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Re: Beyond DNS...

  • From: Daniel Reed
  • Date: Thu Jun 04 18:58:27 1998 
On Thu, 4 Jun 1998, Dean Anderson wrote:
) At 2:51 PM -0400 6/3/98, Phillip Vandry wrote:
) >> 2. Why Web services don't have entries like MX hosts do. Why not be able to
) >> set preferences and fall over hosts?
) Err, they do. Here's how to set it up:  These balance the load, and if one
) machine is down, any browser which handles multiple A records properly will
) still fail over to the other machine.
) 
) www             IN      CNAME   www1.av8.com.
)                 IN      CNAME   www2.av8.com.
That's a Very Bad Idea(tMS). CNAMEs are used as aliases, direct mappings.
For the same reason you can't CNAME a zone's root to something else
(meaning, if I am setting up the zone map for n.ml.org, I can CNAME
web.n.ml.org to narnia.n.ml.org, but I can't CNAME n.ml.org to
narnia.n.ml.org, nor can I CNAME n.ml.org to ftp.microsoft.com, or any
other host), you cannot use multiple CNAMEs. You can *physically* CNAME
multiple times, but you're likely to break things, as it's against the
rules. What you're in fact doing is aliasing one host to two different
ones, which doesn't make sense (because a CNAME says that ThisHost is
actually just a pointer to ThatHost).

) www1            IN      A       198.3.136.144
)                 IN      A       208.156.100.250
) www2            IN      A       208.156.100.250
)                 IN      A       198.3.136.144
That seems rather redundant. Just stick www1's IN A's into www, and forget
about www1 and www2 and funky multiple CNAMEing. There's no need to handle
rotations of the IN A's yourself--BIND, at least, does it all for you.

Do an nslookup on irc.dal.net a couple times, or us.undernet.org. Each
time you do it, the IP's are shifted over to the left by one. Hence, a
user doing /server us.undernet.org might get connected to
LasVegas.NV.US.Undernet.org, while the next person who does /server
us.undernet.org will get connected to Arlington.VA.US.Undernet.Org.

) (What? Your browser won't use multiple A records? Well, in that case, it
) probably will be a much longer time before it supports a new DNS record
) type.)
"Supporting" multiple IN A's shouldn't be necessary for a client to take
advantage of multiple IN A's. Since the IN A's are rotated, load balancing
is implemented at the nameserver level. The client should just always
connect to the first IN A given to it.

--
Daniel Reed <[email protected]> (ask me for my PGP key)
If a jury in a criminal trial stays out for more than twenty-four hours,
 it is certain to vote acquittal, save in those instances where it votes
 guilty -- Joseph C. Goulden