How to use the same network on both sides of an IPsec tunnel

Problem:

I want to establish an IPsec tunnel to a remote office, but the local network there conflicts with the local network at the central office. How can i solve this problem without changing the network on either side?

Solution:

This is possible to solve by address translating the network on both sides to something else when these networks need to talk to each other. Lets say that the conflicting network is 192.168.30.0/24, we create a “fake” network that only exists between these 2 sites. So instead of connecting to the remote host 192.168.30.50 we connect to 172.16.1.50. This modification can be done automatically.

How to accomplish this.

Example:

Main Office Network/IPs:

• Local Network: 192.168.30.0/24
• External IP: 1.1.1.1

Remote Office Network/IPs:

• Local Network: 192.168.30.0/24
• External IP: 2.2.2.2

Main Office IPsec:

• Name: Stockholm_IPsec
• Local Network: 172.16.1.0/24
• Remote Network: 172.16.2.0/24
• Remote Endpoint: 2.2.2.2
• Keep "Add route statically" enabled

Main Office route:

• [core] 172.16.1.0/24

Remote Office IPsec:

• Name: Gothenborg_IPsec
• Local Network: 172.16.2.0/24
• Remote Network: 172.16.1.0/24
• Remote Endpoint: 1.1.1.1
• Keep "Add route statically" enabled

Remote Office route:

• [core] 172.16.2.0/24

Main Office Policy Outbound:

• Action:Allow
  Interface:Lan
  Network:"Lan_net"
  Destination Interface:Stockholm_IPsec
  Destination Network:"Fake_Remote_Network"
  Service:All-Services
  Source Translation:Sat
  Address Action:Transposed
  Base IP Address:172.16.1.0

Main Office Policy Inbound:

• Action:Allow
  Interface:Stockholm_IPsec
  Network:"Fake_Remote_Network"
  Destination Interface:any
  Destination Network:"Fake_Local_Network"
  Service:All-Services
  Source Translation:Nat
  Address Action:Outgoing Interface Address
  Destination Translation:Sat
  Address Action:Transposed
  Base IP Address:192.168.30.0

Note: You can not select i.e 172.16.1.0/24 as Source or destination network on the SAT rule’s address translation tab, you need to type 172.16.1.0 and it will translate correctly from the correct source IP and destination IP. 192.168.30.50 becomes 172.16.1.50 etc.

Note-2: The reason why we [Core] route the 172.16.x.x network is because this network will not exist behind any physical interface. It exists in the Core only (so to speak).

The same rules but the other way around on the remote office.

This enables the same network to still exist on both sides, when clients want to connect to hosts on beyond the IPsec tunnel they use the 172.16.x.x address instead of 192.168.x.x and thus we have bypassed the problem and there is no need to change the local network on either side.

Example flow:

Host 192.168.30.93 on the Main office wants to reach an FTP server on the Remote Office. This FTP server has the IP 192.168.30.55 on the remote office. Host 192.168.30.93 then connects to 172.16.2.93 which will traverse the IPsec tunnel then address translated from 172.16.2.93 to 192.168.30.93 in order to match the machine on the remote office local network.

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