OpenVPN Site-to-Site Configuration Example with SSL/TLS

OpenVPN Site-to-Site Configuration Example with SSL/TLS
A site-to-site connection using SSL/TLS in client/server mode is convenient for managing a large number of remote sites connecting back to a central site in a hub-and-spoke fashion.
Example Configuration Overview
OpenVPN Example Site-to-Site SSL/TLS Network
When configuring a site-to-site OpenVPN connection using SSL/TLS one firewall will be the server and the others will be clients.
Tip
Usually the main location will be the server and the remote offices will act as clients, though if one location has a static IP address and more bandwidth than the main office that may be a more desirable location for the server.
This style of VPN requires a dedicated subnet for the OpenVPN interconnection between networks in addition to the subnets on both ends. Figure OpenVPN Example Site-to-Site SSL/TLS Network shows a depiction of this layout, using 
10.3.101.0/24 as the IPv4 VPN Tunnel Network. This can be any subnet so long as it does not overlap another subnet currently in use on the network.
OpenVPN allocates IP addresses the same way it does for remote access clients. When using a Topology style of subnet, each client obtains one IP address in a common subnet. When using a Topology style of net30, each connecting client gets a /30 subnet to interconnect itself with the server.
See also
The subnet topology style uses address space more efficiently and has less quirks with its behavior in general, but certain very old clients may not be compatible. See Topology for more details.
The following sections describe how to configure the server and client sides of the connection.
Example Configuration Settings
OpenVPN Endpoint Settings - Site A - Server
Site A - Server
Name
Austin Office
WAN Address
198.51.100.3
LAN Subnet
10.3.0.0/24
LAN Address
10.3.0.1
CA Name
S2SCA
Cert CN
serverA
Tunnel Net
10.3.101.0/24
OpenVPN Endpoint Settings - Site B - Client
Site B - Client
Name
London Office
Cert CN
clientB
WAN Address
203.0.113.5
LAN Subnet
10.5.0.0/24
LAN Address
10.5.0.1
OpenVPN Endpoint Settings - Site C - Client
Site C - Client
Name
Colorado Office
Cert CN
clientC
WAN Address
198.51.100.7
LAN Subnet
10.7.0.0/24
LAN Address
10.7.0.1
Configuring SSL/TLS Server Side
The server requires two items to reach the networks behind each client:
A 
route to tell the operating system that OpenVPN knows about a remote network
An internal route (
iroute) in an OpenVPN Client-Specific Override to tell OpenVPN how to route that subnet to a specific client certificate
More detail on this will follow in the example.
See also
Client Specific Overrides
Troubleshooting OpenVPN Internal Routing (iroute)
Tunnel Settings
Create Certificate Structure
The first step is to create a certificate structure for this VPN.
This example uses the names listed in Example Configuration Settings – The CA is named 
S2SCA, the Server CN is named 
serverA, and the clients are 
clientB and 
clientC.
See also
Certificate Management
Create a Certificate Authority
Create a CA unique to this VPN:
Navigate to System > Cert Manager, CAs tab
Click Add to create a new a CA
Enter the settings as follows:
Descriptive Name
S2SCA
Method
Create an internal Certificate Authority
Randomize Serial
Checked
Key Type
RSA, 2048 (or higher)
Digest Algorithm
sha256 (or higher)
Lifetime (days)
3650
Common Name
S2SCA
Subject Component Fields
The remaining fields are optional, but can be set to reflect the location of the CA.
Click Save
Create a Server Certificate
Create a server certificate signed by the VPN CA:
Navigate to System > Cert Manager, Certificates tab
Click Add to create a new certificate
Enter the settings as follows:
Method
Create an internal Certificate
Descriptive Name
serverA
Certificate Authority
S2SCA
Key Type
RSA, 2048 (or higher)
Digest Algorithm
sha256 (or higher)
Lifetime (days)
398
Note
Some current operating systems and software limit server certificates to a maximum lifetime of 
398 days for security reasons. Clients on these platforms may reject a server certificate with a longer lifetime.
Common Name
serverA
Subject Component Fields
The fields contain data copied from the CA and are optional, but can be set to reflect the location of the server.
Certificate Type
Server Certificate
Warning
This setting is critical, do not forget to set this value.
Alternative Names
Optional extra entries, if needed, which specify alternate ways to identify the server. This can be left blank if the certificate will only be used by OpenVPN. Otherwise, add fields with additional information such as alternate hostnames, static IP addresses, and so on which are relevant to this server.
Click Save
Create User Certificates
Create user certificates for each remote site signed by the VPN CA.
Navigate to System > Cert Manager, Certificates tab
Click Add to create a new certificate
Enter the settings as follows:
Method
Create an internal Certificate
Descriptive Name
clientB
Certificate Authority
S2SCA
Key Type
RSA, 2048 (or higher)
Digest Algorithm
sha256 (or higher)
Lifetime (days)
3650
Common Name
clientB
Subject Component Fields
The fields contain data copied from the CA and are optional, but can be set to reflect the location of the client.
Certificate Type
User Certificate
Warning
This setting is critical, do not forget to set this value.
Alternative Names
Optional extra entries which specify alternate ways to identify the client. These can be left blank if the certificate will only be used by OpenVPN. Otherwise, add fields with additional information such as alternate hostnames, static IP addresses, and so on which are relevant to this client.
Click Save
Repeat this process for every client (e.g. 
clientC and any future clients).
Export Certificates
The next task is to export the certificates and keys which the client requires when connecting to the OpenVPN server.
Navigate to System > Cert Manager, CAs tab
Click 
 on the row for the CA to export its certificate
Navigate to System > Cert Manager, Certificates tab
Click 
 on the row for each client certificate to export the certificates
Click 
 on the row for each client certificate to export the private key for the client certificates.
Warning
Do not export the CA key, server certificate, or server key. The client does not need these and copying them unnecessarily significantly weakens the security of the VPN.
Configure the OpenVPN Server Instance
Navigate to VPN > OpenVPN, Servers tab
Click 
 Add to create a new server
Fill in the fields as described below, with everything else left at defaults.
Use values appropriate for this network, or the defaults if unsure.
See also
See Server Configuration Options for details on each of these options.
Description
Enter text to describe the connection, e.g. 
Site-to-Site VPN.
Server Mode
Peer to Peer (SSL/TLS)
DCO (Plus Only)
Check this box to activate the OpenVPN Data Channel Offload (DCO) feature for the server if desired.
See also
See OpenVPN Data Channel Offload (DCO) for additional information.
Device Mode
tun
Protocol
UDP on IPv4 only
Interface
WAN
Local Port
1194
TLS Configuration
Check the Use a TLS Key box to enable TLS authentication which provides protection for the tunnel control channel.
Leave Automatically generate a TLS Key checked so the firewall will generate a new key automatically the first time this entry is saved.
Peer Certificate Authority
Select the CA created at the beginning of this process (
S2SCA)
Peer Certificate Revocation List
Select a CRL for the CA, if one exists.
Server Certificate
Select the server certificate created at the beginning of this process (
serverA)
IPv4 Tunnel Network
Enter the chosen tunnel network, 
10.3.101.0/24
IPv4 Local Network(s)
Enter the LAN subnets for all sites including the server: 
10.3.0.0/24, 10.5.0.0/24, 10.7.0.0/24
Note
If there are more networks on the server side that clients need to reach, such as networks reachable via static routes, other VPNs, and so on, add them as additional entries in the IPv4 Local Network box.
IPv4 Remote Network(s)
Enter only the client LAN subnets: 
10.5.0.0/24, 10.7.0.0/24
Inactive
0 to disable disconnecting idle clients, so that site-to-site connections can stay up indefinitely.
Click Save.
Click 
 to edit the new server instance
Find the TLS Authentication box
Select all of the text inside
Copy the text to the clipboard
Save this to a file or paste it into a text editor such as Notepad temporarily
Create Client-Specific Overrides
Now add Client Specific Overrides for each client site. These tie a client subnet to a particular certificate so that OpenVPN can properly route a subnet to the correct site.
Navigate to VPN > OpenVPN, Client Specific Overrides tab
Click 
 to add a new override
Fill in the fields on this screen as follows:
Common Name
Enter the CN of the first client site. In this example, that is 
clientB.
IPv4 Remote Network/s
The clientB LAN subnet, 
10.5.0.0/24.
Note
This field sets up the internal route (
iroute)
Click Save
Add an override for the second site, adjusting the Common Name and IPv4 Remote Network to match. In the example for site C, these values would be 
clientC and 
10.7.0.0/24 respectively.
See also
Client Specific Overrides
Troubleshooting OpenVPN Internal Routing (iroute)
Tunnel Settings
Firewall Rules
External Traffic (WAN)
Next, add a firewall rule for the WAN interface which allows access to the OpenVPN server.
Navigate to Firewall > Rules, WAN tab
Click 
 Add to create a new rule at the top of the list
Set the options as follows:
Protocol
UDP
Source
any (since multiple sites must connect)
Tip
For extra security, if the clients have static IP addresses, create an alias containing these addresses, then set it as the source on this rule.
Destination
WAN Address
Destination port
1194
Description
OpenVPN Multi-Site VPN
Click Save
Click Apply Changes
Tunneled Traffic
Now add a rule to the OpenVPN tab to pass traffic over the VPN from the Client-side LAN to the Server-side LAN. This can be an “Allow all” style rule or a set of stricter rules. This example allows all traffic using this rule:
Navigate to Firewall > Rules, OpenVPN tab
Click 
 Add to create a new rule at the top of the list
Set the options as follows:
Protocol
any
Source
any
Tip
For extra security, create an alias containing only the remote hosts or subnets which must initiate contact with hosts on the sever LAN, then use that alias as the source on this rule.
Destination
any
Tip
For extra security, create an alias containing only the local hosts or subnets on the server LAN which must accept connections from remote hosts across the VPN, then use that alias as the destination on this rule.
Description
Allow all on OpenVPN
Click Save
Click Apply Changes
That completes the server setup, next, now move on to configure the clients.
Configuring SSL/TLS Client Side
Import CA and Certificate
On the client, import the CA certificate along with the client certificate and key for that site. This is the same CA and client certificate created earlier in this document.
See also
Certificate Management
Import these items at System > Cert Manager.
First import the CA:
Navigate to System > Cert Manager, CAs tab
Click Add to create a new certificate authority
Enter the settings as follows:
Descriptive Name
S2SCA
Method
Import an existing Certificate Authority
Certificate Data
Open the CA certificate file in a text editor on the client PC, select all of the text, and copy it to the clipboard. Then paste it into this field.
Click Save
Next, import the client certificate:
Navigate to System > Cert Manager, Certificates tab
Click Add to create a new certificate
Enter the settings as follows:
Method
Import an existing Certificate
Descriptive Name
clientB VPN Certificate
Certificate Type
X.509 (PEM)
Certificate Data
Open the client certificate file in a text editor on the client PC, select all of the text, and copy it to the clipboard. Then paste it into this field.
Private Key Data
Open the client certificate private key in a text editor on the client PC, select all of the text, and copy it to the clipboard. Then paste it into this field.
Click Save
Repeat these steps on each client firewall.
Configure the OpenVPN Client Instance
After importing the certificates, create the OpenVPN client:
Navigate to VPN > OpenVPN, Client tab
Click 
 Add to create a new client
Fill in the fields as follows, with everything else left at defaults:
See also
See Client Configuration Options for details on each of these options.
Description
Text to describe the connection (e.g. 
Site A VPN)
Server Mode
Peer to Peer (SSL/TLS)
DCO (Plus Only)
Check this box to activate the OpenVPN Data Channel Offload (DCO) feature for the client if desired.
See also
See OpenVPN Data Channel Offload (DCO) for additional information.
Device Mode
tun
Protocol
UDP on IPv4 only
Interface
WAN
Server host or address
The public IP address or hostname of the OpenVPN server (
198.51.100.3 in this example)
Server Port
1194
Enable authentication of TLS packets
Checked
Automatically generate a shared TLS authentication key
Unchecked
TLS Key
Paste in the TLS key copied from the server instance
Peer Certificate Authority
The CA imported at the beginning of this process
Client Certificate
The client certificate imported at the beginning of this process
Click Save
Note
With remote access PKI configurations such as this example, routes and other configuration options are typically pushed from the server and thus not present in the client configuration. If the client side must reach additional networks, configure them in the server settings or a client-specific override as Local Networks.
Firewall Rules
This next step is optional depending on whether or not hosts on the server network or other client sites need to initiate contact with hosts on the client network. If the other sites do not need to initiate contact with this client, then no action is necessary.
If the other sites needs to initiate contact, then this traffic requires a firewall rule on the OpenVPN tab on the client firewall to allow traffic from other VPN sites to reach the Client-side LAN. An “Allow all” style rule is OK in some cases, but a set of stricter rules is the best practice.
This example allows all traffic:
Navigate to Firewall > Rules, OpenVPN tab
Click 
 Add to create a new rule at the top of the list
Set the options as follows:
Protocol
any
Source
any
Tip
For extra security, create an alias containing only the remote hosts or subnets which must initiate contact with hosts on the client LAN, then use that alias as the source on this rule.
Destination
any
Tip
For extra security, create an alias containing only the local hosts or subnets on the client LAN which must accept connections from remote hosts across the VPN, then use that alias as the destination on this rule.
Description
Allow all on OpenVPN
Click Save
Click Apply Changes
Testing the Connection
The configuration is now complete. The OpenVPN client instance automatically starts when created, so it should already be attempting to connect at this point and if the configuration is correct, it will be connected.
Try to ping across to the remote end LAN to verify connectivity.