Help for some Rules

Help for some Rules

[Michael Novelo]

Our Firewall-Administrator is on Vacation for the next 4 Weeks. Now i have
the Problem that he installed a new Firewall (Endian) but he has forgoten to
set certain Rules:

The Main-Office want to get Access to out network without setting on all
Machines a route:

 
Our internal Network is 192.168.171.xxx 
Firewall Adress: 192.168.171.99 
ISDN-Router(Gateway to the Main-Office): 192.168.171.80

Main-Office Network: 192.168.1.xxx

 
now the Main Office want to access our network via an ISDN Router they
dialin in a bintec bingo router which has 192.168.171.80 as ip adress. I
have set a route for their network(route add -net 192.168.1.0 netmask
255.255.255.0 gw 192.168.171.80) everything works fine for me in our
network. i can send a ping to the servers on their network. traceroute works
also fine on all of our machines. But the Main-Office want to get access to
our Machines the only two ways which are working are  
1.) i set on every machine a route via route add 
2.) i change the default Gateway to 192.168.1.80

Can sombody help me to set some rules to route all traffic who comes from
the MainOffice Network the right way?

Greetings 

Novelo