OpenVPN Site To Site

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This article is part of the How-To series.

Connecting two sites using OpenVPN is very simple. One side is configured as a client, and the other as a server. OpenVPN in shared key mode is the recommend method for site to site connections, unless there are a half dozen or more sites. For PKI and advanced options/configurations see OpenVPN Site-to-Site PKI (SSL) and the pfSense book.

The instructions below are for basic site to site connectivity. For other modes such as SSL/TLS, or remote access, look in the Category:OpenVPN OpenVPN category of the doc wiki.

Info

Both IPSec and OpenVPN may be enabled/in use at the same time, however, not for the same subnets. Any IPSec tunnel that references the same pair of subnets configured for use in OpenVPN must be disabled. IPSec and OpenVPN do not conflict otherwise.

The way OpenVPN works is that one end of the tunnel needs to be the “server” and the other the “client”, it does not matter which, though if there is more than one remote site, the main "hub" site should be the server. If one site has a dynamic IP address and the other has a static IP address, then the static IP address end should be the server.

A firewall rule must be created on the Server to allow traffic through to the interface and port where the server is running, otherwise the traffic will be blocked and the VPN will fail to connect. To allow or filter incoming traffic inside the VPN tunnel, add rules to the OpenVPN tab under Firewall > Rules.

For more than 6 site to site connections, SSL/TLS (PKI) can be a better fit for ease of management. Both methods work and require a fair amount of configuration on the server for each site, but there is lower overhead with SSL/TLS since it runs a single server process.

Server Settings

Navigate to VPN > OpenVPN, Servers tab (default tab). Click "+" to create a new OpenVPN server.

Use the following options: Server Mode : Peer to Peer (Shared Key)

Protocol : UDP - TCP is undesirable because every lost packet is retransmitted, and if TCP is used inside the tunnel it will be retransmitted anyway. This will slow down the VPN if there is a lot of lost traffic on the WAN connection. TCP is really only useful to bypass firewalls, in which case the port should be set to 443 as almost no one blocks access to that port. The protocol must match on each side.

Device Mode : tun

Interface : Whichever interface server will use for incoming connections. Typically WAN, but may be an OPT WAN.

Local Port : The port this OpenVPN server will listen on. 1194 is the default OpenVPN port. Each server requires a unique port. Make sure not to use a port in use by another service otherwise problems can occur. If using port 443, ensure the WebGUI is not running on that port first.

Description : A name for this VPN. Shows up in various places where the VPN can be selected from a list, such as Status > Services, or Diagnostics > Packet Capture.

Shared key : The keys can be made in the GUI. Check "Automatically generate a shared key.", and when the settings are saved, a key will be generated. Then copy/paste the key into the client.

Encryption algorithm : This setting must match on both sides. Any of the crypto options are fine, it depends on the user preference. If using ALIX hardware, use aes-128-cbc, see Are cryptographic accelerators supported For most others, aes-256-cbc is a good choice.

Hardware Crypto : If the device has hardware crypto support, choose it from this list. For ALIX and many others, use BSD cryptodev engine to use supported onboard devices.

IPv4 Tunnel Network : The suggested default in the GUI of 10.0.8.0/24 is sufficient, but any random unused network inside of the RFC1918 space is recommended. For site-to-site shared key, only a /30 is used, not a /24, even if /24 is specified.

IPv4 Remote networks : Enter the remote (Client Side) LAN here. To access more than one network, add them all here separated by a comma (e.g. 10.10.10.0/24, 192.168.10.0/24).

Compression : Set this if to compress data on the tunnel. If bulk data or encrypted protocols like https/ssh are used primarily, this may only add unnecessary overhead. Enabled with Adaptive Compression is a good compromise as it will periodically test the compression of data and enable/disable as needed when it can help.

Type-of-Service : Set the TOS IP header value of tunnel packets to match the encapsulated packet value. Useful for traffic shaping on the OpenVPN traffic itself, but it does expose some data about the contents of the packet so it is a potential security risk.

Firewall Rules : Don't forget to add a firewall rule on the WAN tab under Firewall > Rules (or whichever interface the server is running on) to allow traffic to reach the OpenVPN server's IP:port where it is listening. Also don't forget rules on the OpenVPN tab to allow traffic inside the tunnel.

Client Settings

Navigate to VPN > OpenVPN, Clients tab. Click "+" to create a new OpenVPN client.

Server Mode : Peer to Peer (Shared Key)

Protocol : Match the setting from the server side.

Device Mode : tun

Interface : Interface to use for outbound traffic. Typically WAN, but may be an OPT WAN.

Local Port : Leave this blank for a random port. The port this OpenVPN client will use for its side (source port). Each process requires a unique port. Make sure not to use a port in use by another service otherwise problems can occur. Leaving this box blank is best.

Server host or address: IP (203.0.113.19) or FQDN (vpn.example.com)

Server Port: The port used by the OpenVPN server.

Description : A name for this VPN.

Shared key : Copy/paste the key from the server.

Encryption algorithm : Match the setting from the server side.

Hardware Crypto : Hardware specific. See the same setting in the server section.

IPv4 Tunnel Network : Match the setting from the server side.

IPv4 Remote networks : Enter the remote (Server Side) LAN here. To access more than one network, add them all here separated by a comma (e.g. 10.10.9.0/24, 192.168.9.0/24).

Compression : Match the setting from the server side.

Type-of-Service : See the same setting in the server section.

Firewall Rules : Don't forget to add rules to Firewall > Rules on the OpenVPN tab to allow traffic inside the tunnel.