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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

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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 fa-certificate on the row for the CA to export its certificate

  • Navigate to System > Cert Manager, Certificates tab

  • Click fa-certificate on the row for each client certificate to export the certificates

  • Click fa-key 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 fa-plus 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 fa-pencil 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 fa-plus 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 fa-level-up 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 fa-level-up 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 fa-plus 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 fa-level-up 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.

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