redirection and nat

[Payal Rathod <payal-iptables@staticky.com>]

Hi,
In my friend's office where I am trying to put a Linux server, I am
facing some problems.

They have connectivity from 2 ISPs thru' Router A and Router B.
Now in office people from 192.168.10.1 - 192.168.10.7 use Router B to
reach internet and the rest Router A.

I want Linux to be put in such a way so that it accepts connections from
all machines and redirects them to router A and B. Before this they will
be checked against some rules like whether they are allowed to access
external SMTP or not etc. Is this setup possible?

How will I use redirection here? I can redirect for a particular port
but am not getting the syntax for directing any package to particular
router's ip.

Do I need NAT here? If so, kindly tell the logic I have to use this with
NAT. In NAT I have to specify an interface like,
iptables -t nat -A POSTROUTING -o eth0 -j MASQUERADE

But here I will be using 2 interfaces. What will I use then in above
rule? And what should be my default gateway? Does it bind to a
particular ip? BTW, I will be using squid too here. Please remember this
too.

Thanks a lot and eagerly waiting for any input on this.

With warm regards,
-Payal