On 2013-11-13 00:16, Manish Rane wrote:
...
6.Assume if ISP1 goes down, client coming on ISP1 would never be able
to reach; hence as per DNS protocol will try for another link and
come
on ISP2 and then probably get an IP address of Link 2 i.e. 2.2.2.2.
...
I'm not sure about your DNS setup, because I didn't understand how you
described it. But that doesn't matter.
Even if you 100% properly did what you intended to do, it breaks down
at step 6. The DNS protocol definitions only go as far as saying what
your BIND DNS server will return. Importantly (for this answer), it
does NOT say (a) what a remote user's caching/resolving name server will
actually do with your responses, or (b) what the actual application will
do with your responses.
If the application is an SMTP server or another DNS server then, yes,
BY THE DEFINITION OF THAT PROTOCOL, it will try again for another
server.
If the application is a Web browser - which is likely, given that you
mention port 80, presumably TCP - then it will only look at one of the
two IP addresses [for almost all currently available Web browsers]. If
it gets a bad one, it will return the user an error. Because that is
how THAT protocol is defined. Most protocols are not defined to re-try
different servers.
What you are trying to do is what the F5 BigIP GTM does - only return
the IP address for a known-working site. There's a reason that F5 can
sell those boxes - they work where doing this in pure DNS does not.
Joe Yao
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