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.


(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.



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

Migrating from tunnels to connections

Having used the tunnel settings from the early OPNsense days, some terminology might be a bit confusing when moving into the new options offered. This paragraph aims to explain some of the common terms from the tunnel section and their new place in the connections. For a full list of changes, the upstream migration documentation is an interesting read as well.

  • Phase 1 - The general connection settings, like local/remote addressess and general protocol settings. Choices in authentication to use are also part of this, they may involve multiple rounds.

  • Phase 2 - Nowadays Strongswan calls these children, as these define the CHILD_SA subsections in play. This is where you can define the networks on both ends. When multiple segments are being added into the same child, these are being treated as one policy where all of them are able to communicate to eachother.

  • Phase 1 / Tunnel Isolation - This option made sure every network defined in phase 2 would be treated as a child of it’s own (e.g. two phase 2’s would turn into two children)

  • Phase 2 / Manual SPD entries - Manual SPD entries, this has been replaced with it’s own menu option (Security Policy Database) offering more flexibilty and visibilty.


Using DNS for endpoints is possible, but will work a bit different than previously as in most cases the firewall tried to resolve the names and didn’t use the functionality provided by Strongswan. It is however currently not possible to use DNS entries for VTI tunnels due to restrictions in if_ipsec(4) as these type of interfaces can’t be changed dynamically in a reliable way.


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.


Since OPNsense uses the new Strongswan format also for legacy tunnels, it is rather easy to convert a tunnel manually when downloading the swanctf.conf file from the machine. You can find it in /usr/local/etc/swanctl/swanctl.conf and the format is almost identical to the connections gui available in OPNsense.

Combining legacy tunnels and connections

It is possible to combine tunnels and connections, but there are some constraints. As our legacy tunnels force a reqid for each configured child (phase 2), there is a risk the automated numbering from the new connection children overlaps. To prevent these overlaps, its required to set an unused reqid in the connection child.

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.


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.


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.

Firewall rules

When using the legacy tunnels and Disable Auto-added VPN rules is not checked in VPN ‣ IPsec ‣ Advanced Settings some automatic firewall rules are created for remote hosts connecting to this one. The new connections feature does not offer this and (WAN) rules have to be specified manually in order to connect to IPsec on this host.

The relevant protocols and ports for IPsec are the following:


One of the main reasons we are not offering automatic rules is that their either more open than expected (allow IPsec from anywhere) or too closed as the rule engine will “guess” the remote endpoint (in case of a fqdn).

The default behavior of our firewall is to block inbound traffic, which also means traffic using the tunnel should be allowed explicitly, the Firewall ‣ Rules ‣ IPsec menu items offer access to the IPsec traffic policies.

Dead Peer Detection (DPD)

Dead Peer Detection (DPD) is a method of detecting a dead IKE peer by sending periodic R-U-THERE messages to the remote expecting R-U-THERE-ACK messages in return as specified by RFC 3706.

When a peer is assumed dead, an action may be specified, such as closing the CHILD_SA or re-negotiate the CHILD_SA under a fresh IKE_SA.


DPD is disabled by default, when using connections, make sure to specify a DPD delay (s) > 0 to enable the feature. Actions may be specified on its children.

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 sending traffic to a remote location responsible for The advantage of this scenario is the ease of setup, no routes are needed to be configured, when in this example contacts 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 for IPv4 or ::/0 for IPv6).

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


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 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.


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


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.


If you are configuring Radius authentication using the new Connections module, make sure to select the relevant Radius servers in VPN -> IPsec -> Mobile Clients under Radius (eap-radius). This pool of servers will be shared across all connections. This option will not be visibile if you have legacy Radius authentication methods configured.

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.


This paragraph offers examples for some commonly used implementation scenarios.

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


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.

Legacy (VPN -> IPsec -> Tunnel Settings)

The following client setup examples are available in our documentation:


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.

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 (especially when only a single tunnel is being used).

In order to do so, add or change the following tunable in System ‣ Settings ‣ Tunables:


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:


  • 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.

Miscellaneous variables

Path MTU Discovery

When trying to enforce path mtu discovery (PMTU), you need to make sure packets leave the network with the DF set. The kernel offers a tunable net.inet.ipsec.dfbit which offers 3 options, 0, clear the bit on packets leaving the firewall (default), 1, set the DF bit or 2 to copy the bit from the inner header.


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.


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/.


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.


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.


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

General context

The OpenVPN module incorporates different functions to setup secured networks for roadwarriors and side to side connections. Since the start of our project we organized the openvpn menu section into servers and clients, which actually is a role for the same OpenVPN process. As our legacy system has some disadvantages which are difficult to fix in a migration, we have chosen to add a new component named Instances in version 23.7 which offers access to OpenVPN’s configuration in a similar way as the upstream documentation describes it. This new component will eventually replace the existing client and server options in a future version of OPNsense, leaving enough time to migrate older setups.


When upgrading into a new major version of OPNsense, always make sure to read the release notes to check if your setup requires changes.


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

The following functions are available in the menu (as of OPNsense 23.7):

  • Instances

    • New instances tool offering access to server and client setups

  • Servers

    • Legacy server configuration tool

  • Clients

    • Legacy client configuration tool

  • Client Specific Overrides

    • Set client specific configurations based on the client’s X509 common name.

  • Client Export

    • Export tool for client configurations, used for server type instances

  • Connection Status

    • Show tunnel statusses

  • Log File

    • Inspect log entries related to OpenVPN

Public Key Infrastructure (X.509)

OpenVPN is most commonly used in combination with a public key infrastructure, where we use a certificate autority which signs certificates for both server and clients (Also know as TLS Mode). More information about this topic is available in our Trust section.


As of version 24.1 OPNsense is able to use OCSP to validate client certificates when using the new Instances. Make sure Use OCSP (when available) is enabled in the trust section of the server instance and the CA used contains a proper AuthorityInfoAccess extension as described in our Trust section.

Firewall rules

To allow traffic to the tunnel on any interface, a firewall rule is needed to allow the tunnel being established. The default port for OpenVPN is 1194 using protocol UDP.

After communication has been established, it’s time to allow traffic inside the tunnel. All OpenVPN interfaces defined in OPNsense are grouped as OpenVPN.


In order to use features as policy based routing or manual routes, you can assign the underlying devices and use them in a similar fashion as physical interfaces.

High availability [CARP]

When operating an OpenVPN server, there’s not much needed to allow an active/passive setup for your environment other then using a virtual (CARP) address. As the server will stop receiving traffic when the virtual address doesn’t it, the backup will eventually become out of service automatically.

In client mode, the OpenVPN instance needs to stop trying to reconnect when it’s not in MASTER mode, the legacy client module shutsdown all instances directly attached to the interface. Our new instances module allows to select the vhid to track. In most cases an explicit bind isn’t needed for a client, the default for a client is to use the nobind option.


It’s not possible to move between machines fully seamless as the client will have to reconnect in order to reach a valid state again.


This paragraph offers examples for some commonly used implementation scenarios.


When using a 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.

Legacy (VPN -> OpenVPN -> Client|Server)

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.


Client specific overwrites will be written after authentication or client connect (depending on the type of setup). This in order for authentication services like RADIUS to be able to provision additional properties, such as tunnel and local networks.

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

Client Specific Overrides




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


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


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 network you are able to define a client specific one like

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.


When using topology “subnet” the netmask usually equals the one defined in the instance itself as the gateway being pushed to the client is the first adress in the network and otherwise unreachable.

Troubleshooting common issues

The most common causes for non functional overwrites are caused by mismatches, in order to debug these, make sure to check the logs for messages like the following:

  • Locate overwrite for 'XXX' using server 'XXX' (vpnid: XXX) << trying to find an overwrite (user authentication))

    • Usually followed by user 'XXX' authenticated using 'XXX' XXX showing username, authenticator used and optionally the overwrite type and filename.

  • client config created @ XXX << file written on client connect (without user authentication)

  • unable to write client config for XXX, missing target filename << no matching overwrite found (without user authentication)

By default overwrites are matched by certificate common name, when Force CSO Login Matching (legacy) or Username as CN (instances) are set the username will be used instead.


General context

WireGuard® is a simple yet fast and modern VPN solution, which in some cases is more convenient than IPsec or OpenVPN, certainly in terms of options you need to configure. In our experience IPsec is the fastest solution for site-to-site connections, but Wireguard is the simplest option to setup.

A wireguard setup on our end exists of the following main components:

  • Instances: in the wireguard configuration these are called “interfaces” and they describe how the virtual wgX device on our end is configured in terms of addressing and cryptography.

  • Peers: these are the clients that are allowed to connect to us, described by their optional remote address including the networks that are allowed to pass through the tunnel. Peers belong to one or more instances.


In order to configure an instance, we start by adding one in the gui and generate a keypair. The public key is usually required for the other end of the tunnel (peer). An unused port to listen on is required as well. The tunnel addresses are configured on the wgX device (which is always visible in Interfaces ‣ Overview).

By default, when “Disable routes” is not set, routes are created for each connected peer to the networks selected in “Allowed IPs”, optionally only a single gateway route might be configured as well.


When choosing tunnel addresses, make sure the network defined includes the addresses being used by the peers. For example when choosing the wgX instance has this address configured and is able to accept a peer using


Remember to create a firewall rule to allow traffic to the configured port and inside the tunnel.


Peers define the hosts that we exchange information with, which might be a road-warrior type or a static destination, in which case you either provide or omit an “Endpoint Address and Port”. At minimum you need the public key of the other party, optionally you may offer a pre-shared key as additional security measure. The “Allowed IPs” define the networks that are allowed to pass the tunnel.


In most cases the “Allowed IPs” list contains the networks used on the remote host and the peer ip address (instance/tunnel address) configured on the other end.


When NAT and firewall traversal persistence is required, the :code:` Keepalive interval` can be used to exchange packets every defined interval ensuring states will not expire.

High availability (using CARP)

When using wireguard on active/passive high availability clusters, only one instance at a time is allowed to communicate to the other party. In OPNsense this can be reached by selecting a vhid to track as instance dependancy {Depend on (CARP)}.

If an instance depends on a CARP vhid, it will query the current status and determine if the interface should be usable (when MASTER), the interface status (up/down) will be toggled accordingly.


As the interface itself will not change, all of its addresses and routes remain when not being active. This ensures a relatively quick switch between roles.


Because the carp dependancy is managed per instance, you are able to keep tunnels available selectively, for example to manage the machines remotely.

Diagnostics and debugging

In VPN ‣ WireGuard ‣ Status you can find the configured instances and peers including their last known handshake and the amount of data being exchanged. For Instances you are also able to see if the device underneath (wgX) is up or down, depending on the carp status described in the previous chapter.


Althought wireguard itself offers very limit logging, our setup process will make a note of errors and signal about certain events. When having issues configuring an instance or peer, always make sure to check the logs in VPN ‣ WireGuard ‣ Log File first.


When having issues exchanging packets between both ends of the tunnel, always make sure to check if the “Allowed IPs” in the peer configurations contain the proper networks. In case traffic is not allowed when traveling in, its dropped silently (a capture will not show it), roughly the same happens when traveling out, a capture will show it, but nothing will be send out.


Runtime debugging from the console is possible using the ifconfig command, for more information see the upstream manual page

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, free for up to 100 devices.