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¶

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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

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

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.

See also

Troubleshooting OpenVPN.