Choosing proxy on location

Hi. I have a situation where we have three offices. The main office has
a connection to the internet and the other two connect directly to this
office. We have always had a squid proxy in the main office and have
connected all offices through this one proxy.

Recently we have put a proxy server in each of the offices. What I am
after is a way to pick which proxy server a person connects to depending
on the office they are in, but this needs to be dynamic. We thought of
hard coding it to the local one, but there is a number of users that
travel between the offices, and if they connected back to the main proxy
it would harm the link between the offices.

Can you please give me some ideas of how to do this. I know it probably
could be done via auto configuration, but am not sure I would have the
necessary skill to implement this. I also thought of doing it via a dns
alias (have it pointing to the proxy server in each state depending on
which DNS server they connected to as this is dynamically assigned for
each office via DHCP, but this was ruled out due to replication done via
the server).

Any help would be appreciated.

Phillip Malone