Dynamic DNS (address takeover-type-things)
andy at globalauctions.com
Mon Oct 19 22:06:34 MDT 1998
On Mon, 19 Oct 1998 alanr at bell-labs.com wrote:
> Let me elaborate:
> In DNS one can have a round-robin scheme so that when a user requests a particular
> machine (like www.netscape.com), that it will be redirected to one of a set of
> machines in a round-robin fashion.
Technically, it's not a "redirection"... more like a direction. :-)
> In Dynamic DNS, one can dynamically change the
> mapping of machine names to IP addresses. Ultimately, if we add a machine to or
> remove a machine from a cluster served by this kind of DNS addressing, we would
> want to be able to update the DNS round-robin list as well.
BIND had code in place, nearly from the beginning, to allow dynamic
updates. However, it always used to be marked "don't use this" or something
like that. Haven't looked at it in about a year though.
I know that there are, out there, miniature DNS servers that do
load-balanced round-robining, with one even being written in (urgh) Perl.
You tell your mail DNS server that this special server is authoritative for
that particular name (or set of names) and the rest takes care of itself.
Two important points, though...
The first is that DNS is highly cached. That is to say, other DNS servers
are caching your answer (albeit for as long as you tell them to) so that you
cannot effect an immediate change. Maybe something more like 15 minutes
would be effective.
The second is, in your given example (web service) you have a problem in
that browsers cache IP addresses practically indefinitely. They are not
aware of (or choose to ignore) the TTL specified in the DNS data and cache
the IP address as long as they feel like (which could be days in my
I guess my point is that an IP fail-over is still a critical part of any
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