IPsec - Site to Site tunnel

Site to site VPNs connect two locations with static public IP addresses and allow traffic to be routed between the two networks. This is most commonly used to connect an organization’s branch offices back to its main office, so branch users can access network resources in the main office.

Before you start

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

Note

For the sample we will use a private IP for our WAN connection. This requires us to disable the default block rule on wan to allow private traffic. To do so, go to Interfaces ‣ [WAN] and uncheck “Block private networks”. (Dont forget to save and apply)

../../_images/block_private_networks.png

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	172.10.1.1/24
LAN Net	192.168.1.0/24
LAN IP	192.168.1.1/24
LAN DHCP Range	192.168.1.100-192.168.1.200

Site B

Hostname	fw2
WAN IP	172.10.2.1/24
LAN Net	192.168.2.0/24
LAN IP	192.168.2.1/24
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)

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	172.10.2.1	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

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:

Now you should see the following screen:

Step 2 - Phase 2 Site A

Press the button + in front of the phase 1 entry to add a new phase 2.

General information

Mode	Tunnel IPv4	Select Tunnel mode
Description	Local LAN Site B	Freely chosen description

Local Network

Local Network

LAN subnet

Route the local LAN subnet

Remote Network

Type	Network	Route a remote network
Address	192.168.2.0/24	The remote LAN subnet

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:

Enable IPsec for Site A, Select:

../../_images/ipsec_s2s_vpn_p1a_enable.png

Save:

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 we’ll address 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	172.10.1.1	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

Disable Rekey	Unchecked	Renegotiate when connection is about to expired
Disable Reauth	Unchecked	For IKEv2 only re-authenticate peer on rekeying
NAT Traversal	Disable	For IKEv2 NAT traversal is always enabled
Dead Peer Detection	Unchecked

Save your setting by pressing:

Now you should see the following screen:

Step 4 - Phase 2 Site B

Press the button + in front of the phase 1 entry to add a new phase 2.

General information

Mode	Tunnel IPv4	Select Tunnel mode
Description	Local LAN Site A	Freely chosen description

Local Network

Local Network

LAN subnet

Route the local LAN subnet

Remote Network

Type	Network	Route a remote network
Address	192.168.1.0/24	The remote LAN subnet

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:

Enable IPsec for Site B, Select:

Save:

And Apply changes:

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. Press on the (i) to see the details of the phase 2 tunnel(s), like this:

Note

If the tunnel did not come up, try to restart the service on both ends.

Sample configuration

For test purposes we used two OPNsense boxes integrated into one unit and a cross-cable between the WAN ports.

To route traffic the WAN interfaces have been configured to use a /16 segment and they are each others default gateway. Other than that the sample is equal to this how-to.

Configuration Site A: Config.xml Site A
Configuration Site B: Config.xml Site B

Trouble shooting

Phase 1 won’t come up

That is a difficult one. First check you firewall rules to see if you allow the right ports and protocols (ESP, UDP 500 & UDP 4500) for the WAN interface.

Check your ipsec log to see if that reviels a possible cause.

Common issues are unequal settings. Both ends must use the same PSK and encryption standard.

Phase 1 works but no phase 2 tunnels are connected

Did you set the correct local and remote networks. A common mistake is to fill in the IP address of the remote host instead of its network ending with x.x.x.0

Common issues are unequal settings. Both ends must use the same encryption standard.

Note

If you are testing locally with your pc connected to one of the two test boxes as in the sample configuration, then make sure you have no other network connections (Wi-Fi, for example).