[ubuntu-uk] Setting up mailserver

[Sean Miller]

On Sun, Dec 28, 2008 at 10:31 AM, Martin Meredith <mez at ubuntu.com> wrote:
> Dyndns etc provide services that allow you to setup a domain name that will
> change it's DNS when contacted by you.
>
> Put simply, a dynamic DNS service needs to be told every time your IP address
> changes. When this happens, it updates the record for your domain name to point
> to the new address. Generally, these use a low TTL (time to live) so that DNS
> servers at, for example, your ISP, will query their DNS servers (with the
> up-to-date record for your IP address) on a regular basis, rather than caching
> it (which is why you generally find when you make DNS changes on a "control
> panel" style site, it says that you may have to wait up to 48 hours - this is
> for the TTL to expire, so the DNS server checks against the server holding the
> record, rather than their internal cache)

I think we have two completely different things here.

One is the concept of DNS propogation, but the other is what DynDNS
does which is forward traffic to a given IP.

There is no need for low TTL etc. in the forwarding scenario... the
nameservers do not change... they are still DynDNS.    By doing that
they can instantly change what IP address the site is hosted on,
because it completely bypasses the whole DNS thing and goes straight
to the server.    The nameservers (which have to propogate) are
DynDNS's, but once in place they'll stay there regardless of TTL or
whatever else.

Think of it as a person sitting in a Tourist Information Office.  You
phone her and say "I've moved my B&B to this new address, Gladys" and
she then tells folks where to go.  DNS propogation is more like
word-of-mouth... "did you realise that B&B has moved Mrs Smith?" "well
I never!"...

Sean