Change of IP - Update time

[Jeff Lasman]

Peter Pfannenschmid wrote:

> I have the following problem: I would like to use a name server (which
> is running fine and without problems) for some sort of dynamic DNS.
> Let's assume we have the host test.mydomain.com with IP address
> a.b.c.50; if I change this to a.b.c.51 and reconfigure the DNS server,
> there may be a delay up to two days until some client at the other end
> of the world gets the changed IP address when querying the host name.

There are several reasons for the delay:

1) Your TTL for the record in question is overly long.

2) Your end-user is (probably unbeknownst to him) cacheing your DNS
either in his client program (browser) or operating system.

3) Your end-user is on a network that's cacheing (again, probably
unbeknownst to him) your DNS.

4) your end-user is getting DNS from a nameserver that caches DNS for
overly long time periods ignoring your TTL.

Of these, you can only control #1.

> I suspect that the reason for this is the fact that multiple name
> servers can be involved in the resolution of a client's query, and
> every one of them could have the old IP address cached. So it will
> take some time until the new IP address is propagated. Please correct
> me if I am wrong here.

See my notes above.

> On the other hand, there are services like dyndns.org which are
> specialized in this kind of stuff; these guys manage to propagate a
> changed IP address (a changed DNS record) throughout the world within
> five minutes after the changes have happened on their DNS servers. My
> question is: How do they achieve this?

They, like you, can only control #1.

> I am planning to experiment with the TTL and other parameters of my
> zone file. But before I do so, I would like to hear the opinions of
> the experts, since a too low TTL time may be considered as bad
> practice by the community.

If you want to serve dynamic DNS you have no option than to use a low
TTL, perhaps only seconds.  As you can see from reading my points above,
that won't help in all situations.

Dynamic DNS is a tradeoff at best, and only works even reasonably well
for most people when the dynamic IP# is changed only occasionally.

> So what? Is there any method to achieve my goal, or should I set the
> TTL parameter to five minutes?

Perhaps even less.

> If the latter is the case, is there a
> way for changing the TTL of the record for only one certain host of a
> domain (other than making a new zone)? I am using bind 9.1.3.

Yes... read the fine manual <smile>.

Here's a hint, using my own domain name and a fictitious host:

fictitious.nobaloney.net.	1M IN A		63.108.93.101

The above example would give fictitious.nobaloney.net a ttl of one
minute.

Jeff
-- 
Jeff Lasman, nobaloney.net, P. O. Box 52672, Riverside, CA  92517 US
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