Setup SSL VPN 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.

Note

When using the site to site example with SSL/TLS instead of a shared key, make sure to configure “client specific overrides” as well to correctly bind the remote networks to the correct client.

Before you start

Before starting with the configuration of an OpenVPN SSL tunnel you need to have a working OPNsense installation with a unique LAN IP subnet for each side of your connection (your local network needs 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”. (Don’t forget to save and apply)

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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 - Server

Hostname	fw1
WAN IP	172.10.1.1/16
LAN IP	192.168.1.1/24
LAN DHCP Range	192.168.1.100-192.168.1.200
Tunnel Network	10.10.0.0/24

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Network Site B

Site B - Client

Hostname	fw2
WAN IP	172.10.2.1/16
LAN Net	192.168.2.0/24
LAN DHCP Range	192.168.2.100-192.168.2.200
Tunnel Network	10.10.0.0/24

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Full Network Diagram Including SSL VPN Tunnel

SSL VPN Site-to-Site tunnel network

Step 1 - Add SSL Server

Adding a new SSL VPN server is relatively simple. We’ll start by adding a server that uses a shared key. This setup offers a good protection and it is easy to setup.

Go to VPN ‣ OpenVPN ‣ Servers and click on click Add in the top right corner of the form.

For our example will use the following settings (leave everything else on its default):

Server Mode	Peer to Peer (Shared Key)
Protocol	UDP
Device Mode	tun
Interface	WAN
Local port	1194
Description	SSL VPN Server
Shared Key	Leave on enabled (checked) to create a new key
DH Parameters Length	4096
Encryption algorithm	AES-256-CBC (256-bit)
Auth Digest Algorithm	SHA512 (512-bit)
IPv4 Tunnel Network	10.10.0.0/24
IPv4 Local Network/s	192.168.1.0/24
IPv4 Remote Network/s	192.168.2.0/24
Compression	Enabled with Adaptive Compression

Click Save to add the new server.

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Step 2 - Copy Shared Key

To copy the newly created shared key, click on the pencil icon next to the newly created SSL VPN server.

You will see the shared key, copy this and keep it safe!

Sample key:

#
# 2048 bit OpenVPN static key
#
-----BEGIN OpenVPN Static key V1-----
0960c87c3aafa8f306fe270c1564380b
7922543563a17b5d2636b4ef9412dd09
9ad44974ca1b293963e0f8ac9cbdd97c
2c31bf35f0df45c9e928ccb033e6d51d
2caaec02d649ad081c68d7bc7d28030e
9182c9597a83024097bea860e52d9c66
1b9e0048fbf951ce8659bc56edb7f9a1
14f7740fc9231a3750557e02eb112712
ac4b9980d4c740ec96a4357f3940ed90
d1bbf8eed3de135c886fe2eff8e8b943
ab1f52b59def4c9ebeacc5eb48425189
c43887a6237c29e0724f5f45a0f70635
10680bec8bfb67c21bf2b4866268594c
9ba093668064f9a898e6a6ad103b401d
b2047132f0dc8db2230db38444d689fa
ddba46bf6f892ae90c59415f94b82750
-----END OpenVPN Static key V1-----

Step 3 - Server Firewall Rules

To allow SSL VPN client connections, we should allow access to the OpenVPN server port on the WAN interface. When using multiple servers we need to open up each port.

For our configuration we only use one server accessible on UDP port 1194.

Next we also need to allow traffic from the VPN client network (192.168.2.0/24). For our example we will allow client to access anything on our local network(s), however you may decide just to allow traffic to one or more IPs.

You are done configuring Site A.

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Step 4 - Site B Client

Now we will have to setup the client. Login to the second firewall, go to VPN ‣ OpenVPN ‣ Clients and click on add client in the upper right corner of the form.

Now enter the following into the form (and leave everything else default):

Server Mode	Peer to Peer (Shared Key)
Protocol	UDP
Device Mode	tun
Interface	WAN
Server host or address	172.10.1.1
Server port	1194
Description	SSL VPN Client
Shared Key	Uncheck to paste the shared key
…	Paste your shared key
Server Certificate	SSLVPN Server Certificate (CA: SSL VPN CA)
DH Parameters Length	4096
Encryption algorithm	AES-256-CBC (256-bit)
Auth Digest Algorithm	SHA512 (512-bit)
Hardware Crypto	No Hardware Crypto Acceleration
IPv4 Tunnel Network	10.10.0.0/24
IPv4 Remote Network/s	192.168.1.0/24
Compression	Enabled with Adaptive Compression

Now click on Save to apply your settings.

The Connection Status can be viewed under VPN ‣ OpenVPN ‣ Connection Status

Step 5 - Client Firewall Rules

To allow traffic from the remote network just add a rule under Firewall ‣ Rules OpenVPN tab.

Done