How to connect two offices into a single local network?

Question

linux

Hello.
The question is not original, but I could not find a solution.
There are two parts of the local network in different cities, with internet (different providers, so the VLAN is cancelled). The local area network of city 1 has DHCP, DNS, domain controller server, file server, mail server, VPN server (Windows) and workstations. In the city, 2 workplaces are simply connected to a local network and there is a computer with two network cards running Debian Wheesy (connected to the Internet through an external router).
In city 2, I managed to set up a VPN tunnel (via the pptp-linux utility) on a Debian machine and get access to all the resources of the local network of city 1. The question is how to create a network bridge between the second NIC and the VPN tunnel in order to did all the other machines in city 2 become part of the LAN in city 1?
I tried to combine a second network card and a VPN connection using bridge-utils, but apparently this was not the right approach at all.
I tried to create a network bridge using iptables:

iptables -A FORWARD -i eth0 -o ppp0 -j ACCEPT
iptables -A FORWARD -i ppp0 -o eth0 -j ACCEPT

Did not work. Apparently due to the fact that machines in city 2 do not have their own network, but must receive settings from city 1.
I found a solution to this problem using OpenVPN, but it is not possible to install an OpenVPN server in city 1.
On the Debian machine, the eth1 network card gets its settings via DHCP from the router, the eth0 network card has no settings, the VPN connection is ppp0.
Knowledge on this issue is sorely lacking, tell me at least in which direction to move?
Thank you.

Answer 1

You need either EoIP to get a network at the L2 level (but without encryption), or IPSec in tunnel mode to get a network at the l3 level (but without general DHCP and with different addressing), or IPSec in transport mode plus an EoIP tunnel built on top of this transport - get a secure flat network, including with possible general addressing.

Answer 2

And what prevents in the city 2 to use the subnet?
Everything will become easier.