What should I do?

Dear Squidders,

I'm in charge of setting a proxy server for my corporate (ISP). Basically
our network look like this

        +--------+ +--------+
        | Site B |-------- 2M Link ---------| Site A |--->Internet
        | | | (HQ) |
        +--------+ +--------+

At the moment, we have one squid server running at SiteA which serve
around 400 dialin users and most of our leased line users. On SiteB we
have around 300 dialin users and some of our leased line users. We're
planning to install 1000(maybe more) more number at SiteB so we're now
consider install a proxy server at that site.

The problem is that, we allow all of our dialin users dial in to both
site. Currently we ask them to set a proxy preference of their browser to
proxy.siteA.my.dom. If we install a new proxy server at SiteB
(proxy.siteB.my.dom) It would be very inconvenience for users to change
their preference to proxy.siteB.my.dom whenever they dialin in to SiteB
and change to proxy.siteA.my.dom whenever they dialin to SiteA.

Does anyone know how to solve this problem if we don't want users to
change anything in their setting and still be able to use a nearest proxy
server for best performance?

I can think of 2 choice at the moment.

1. User Rotary DNS server. This maybe not a good choice since all leased
   line user at SiteA will get lower performance.

2. User Linux redirector feature. I've heard that we can config Linux to
   be a transparent proxy. Maybe I can adapt the same idea to my problem.

I'm sorry if this is not a proper group for this question. Please point me
to the proper group then. But I think some of you may have the same
network config as me and be able to help.

Any help would be greatly appreciate.

--
aet