On the fly TTL modification on a cache DNS server

[Francois Goudal]

Hello,

I'm currently making a linux software for an avionics embedded router.
The charge for each minute is about 10$ so I made some things for the 
user to disable and block any outgoing connections.
I would like, when it is blocked, every http connection to be redirected 
to the internal webserver, to display a splashscreen.
On currently existing softwares that provides splashscreens, it is much 
easier cause the DNS resolution can be done because the connection is 
permanent.
Here, I will have to "hack" the DNS resolution for this redirection.
Currently, bind is installed as a DNS cache server on the router.
I would like it to serve also as a nameserver that will reply to every 
request it gets without referring to another DNS server, always replying 
the router's local IP address.
Currently, I have made some successful tests about that but, my problem 
is about TTLs.
When for example internet is disabled and i want for example to access 
google.com, it is resolved as 192.168.1.1 (the router's address), so I 
can see the splashscreen.
Now if I activate the internet, I can't go to google.com cause it's not 
resolved again (in the client cache, it's still 192.168.1.1.
I solved this problem, now, the TTL is quite small and it works.
But :
When internet is enabled, the bind just acts as a dns cache server so if 
I get google.com, it is resolved as its real IP and put in the client's 
cache (this TTL is provided by google's nameservers). So if I close the 
connection, now the client still refers to google's real IP address so 
the splashscreen doesn't appears as it should.
I would like the bind cache to do an on the fly alteration of the DNS 
answers, to set a very low TTL for the client's answers.
Do you know a way to do that ?

Thank's !

-- 
Francois Goudal
Epita promo 2008 - Ing2 - President Evolutek
francois at goudal.net