check Availability before sending response

[Sam Wilson]

In article <gh67eu$2rjm$1 at sf1.isc.org>,
 Stephane Bortzmeyer <bortzmeyer at nic.fr> wrote:

> On Wed, Dec 03, 2008 at 10:53:43PM +0800,
>  Ken DBA <mysqld at yahoo.cn> wrote 
>  a message of 21 lines which said:
> 
> > ie, given the domain name www.site.com was pointed to 1.1.1.1 and
> > 2.2.2.2 in Bind.  When a client query for www.site.com, Bind will
> > check the health status for these two servers. If one is
> > unavailable,Bind shouldn't direct client's requests to it.
> 
> How BIND could:
> 
> * Know what protocol to test? www.site.com is probably for HTTP but
> mail.site.com ? POP ? IMAP ?
> 
> * Embed all these protocols? HTTP, HTTPS, POP, IMAP, BitTorrent, DNS,
> whois, FTP, SSH, SMTP...

And how could the nameserver know whether the server was accessible from 
the client, even if it was accessible from the nameserver, and vice 
versa?

Sam