Setup a routed IPSec Tunnel

Most Site-to-Site VPNs are policy-based, which means you define a local and a remote network (or group of networks). Only traffic matching the defined policy is pushed into the VPN tunnel. As the demands for more complex and fault tolerant VPN scenarios growed over the years, most major router vendors implemented a kind of VPN, the route-based IPSec.

The difference is that local and remote network is just 0.0.0.0/0, so anything can travel through the tunnel, it just needs a route. A new Virtual Tunnel Interface (VTI) has to be used for this.

There are two benefits for this kind of VPN:

First, you can set up two tunnels to the same gateway and failover when one line goes down. Second, you can run dynamic routing protocols over the tunnel to create more redundant, or software-defined networks.

Before you start

Before starting with the configuration of an IPsec tunnel you need to have a working OPNsense installation wit a unique LAN IP subnet for each side of your connection (your local network needs a different one than the remote network).

Sample Setup

For the sample configuration we use two OPNsense boxes to simulate a site to site tunnel, with the following configuration:

Site A

Hostname fw1
WAN IP 1.2.3.4/24
LAN IP 192.168.1.1/24
LAN DHCP Range 192.168.1.100-192.168.1.200





Site B

Hostname fw2
WAN IP 4.3.2.1/24
LAN Net 192.168.2.0/24
LAN DHCP Range 192.168.2.100-192.168.2.200





Full Network Diagram Including IPsec Tunnel

IPsec Site-to-Site tunnel network

Firewall Rules Site A & Site B (part 1)

To allow IPsec tunnel connections, the following should be allowed on WAN for on sites (under Firewall ‣ Rules ‣ WAN):

  • Protocol ESP
  • UDP Traffic on port 500 (ISAKMP)
  • UDP Traffic on port 4500 (NAT-T)
../../_images/ipsec_wan_rules.png

Note

You can further limit the traffic by the source IP of the remote host.

Step 1 - Phase 1 Site A

(Under VPN ‣ IPsec ‣ Tunnel Settings Press +) We will use the following settings:

General information

Connection method default Default is “Start on traffic”
Key Exchange version V2  
Internet Protocol IPv4  
Interface WAN Choose the interface connected to the internet
Remote gateway 4.3.2.1 The public IP address of your remote OPNsense
Description Site B Freely chosen description

Phase 1 proposal (Authentication)

Authentication method Mutual PSK Using a Pre-shared Key
My identifier My IP address Simple identification for fixed IP
Peer identifier Peer IP address Simple identification for fixed IP
Pre-Shared Key At4aDMOAOub2NwT6gMHA Random key. CREATE YOUR OWN!

Phase 1 proposal (Algorithms)

Encryption algorithm AES For our sample we will use AES/256 bits
Hash algoritm SHA512 Use a strong hash like SHA512
DH key group 14 (2048 bit) 2048 bit should be sufficient
Lifetime 28800 sec Lifetime before renegotiation

Advanced Options

Install Policy Unchecked This has to be unchecked since we want plain routing
Disable Rekey Unchecked Renegotiate when connection is about to expire
Disable Reauth Unchecked For IKEv2 only re-authenticate peer on rekeying
NAT Traversal Disabled For IKEv2 NAT traversal is always enabled
Dead Peer Detection Unchecked  

Save your setting by pressing:

../../_images/btn_save.png

Step 2 - Phase 2 Site A

Press the button that says ‘+ Show 0 Phase-2 entries’

../../_images/ipsec_s2s_vpn_p1a_show_p2.png

You will see an empty list:

../../_images/ipsec_s2s_vpn_p1a_p2_empty.png

Now press the + at the right of this list to add a Phase 2 entry. As we do not define a local and remote network, we just use tunnel addresses, you might already know from OpenVPN. In this example we use 10.111.1.1 and 10.111.1.2. These will be the gateway addresses used for routing

General information

Mode Route-based Select Route-based
Description Local LAN Site B Freely chosen description

Tunnel Network

Local Address Local Tunnel IP Set IP 10.111.1.1
Remote Address Remote Tunnel IP Set IP 10.111.1.2

Phase 2 proposal (SA/Key Exchange)

Protocol ESP Choose ESP for encryption
Encryption algorithms AES / 256 For the sample we use AES 256
Hash algortihms SHA512 Choose a strong hash like SHA512
PFS Key group 14 (2048 bit) Not required but enhanced security
Lifetime 3600 sec  

Save your settings by pressing:

../../_images/btn_save.png

Enable IPsec for Site A, select:

../../_images/ipsec_s2s_vpn_p1a_enable.png

Save:

../../_images/btn_save.png

And apply changes:

../../_images/ipsec_s2s_vpn_p1a_apply.png
../../_images/ipsec_s2s_vpn_p1a_success.png

You are almost done configuring Site A (only some firewall settings remain, which will be addressed later). We will now proceed setting up Site B.


Step 3 - Phase 1 Site B

(Under VPN ‣ IPsec ‣ Tunnel Settings Press +) We will use the following settings:

General information

Connection method Default Default is ‘Start on traffic’
Key Exchange version V2  
Internet Protocol IPv4  
Interface WAN Choose the interface connected to the internet
Remote gateway 1.2.3.4 The public IP address of your remote OPNsense
Description Site A Freely chosen description

Phase 1 proposal (Authentication)

Authentication method Mutual PSK Using a Pre-shared Key
My identifier My IP address Simple identification for fixed ip
Peer identifier Peer IP address Simple identification for fixed ip
Pre-Shared Key At4aDMOAOub2NwT6gMHA Random key. CREATE YOUR OWN!

Phase 1 proposal (Algorithms)

Encryption algorithm AES For our sample we will use AES/256 bits
Hash algoritm SHA512 Use a strong hash like SHA512
DH key group 14 (2048 bit) 2048 bit should be sufficient
Lifetime 28800 sec Lifetime before renegotiation

Advanced Options

Install Policy Unchecked This has to be unchecked since we want plain routing
Disable Rekey Unchecked Renegotiate when connection is about to expire
Disable Reauth Unchecked For IKEv2 only re-authenticate peer on rekeying
NAT Traversal Disabled For IKEv2 NAT traversal is always enabled
Dead Peer Detection Unchecked  

Save your setting by pressing:

../../_images/btn_save.png

Step 4 - Phase 2 Site B

Press the button that says ‘+ Show 0 Phase-2 entries’

../../_images/ipsec_s2s_vpn_p1a_show_p2.png

You will see an empty list:

../../_images/ipsec_s2s_vpn_p1a_p2_empty.png

Now press the + at the right of this list to add a Phase 2 entry.

General information

Mode Route-based Select Route-based
Description Local LAN Site A Freely chosen description

Tunnel Network

Local Address Local Tunnel IP Set IP 10.111.1.2
Remote Address Remote Tunnel IP Set IP 10.111.1.1

Phase 2 proposal (SA/Key Exchange)

Protocol ESP Choose ESP for encryption
Encryption algorithms AES / 256 For the sample we use AES 256
Hash algortihms SHA512 Choose a strong hash like SHA512
PFS Key group 14 (2048 bit) Not required but enhanced security
Lifetime 3600 sec  

Save your setting by pressing:

../../_images/btn_save.png

Enable IPsec for Site B, Select:

../../_images/ipsec_s2s_vpn_p1a_enable.png

Save:

../../_images/btn_save.png

And apply changes:

../../_images/ipsec_s2s_vpn_p1a_apply.png
../../_images/ipsec_s2s_vpn_p1a_success.png

Firewall Rules Site A & Site B (part 2)

To allow traffic passing to your LAN subnet you need to add a rule to the IPsec interface (under Firewall ‣ Rules ‣ IPsec).

../../_images/ipsec_ipsec_lan_rule.png

IPsec Tunnel Ready

The tunnel should now be up and routing the both networks. Go to VPN ‣ IPsec ‣ Status Overview to see current status.

Step 5 - Define Gateways

Now that you have the VPN up and running you have to set up a gateway. Go to System ‣ Gateways ‣ Single and add a new gateway.

Gateway Site-A

Name VPNGW Set a name for your gateway
Interface IPSEC1000 Choose the IPsec interface
IP Address 10.111.1.2 Set the peer IP address
Far Gateway Checked This has to be checked as it is a point-to-point connection

Gateway Site-B

Name VPNGW Set a name for your gateway
Interface IPSEC1000 Choose the IPsec interface
IP Address 10.111.1.1 Set the peer IP address
Far Gateway checked This has to be checked as it is a point-to-point connection

Step 5 - Add Static Routes

When gateways are set up you can add a route for the remote network pointing to the new gateway. On Site-A add a route to Site-B and vice versa. Go to System ‣ Routes ‣ Configuration.

Route Site-A

Network Address 192.168.2.0/24 Set the network of Site-B
Gateway VPNGW Select the VPN gateway

Gateway Site-B

Network Address 192.168.1.0/24 Set the network of Site-A
Gateway VPNGW Select the VPN gateway

Now you are all set!