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.

Note

For a stable setup, we highly advise using standard IPv4 / IPv6 addresses, although the web interface allows the use of fully qualified domain names (e.g. my.own.domain.xyz), this will have side affects (the tunnel device won’t react on name changes for example).

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:

Network Site A

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

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Network Site B

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

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Full Network Diagram Including IPsec Tunnel

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)

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:

Step 2 - Phase 2 Site A

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

You will see an empty list:

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:

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Enable IPsec for Site A, select:

Save:

And apply changes:

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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.

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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:

Step 4 - Phase 2 Site B

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

You will see an empty list:

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:

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Enable IPsec for Site B, Select:

Save:

And apply changes:

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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).

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!