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From "Ben Yau" <>
Subject RE: [users@httpd] backup www server
Date Tue, 02 Mar 2004 23:19:43 GMT

> I like the second idea a lot, so if I get what you mean correctly I have
> currently have this for my www server in my dns zone file:
>           IN A
> I would simply add:
>          IN    A    yyy.yyy.yyy.yyy
> Thanks
> Matt.

I would recommend caution with  this.  It is a great idea for clustering.
First, realize that both machines wlil be serving pages so it isn't true
failover.  If you have the resources for both machines to serve, then that's

However, if one server goes down, and you have to take it down, realize that
some requests will still attempt to go to the old IP because of the way DNS

If you do this, two more things to keep in mind:
1) if one goes down, you need the other to still take over the IP of the old
one via a virtual IP or having a second NIC already installed and ready to
take over the IP.


2) set the TTL in the DNS to be somehting very short so that DNS servers do
not cache the IP for very long (this wll put more load on your network and
DNs server)

Clustering (not using DNS) is a better solution.  I haven't used mod_backend
(which the prevoius poster suggested).  However, many firewalls do
clustering.  I used to work with Netscreen which did a great job of
clustering.   You could cluster multiple internal IP behind one extenral IP
so DNS always pointed user browsers to the one external IP.  Behind the
scenes at any time you could add/remove internal clients from the external
IP cluster.  I imagine most firewalls are able to do this kind of
combination of load-balancing/clustering/NAT


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