Virtual Private Networking

A virtual private network secures public network connections and in doing so it extends the private network into the public network such as internet. With a VPN you can create large secure networks that can act as one private network.

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(picture from wikipedia)

Companies use this technology for connecting branch offices and remote users (road warriors).

OPNsense supports VPN connections for branch offices as well as remote users.

Creating a single secured private network with multiple branch offices connecting to a single site can easily be setup from within the graphical user interface. For remote users, certificates can be created and revoked and a simple to use export utility makes the client configuration a breeze.

OPNsense offers a wide range of VPN technologies ranging from modern SSL VPNs to well known IPsec as well as WireGuard and Zerotier via the use of plugins.

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IPsec

Since IPsec is used in many different scenario’s and sometimes has the tendency to be a bit complicated, we will describe different usecases and provide some examples in this chapter.

General context

The IPsec module incorporates different functions, which are grouped into various menu items. Since the start of our project we have been offering IPsec features based on the legacy ipsec.conf format, which we are migrating to swantcl.conf as of version 23.1. While migrating the existing featureset we came to the conclusion that the world has changed quite a bit and in order to offer better (api) access to the featureset available we decided to plan for deprecation of the legacy “Tunnel settings” as they have existed since we started. No timeline has been set, only a feature freeze on tunnels using the “Tunnel settings” menu item.

One of the main goals for the long run is to better align the gui components so they reflect the reality underneath, as we use strongswan, our aim is to follow their terminology more closely than we previously did.

The following functions are available in the menu (as of OPNsense 23.1):

  • Connections

    • New configuration tool offering access to the connections and pools sections of the swanctl configuration

  • Tunnel Settings

    • Legacy IPsec configuration tool

  • Mobile Clients

    • Offering access to various options of the attr plugin and pool configurations for legacy tunnels

  • Pre-Shared Keys

    • Define secrets to be used for local authentication.

  • Key Pairs

    • For public key authentication collect public and private keys.

  • Advanced Settings

    • Define passthrough networks (to exclude from kernel traps), logging options and some generic options

  • Status Overview

    • Shows tunnel statusses

  • Lease Status

    • For mobile clients, show address leases for various pools configured

  • Security Association Database

    • Shows security associations, the fundamental concept of IPsec describing a relationship between two or more entities

  • Security Policy Database

    • Installed security policies describing which traffic is allowed to pass a tunnel

  • Virtual Tunnel Interfaces

    • Edit or create new if_ipsec(4) interfaces and show the ones created by legacy tunnels

  • Log File

    • Inspect log entries related to IPsec

Note

When migrating Pre-Shared Key type tunnels to connections, make sure to add an entry in the “Pre-Shared Keys” module as well. If both ends should use their own identifier, fill in both local and remote values. The legacy module requested this information in the phase 1 page and wrote the same information to the secrets.

Security policies and routing

In order to pass traffic over an IPsec tunnel, we need a policy matching the traffic. By default when adding a phase 2 (or child) policy a “kernel route” is installed as well, which traps traffic before normal routing takes place.

Note

Without a policy in place for the tunnel, traffic won’t be accepted, in case a policy with a kernel route overlaps a local or locally routed network the traffic will not be received by the host in question.

Tip

When matching overlapping networks in a policy (VTI or overlapping networks), make sure to exclude your own network segments in the Passthrough networks option in VPN -> IPsec -> Advanced Settings to prevent traffic being blackholed.

Implementation schemes

When setting up IPsec VPNs there are two main types of scenario’s with their own advantages and disadvantages.

Policy based

The first one is the standard policy based tunnel, which guards the security of the tunnel with policies and installs kernel traps to send traffic over the tunnel in case it matches these policies. For example a local network 192.168.1.0/24 sending traffic to a remote location responsible for 192.168.2.0/24. The advantage of this scenario is the ease of setup, no routes are needed to be configured, when in this example 192.168.1.10 contacts 192.168.2.10 the packets are seamlessly forwarded over the tunnel to the remote location.

When local traffic doesn’t match the policies in question due to the tunnel needing Network Address Translation, that’s also possible as long as policies are manually added to the security policy database, this is also referred to as “NAT before IPsec”.

Route based (VTI)

Route based, also known as VTI, tunnels are using a virtual interface known as if_ipsec(4), which can be found under VPN -> IPsec -> Virtual Tunnel Interfaces. This links two ends of the communication for routing purposes after which normal routing applies. The “(Install) Policies” checkmark needs to be disabled in this case for the child (phase 1 in the legacy tunnel configuration) definition. Usually the communication policy (phase 2 or child) is set to match all traffic (either 0.0.0.0/0 for IPv4 or ::/0 for IPv6).

So the same example as the policy based option would need (static) routes for the destinations in question (192.168.1.0/24 needs a route to 192.168.2.0/24 and vice versa), peering happens over a small network in another subnet (for example 10.0.0.1 <-> 10.0.0.2) bound to the tunnel interface.

The advantage of this type of setup is one can use standard or advanced routing technologies to forward traffic around tunnels.

Note

In order to filter traffic on the if_ipsec(4) device some tunables need to be set. Both net.inet.ipsec.filtertunnel and net.inet6.ipsec6.filtertunnel need to be set to 1 and net.enc.in.ipsec_filter_mask and net.enc.out.ipsec_filter_mask need to be set to 0 in order to allow rules on the device. The downside is that policy based tunnels (enc0) can not be filtered anymore as this changes the behaviour from filtering on the enc0 device to the if_ipsec(4) devices.

Warning

Currently it does not seem to be possible to add NAT rules for if_ipsec(4) devices.

Warning

In order to reliably setup a VTI tunnel, both ends should use static ip addresses. Although in the legacy configuration it was possible to resolve hostnames, this will never lead to a stable configuration as the if_ipsec(4) device matches both source and destination [#] before accepting the traffic and has no knowledge about any external changes.

Road Warriors / Mobile users

IPsec may also be used to service remote workers connecting to OPNsense from various clients, such as Windows, MacOS, iOS and Android. The type of client usually determines the authentication scheme(s) being used.

In case clients should be offered default settings, these can be configured from VPN -> IPsec -> Mobile Clients. Pool options (Virtual IPvX Address Pool) on this page will be used by the legacy tunnel configuration only, when using the new connections module one may configure different pools per connection.

The examples section contains various options available in OPNsense. When using the new “connections” option available as of OPNsense 23.1, different examples from Strongswan are usually quite easy to implement as we follow the swantcl.conf format quite closely in the new module.

The following client setup examples are available in our documentation:

Examples

This paragraph offers examples for some commonly used implementation scenarios.

Legacy (VPN -> IPsec -> Tunnel Settings)

Note

Using Network Address Translation in policy based tunnels is different, due to the fact that the installed IPsec policy should accept the traffic in order to encapsulate it. The IPSec BINAT document will explain how to apply translations.

New > 23.1 (VPN -> IPsec -> Connections)

Tip

The number of examples for the new module on our end is limited, but for inspiration it’s often a good idea to walkthrough the examples provided by Strongswan. Quite some swanctl.conf examples are easy to implement in our new module as we do follow the same terminology.

Tuning considerations

Depending on the workload (many different IPsec flows or a single flow), it might help to enable multithreaded crypto mode on ipsec, in which case cryptographic packets are dispatched to multiple processors. In order to do so, add or change the following tunable in System ‣ Settings ‣ Tunables:

Note

net.inet.ipsec.async_crypto = 1

To distribute load better over available cores in the system, it may help to enable receive side scaling. In which case the following tunables need to be changed:

Note

  • net.isr.bindthreads = 1

  • net.isr.maxthreads = -1 <– equal the number of cores in the machine

  • net.inet.rss.enabled = 1

  • net.inet.rss.bits = X <– see rss document.

Diagnostics

In order to keep track of the connected tunnels, you can use the VPN -> IPsec -> Status Overview to browse through the configured tunnels.

The VPN -> IPsec -> Security Policy Database is also practical to gain insights in the registered policies, when NAT is used, the additional SPD entries should be visible here as well.

When troubleshooting problems with your firewall, it is very likely you have to check the logs available on your system. In the UI of OPNsense, the log files are generally grouped with the settings of the component they belong to. The log files can be found in the “Log file” menu item.

Tip

When trying to debug various issues, the amount of log information gathered can be configured using the settings in VPN -> IPsec -> Advanced Settings.

Custom configurations

In some (rare) cases one might want to add custom configuration options not available in the user interface, for this reason we do support standard includes.

While the swanctl.conf and the legacy ipsec.conf configuration files are well suited to define IPsec-related configuration parameters, it is not useful for other strongSwan applications to read options from these files. To configure these other components, it is possible to manually append options to our default template, in which case files may be placed in the directory /usr/local/etc/strongswan.opnsense.d/ using the file extention .conf

IPsec configurations are managed in swantcl.conf format (as of 23.1), merging your own additions is possible by placing files with a .conf extension in the directory /usr/local/etc/swanctl/conf.d/.

Warning

Files added to these directories will not be mainted by the user interface, if you’re unsure if you need this, it’s likely a good idea to skip adding files here as it might lead to errors difficult to debug.

Note

Prior to version 23.1 it was also possible to add secrets and ipsec configurations in /usr/local/etc/ipsec.secrets.opnsense.d/ and /usr/local/etc/ipsec.opnsense.d/, with the switch to 23.1 these files are deprecated and should be manually migrated into swanctl.conf format.

OpenVPN (SSL VPN)

One of the main advantages of OpenVPN in comparison to IPsec is the ease of configuration, there are less settings involved and it’s quite simple to export settings for clients.

Site 2 Site

OpenVPN on OPNsense can also be used to create a tunnel between two locations, similar to what IPsec offers, generally the performance of IPsec is higher which usually makes this a less common choice.

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.

Road Warriors / Mobile users

Mobile usage is really where OpenVPN excells, with various (multifactor) authentication options and a high flexibility in available network options.

Client Specific Overrides

The mechanism of client overrides utilises OpenVPN client-config-dir option, which offer the ability to use specific client configurations based on the client’s X509 common name.

It is possible to specify the contents of these configurations in the gui under VPN -> OpenVPN -> Client Specific Overrides. Apart from that, an authentication server (System -> Access -> Servers) can also provide client details in special cases when returning Framed-IP-Address, Framed-IP-Netmask and Framed-Route properties.

Tip

Radius can be used to provisioning tunnel and local networks.

A selection of the most relevant settings can be found in the table below.

Client Specific Overrides

Parameter

Purpose

Disabled

Set this option to disable this client-specific override without removing it from the list

Servers

Select the OpenVPN servers where this override applies to, leave empty for all

Common name

The client’s X.509 common name, which is where this override matches on

IPv[4|6] Tunnel Network

The tunnel network to use for this client per protocol family, when empty the servers will be used

IPv[4|6] Local Network

The networks that will be accessible from this particular client per protocol family.

IPv[4|6] Remote Network

These are the networks that will be routed to this client specifically using iroute, so that a site-to-site VPN can be established.

Redirect Gateway

Force the clients default gateway to this tunnel

Note

When configuring tunnel networks, make sure they fit in the network defined on the server tunnel itself to allow the server to send data back to the client. For example in a 10.0.0.0/24 network you are able to define a client specific one like 10.0.0.100/30.

To reduce the chances of a collision, also make sure to reserve enough space at the server as the address might already be assigned to a dynamic client otherwise.

Plugin VPN options

Via plugins additional VPN technologies are offered, including:

  • OpenConnect - SSL VPN client, initially build to connect to commercial vendor appliances like Cisco ASA or Juniper.

  • Stunnel - Provides an easy to setup universal TLS/SSL tunneling service, often used to secure unencrypted protocols.

  • Tinc - Automatic Full Mesh Routing

  • WireGuard - Simple and fast VPN protocol working with public and private keys.

  • Zerotier - seamlessly connect everything, requires account from zerotier.com, free for up to 100 devices.