Configuring the FortiGate unit
There are several steps to the GRE-over-IPsec configuration:
- Enable overlapping subnets. This is needed because the IPsec and GRE tunnels will use the same addresses.
- Configure a route-based IPsec VPN on the external interface.
- Configure a GRE tunnel on the virtual IPsec interface. Set its local gateway and remote gateway addresses to match the local and remote gateways of the IPsec tunnel.
- Configure security policies to allow traffic to pass in both directions between the GRE virtual interface and the IPsec virtual interface.
- Configure security policies to allow traffic to pass in both directions between the protected network interface and the GRE virtual interface.
- Configure a static route to direct traffic destined for the network behind the Cisco router into the GRE-over-IPsec tunnel.
Enabling overlapping subnets
By default, each FortiGate unit network interface must be on a separate network. The configuration described in this chapter assigns an IPsec tunnel end point and the external interface to the same network. Enable subnet overlap as follows:
config system settings
set allow-subnet-overlap enable end
Configuring the IPsec VPN
A route-based VPN is required. It must use encryption and authentication algorithms compatible with the Cisco equipment to which it connects. In this chapter, preshared key authentication is shown.
To configure the IPsec VPN – web-based manager
1. Define the Phase 1 configuration needed to establish a secure connection with the remote Cisco device. Enter these settings in particular:
Name Enter a name to identify the VPN tunnel, tocisco for example. This is the name of the virtual IPsec interface. It appears in Phase 2 configurations, security policies and the VPN monitor.
Remote Gateway Select Static IP Address.
IP Address Enter the IP address of the Cisco device public interface. For example, 192.168.5.113.
Local Interface Select the FortiGate unit’s public interface. For example, 172.20.120.141.
Mode Select Main (ID Protection).
Authentication Method Preshared Key
Pre–shared Key Enter the preshared key. It must match the preshared key on the Cisco device.
Advanced Select the Advanced button to see the following settings.
Phase 1 Proposal 3DES–MD5
At least one proposal must match the settings on the Cisco unit.
For more information about these settings, see Phase 1 parameters on page 1624.
2. Define the Phase 2 parameters needed to create a VPN tunnel with the remote peer. For compatibility with the Cisco router, Quick Mode Selectors must be entered, which includes specifying protocol 47, the GRE protocol. Enter these settings in particular:
Phase 2 Proposal 3DES–MD5
At least one proposal must match the settings on the Cisco unit.
Quick Mode Selector
Source Address Enter the GRE local tunnel end IP address.
For example 172.20.120.141.
Source Port 0
Destination Address Enter the GRE remote tunnel end IP address.
For example 192.168.5.113.
Destination Port 0
Protocol 47
For more information about these settings, see Phase 2 parameters on page 1642.
3. If the Cisco device is configured to use transport mode IPsec, you need to use transport mode on the FortiGate VPN. You can configure this only in the CLI. In your Phase 2 configuration, set encapsulation to transport-mode as follows:
config vpn phase2-interface edit to_cisco_p2
set encapsulation transport-mode end
To configure the IPsec VPN – CLI
config vpn ipsec phase1-interface edit tocisco
set interface port1
set proposal 3des-sha1 aes128-sha1 set remote-gw 192.168.5.113
set psksecret xxxxxxxxxxxxxxxx end
config vpn ipsec phase2-interface
edit tocisco_p2
set phase1name “tocisco” set proposal 3des-md5
set encapsulation tunnel-mode // if tunnel mode
set encapsulation transport-mode // if transport mode
set protocol 47
set src-addr-type ip
set dst-start-ip 192.168.5.113 set src-start-ip 172.20.120.141
end
Adding IPsec tunnel end addresses
The Cisco configuration requires an address for its end of the IPsec tunnel. The addresses are set to match the GRE gateway addresses. Use the CLI to set the addresses, like this:
config system interface edit tocisco
set ip 172.20.120.141 255.255.255.255 set remote-ip 192.168.5.113
end
Configuring the GRE tunnel
The GRE tunnel runs between the virtual IPsec public interface on the FortiGate unit and the Cisco router. You must use the CLI to configure a GRE tunnel. In the example, you would enter:
config system gre-tunnel edit gre1
set interface tocisco
set local-gw 172.20.120.141 set remote-gw 192.168.5.113
end
interface is the virtual IPsec interface, local-gw is the FortiGate unit public IP address, and remote-gw is the remote Cisco device public IP address
Adding GRE tunnel end addresses
You will also need to add tunnel end addresses. The Cisco router configuration requires an address for its end of the GRE tunnel. Using the CLI, enter tunnel end addresses that are not used elsewhere on the FortiGate unit, like this:
config system interface edit gre1
set ip 10.0.1.1 255.255.255.255 set remote-ip 10.0.1.2
end
Configuring security policies
Two sets of security policies are required:
- Policies to allow traffic to pass in both directions between the GRE virtual interface and the IPsec virtual interface.
- Policies to allow traffic to pass in both directions between the protected network interface and the GRE virtual interface.
To configure security policies – web-based manager
1. Define an ACCEPT firewall security policy to permit communications between the protected network and the GRE
tunnel:
Incoming Interface Select the interface that connects to the private network behind this FortiGate unit.
Source Address All
Outgoing Interface Select the GRE tunnel virtual interface you configured.
Destination Address All
Action ACCEPT
Enable NAT Disable
2. To permit the remote client to initiate communication, you need to define a firewall address security policy for communication in that direction:
Incoming Interface Select the GRE tunnel virtual interface you configured.
Source Address All
Outgoing Interface Select the interface that connects to the private network behind this FortiGate unit.
Destination Address All
Action ACCEPT
Enable NAT Disable
3. Define a pair of ACCEPT firewall address security policies to permit traffic to flow between the GRE virtual interface and the IPsec virtual interface:
Incoming Interface Select the GRE virtual interface. See Configuring the GRE tunnel on page 1792.
Source Address All
Outgoing Interface Select the virtual IPsec interface you created. See Configuring the IPsec VPN on page 1790.
Destination Address All
Action ACCEPT
Enable NAT Disable
Incoming Interface Select the virtual IPsec interface you created. See Configuring the IPsec VPN on page 1790.
Source Address All
Outgoing Interface Select the GRE virtual interface.See Configuring the GRE tunnel on page 1792.
Destination Address All
Action ACCEPT
Enable NAT Disable
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