Re: [cobalt-users] IMPORTANT Double DNS entries

[Jeff Lasman <jblists@xxxxxxxxxxxxx>]

At 04:18 PM 3/3/00 +0100, you wrote:

> Therefore he has setup his dns so that all hostnames (e.g. www) are pointing
> to the old AND the new ips, e.g.
>
>         www.domain.com                A record                old.ip.is.here
>         www.domain.com                A record                new.ip.is.here
>
> Now he has problems but it is difficult to say if the datacenter (the old,
> the move is coming in the middle of march) makes the problems or these
> entries.

The problems are ocurring because of the way DNS works.  Whoever advised 
him to do it this way had no idea how it works, I'd imagine.

Here's what the O'Reilly book, "DNS and Bind" says about this example:

First the example from the book:

> ;
> ; Multi-home hosts
> ;
> wormhole.movie.edu.    IN A   192.249.249.1
> wormhole.movie.edu.    IN A   192.253.253.1

Now the text from the book:

> _wormhole_ acts as a router.  It has two addresses associated with its 
> name and therefore two address records.  Unlike host table lookups, a DNS 
> lookup can return more than one address for a name.  A lookup of 
> _wormhole_ will return two.  If the requestor and name server are on the 
> same network, some name servers will place the "closest" address first in 
> the response for better performance.  This feature is called 
> _address_sorting_ and is covered in Chapter 10, 
> _Advanced_Features_and_Security_.  If address sorting does not apply, the 
> addresses are _rotated_ between queries so subsequent responses list them 
> in a different order.  This "round robin" feature shows up first in BIND 4.9.

See the problem?  Every other lookup to DNS is getting the address that's 
not working yet.  It's not a bug.  It's a feature.

There are ways to minimize downtime when moving a server.  They're not all 
that effective though, now that big ISPs routinely ignore TTL headers.

What you need is a good understanding of DNS, and a well-thumbed (by you) 
copy of O'Reilly's "DNS and Bind".

Or a good DNS consultant <big grin>.

Or someone else (us?) to host your DNS and do it right <bigger grin>.

Jeff

--
Jeff Lasman <jblists@xxxxxxxxxxxxx>