Can one mirror a web server ?

[John Wobus]

My "news" in this area is old, but as you might imagine, DNS servers
that do this have been created and are used extensively.
F5's 3-DNS is a commercial product to do this.
The technique can be and is done, but can present a variety of problems,
and the best solution needs to fit the situation.  For obvious reasons, 
static
and other stateless websites have the fewest issues.

My recollection from a few years ago is that browsers typically reused
name/number associations for thirty minutes after they got them 
(ignoring TTLs),
which meant that someone using a site right before the fail-over
might find they can't reach it for up to 30 minutes after the fail-over
unless they reset their browser somehow.  Depending upon who
uses your site and for what, this might be a big issue for you.  It 
seems
like an obvious area for browser makers to improve their software
so perhaps current browsers do better, but if you depend upon that,
you still may consider the consequences of ignoring those who
don't run the latest browser software.

To give your site maximum redundancy, you typically use such DNS
methods along with BigIP/Localdirector type load balancing.

John Wobus

On Feb 3, 2005, at 1:54 AM, Jim Popovitch wrote:

> On Wed, 2005-02-02 at 20:17 -0800, Pete Tenereillo wrote:
>> Jim, I agree the default TCP timeout of 1 minute on Netscape/Firefox 
>> seems
>> high... a good question to post over there. I also agree that  "works 
>> like a
>> charm" was stretching it. Still better than counting on BGP (or 
>> nothing)
>> though, wouldn't you agree?
>
> Yes, it does beat nothing.   I'm toying with the idea of creating a 
> Bind
> wrapper (listens on external interface, queries Bind on localhost) that
> will perform a test (http/ftp/icmp/smtp/etc) to verify queries before
> responding.  It could even cache statuses of tests so that inline
> latency is near zero.  Now, if I could find a way to do it all based on
> DNS TXT records I think that I'll have a winner.  Any input?  Anyone?
> Bueller?
>
> -Jim P.
>
>>
>> ----- Original Message -----
>> From: "Jim Popovitch" <jimpop at yahoo.com>
>> To: "Pete Tenereillo" <pt_bind at hotmail.com>
>> Cc: "Kevin Darcy" <kcd at daimlerchrysler.com>;
>> <comp-protocols-dns-bind at isc.org>
>> Sent: Wednesday, February 02, 2005 7:10 PM
>> Subject: Re: Can one mirror a web server ?
>>
>>
>>> On Wed, 2005-02-02 at 18:16 -0800, Pete Tenereillo wrote:
>>>> Kevin, why do call out the browser multiple A record failover 
>>>> process as
>>>> "slow"? Slow relative to what? It takes between 12 and 21 seconds 
>>>> with IE
>>>> at
>>>> the default settings (clearly the most common, time depends on 
>>>> version),
>>>> and
>>>> a minute or so with Netscape (and I think FireFox). In neither case 
>>>> is a
>>>> "site unavailable" type dialog box displayed to the user. It just
>>>> silently
>>>> connects. I thought that was the intent of multiple A records??? 
>>>> Seems to
>>>> me
>>>> it works like a charm.
>>>
>>> Working like a charm is a bit of a stretch with Firefox on Linux 
>>> (kudos
>>> to IE for getting it right).  Firefox seems to wait at least a 
>>> minute or
>>> more, and then forgets the IP address that worked and retries from 
>>> the
>>> top of the list when navigating to another page on the same hostname.
>>> Firefox makes it impossible to consider it working or charming. ;-)  
>>> I
>>> will however retract my earlier statement that this yields a page
>>> timeout.
>>>
>>> -Jim P.
>>>
>>>
>>>
>
>