would need to do this on? I am not a supper dns
> guru so I am not sure where to look.
>
> I am worried about this be cause the x.x.x.21 and
> x.x.x.20 are nt boxes (not my choice) but we need the
> fail over for them.

Because most DNS servers out on the internet at large cache their entries,
it can take days for a DNS change that you make to take effect, even if your
DNS server is the authoritative one for the domain.  So it's probably
easier to have www.x.com point to a load-balancing "dispatcher" server,
which most of the time redirects half the traffic to www1.x.com and the other 
half to www2.x.com, but if one is down then the other gets all the traffic.
Users hit www.x.com and it sends an http redirect to whichever server.  
However I'm not sure what software to use to do this.  If all else fails
a CGI script could send the redirect, based on a config file which is 
written to by a cron job that is frequently checking the two servers to
see if they're up.  But I'm sure there is a prepackaged solution too; most
likely there is an apache module to do round-robin redirect.  homebid.com
(where I work) has an arrangement like this but they're doing it the 
expensive way with Solaris and probably some commercial load-balancing
thingy.
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