2nd site for instant backup

Kevin Darcy kcd at daimlerchrysler.com
Wed Feb 6 00:46:37 UTC 2002

DNS offers no *good* solutions to this problem...

Approach "A" would be to have www.mysite.com resolve to both
IP addresses. Most web browsers are smart enough to fail over to other
addresses, after a delay, if the first one is unavailable. The big
drawback of this approach is that you'll get a lot of "spillover" to the
backup server even when the main server is available. You can minimize
this spillover to a certain degree by using a "fixed" rrset-order (see
"rrset-order" in the BIND documentation) on all of your authoritative
servers, but even so, you'd have to reduce your TTL values to something
ridiculously small, which will waste a lot of query traffic and
nameserver/network resources.

Approach "B" is to have the name resolve only to the main webserver
normally and then change it to point to the backup webserver if the main
one goes down. Again, to make this work acceptably, you have to reduce
your TTL values and cause waste. Approach "B" doesn't leverage the
ability of some clients to fail over (as Approach "A" does), and it also
requires some sort of intervention in the case of failover, as well as a
"fall-forward" step when the main webserver is back in operation. This
makes it generally less manageable than Approach "A", but possibly a
better fit if many of your clients don't have failover capability and/or
if the length of the failover delay is unacceptable in your environment.

- Kevin

Chris Clock wrote:

> Hi,
> My friend's company's Web site, www.mysite.com, is currently hosted in
> an ISP's server.  They would like to duplicate this site in their
> office as the 2nd site for instant backup.  That is if there is any
> problem on the site in ISP's server, the connection of www.mysite.com
> will go to the site in their office.  Is there any solution or
> software to set this up ?
> Thank you.
> Chris

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