[lug] Clustering for Load-Balancing and Fault-Tolerance??

Alan Robertson alanr at unix.sh
Mon Jan 28 21:32:29 MST 2002

Dave Anselmi wrote:
> Shannon Johnston wrote:
> > Hello all!
> > I'm looking for opinions here...
> > I need load-balancing, fault-tolerant DNS servers. (Not load-balancing
> > for http, but distributing DN resolution requests.) I've never worked
> > with clusters before so I would like to know where a good starting point
> > would be, and if anybody has any suggestions as to what to use.
> Do you really?  I suppose you could look at the sites mentioned and put a
> load balancing machine in front of several DNS servers.
> When you list several name servers as authoritative for a domain, I would
> guess that other name servers will pick between them at random.  You don't
> need any session sharing type fault tolerance because DNS only uses one
> packet each direction.
> The only thing clustering will buy you is that all the name servers could
> share one IP, so if one goes down there's no delay from asking it for a
> lookup.  Unless you're talking about load balancing recursive requests
> (i.e., from resolver clients rather than name servers).
> I'm curious what setup you have and why you think clustering is the way to
> go.

DNS has minimal useful fault tolerance.  When a server goes down, sites and
clients that have the dead server IP cached get the shaft.  People who use
this solution typically dislike the result.  Microsoft clients are
notoriously slow to bypass a dead server to go to the next one.  Delays of a
minute or more are not uncommon.  That's not very fault tolerant.

You don't actually need a load balancer, but you either need a load balancer
or IP address takeover.  Take your pick.  For just two machines, IP takeover
is probably the way to go.  For "n" machines, where "n" is a large number,
the load balancer is probably a better choice.

The CPU overhead from running one or both of these solutions is pretty
small.  You don't need to add more machines to do it.  It *is* more
complicated to configure, but it works in a much more satisfactory way.

If you're my ISP and I run microsoft clients, and you just let DNS fault
tolerance take care of it, I'm going to be very testy about these failures. 
If you use load balancing or IP failover, I won't even know.

	-- Alan Robertson
	   alanr at unix.sh

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