---
slug: opnsense-virtualization-highly-available
title: Build a Highly Available OPNsense Cluster on Proxmox VE
description: A proof of concept showing how to virtualize OPNsense on Proxmox VE, configure high availability with CARP and pfSync and handle a single WAN IP.
date: 2025-09-29
draft: false
tags:
  - opnsense
  - proxmox
  - high-availability
categories:
  - homelab
---
## Intro

I recently encountered my first real problem, my physical **OPNsense** box crashed because of a kernel panic, I've detailed what happened in that [post]({{< ref "post/10-opnsense-crash-disk-panic" >}}). 

That failure made me rethink my setup. A unique firewall is a single point of failure, so to improve resilience I decided to take a new approach: **virtualize OPNsense**.

Of course, just running one VM wouldn’t be enough. To get real redundancy, I need two OPNsense instances in **High Availability**, with one active and the other standing by.

Before rolling this out in my network, I wanted to demonstrate the idea in my homelab. In this post, I’ll walk through the proof of concept: deploying two OPNsense VMs inside a **Proxmox VE** cluster and configuring them to provide a highly available firewall.

---
## Current Infrastructure

On top of my setup, my ISP modem, a *Freebox* in bridge mode, connects directly to the `igc0` interface of my OPNsense box, serving as the **WAN**. On `igc1`, the **LAN** is linked to my main switch using a trunk port, with VLAN 1 as the native VLAN for my management network.

The switch also connects my three Proxmox nodes, each on trunk ports with the same native VLAN. Every node has two NICs: one for general networking and the other dedicated to the Ceph storage network, which runs through a separate 2.5 Gbps switch.

Since the OPNsense crash, I’ve simplified things by removing the LACP link, it wasn’t adding real value:
![Current homelab network diagram](img/homelan-current-physical-layout.png)


Until recently, Proxmox networking on my cluster was very basic: each node was configured individually with no real overlay logic. That changed after I explored Proxmox SDN, where I centralized VLAN definitions across the cluster. I described that step in [this article]({{< ref "post/11-proxmox-cluster-networking-sdn" >}}).

---
## Proof of Concept

Time to move into the lab. Here are the main steps:
1. Add some VLANs in my Homelab
2. Create Fake ISP router
3. Build two OPNsense VMs
4. Configure high availability
5. Test failover

![Diagram of the POC for OPNsense high availability](img/poc-opnsense-diagram.png)

### Add VLANs in my Homelab

For this experiment, I create 3 new VLANs:
- **VLAN 101**: *POC WAN* 
- **VLAN 102**: *POC LAN*
- **VLAN 103**: *POC pfSync*

In the Proxmox UI, I navigate to `Datacenter` > `SDN` > `VNets` and I click `Create`:
![Create POC VLANs in the Proxmox SDN](img/proxmox-sdn-create-poc-vlans.png)

Once the 3 new VLAN have been created, I apply the configuration.

Additionally, I add these 3 VLANs in my UniFi Controller. Here only the VLAN ID and name are needed, since the controller will propagate them through the trunks connected to my Proxmox VE nodes.

### Create “Fake ISP Box“ VM

To simulate my current ISP modem, I built a VM named `fake-freebox`. This VM routes traffic between the *POC WAN* and *Lab* networks and runs a DHCP server that serves only one lease, just like my real Freebox in bridge mode.

This VM has 2 NICs, I configure Netplan with:
- `eth0` (*POC WAN* VLAN 101): static IP address `10.101.0.254/24`
- enp6s19 (Lab VLAN 66): DHCP address given by my current OPNsense router, in upstream
```yaml
network:
  version: 2
  ethernets:
    eth0:
      addresses:
        - 10.101.0.254/24
    enp6s19:
      dhcp4: true
```

I enable packet forward to allow this VM to route traffic:
```bash
echo "net.ipv4.ip_forward=1" | sudo tee -a /etc/sysctl.conf
sudo sysctl -p
```

Then I set up masquerading so packets leaving through the lab network wouldn’t be dropped by my current OPNsense:
```bash
sudo iptables -t nat -A POSTROUTING -o enp6s19 -j MASQUERADE
sudo apt install iptables-persistent -y
sudo netfilter-persistent save
```

I install `dnsmasq` as a lightweight DHCP server:
```bash
sudo apt install dnsmasq -y
```

In `/etc/dnsmasq.conf`, I configure to serve exactly one lease (`10.101.0.150`) with DNS pointing to my current OPNsense router, in the *Lab* VLAN:
```
interface=eth0
bind-interfaces
dhcp-range=10.101.0.150,10.101.0.150,255.255.255.0,12h
dhcp-option=3,10.101.0.254      # default gateway = this VM
dhcp-option=6,192.168.66.1      # DNS server  
```

I restart the `dnsmasq` service to apply the configuration:
```bash
sudo systemctl restart dnsmasq
```

The `fake-freebox` VM is now ready to serve DHCP on VLAN 101 and serve only one lease.

### Build OPNsense VMs

First I download the OPNsense ISO and upload it to one of my Proxmox nodes:
![Upload de l’ISO OPNsense dans Proxmox](img/proxmox-upload-opnsense-iso.png)

#### VM Creation

I create the first VM `poc-opnsense-1`, with the following settings:
- OS type: Linux (even though OPNsense is FreeBSD-based)
- Machine type: `q35`
- BIOS: `OVMF (UEFI)`, EFI storage on my Ceph pool
- Disk: 20 GiB also on Ceph
- CPU/RAM: 2 vCPU, 2 GiB RAM
- NICs:
	1. VLAN 101 (POC WAN)
	2. VLAN 102 (POC LAN)
	3. VLAN 103 (POC pfSync)
![OPNsense VM settings in Proxmox](img/proxmox-create-poc-vm-opnsense.png)

ℹ️ Before booting it, I clone this VM to prepare the second one: `poc-opnsense-2`

On first boot, I hit an “access denied” error. To fix this, I enter the BIOS, go to **Device Manager > Secure Boot Configuration**, uncheck _Attempt Secure Boot_, and restart the VM:
![Disable Secure Boot in Proxmox BIOS](img/proxmox-disable-secure-boot-option.png)

#### OPNsense Installation

The VM boots on the ISO, I touch nothing until I get into the login screen:
![OPNsense CLI login screen in LiveCD](img/opnsense-vm-installation-welcome.png)

I log in as `installer` / `opnsense` and launch the installer. I select the QEMU hard disk of 20GB as destination and launch the installation:
![Barre de progression de l’installation OPNsense](img/opnsense-vm-installation-progress-bar.png)

Once the installation is finished, I remove the ISO from the drive and restart the machine.

#### OPNsense Basic Configuration

After reboot, I log in as `root` / `opnsense` and get into the CLI menu:
![Menu CLI après une installation fraîche d’OPNsense](img/opnsense-vm-installation-cli-menu.png)

Using option 1, I reassigned interfaces:
![Configuration des interfaces dans OPNsense via le CLI](img/opnsense-vm-installation-assign-interfaces.png)

The WAN interface successfully pulled `10.101.0.150/24` from the `fake-freebox`. I set the LAN interface to `10.102.0.2/24` and configured a DHCP pool from `10.102.0.10` to `10.102.0.99`:
![Interface WAN OPNsense recevant une IP depuis la VM `fake-freebox`](img/opnsense-vm-installation-interfaces-configured.png)

✅ The first VM is ready, I start over for the second OPNsense VM, `poc-opnsense-2` which will have the IP `10.102.0.3`

### Configure OPNsense Highly Available

With both OPNsense VMs operational, it’s time to configure them from the WebGUI. To access the interface, I connected a Windows VM into the _POC LAN_ VLAN and browsed to the OPNsense IP on port 443:
![OPNsense WebGUI depuis une VM Windows](img/opnsense-vm-webgui-from-poc-lan.png)

#### Add pfSync Interface

The third NIC (`vtnet2`) is assigned to the _pfSync_ interface. This dedicated network allows the two firewalls to synchronize states on the VLAN *POC pfSync*:
![Add pfSync interface in OPNsense](img/opnsense-vm-assign-pfsync-interface.png)

I enable the interface on each instance and configure it with a static IP address:
- **poc-opnsense-1**: `10.103.0.2/24`
- **poc-opnsense-2**: `10.103.0.3/24`

Then, I add a firewall rule on each node to allow all traffic coming from this network on that *pfSync* interface:
![Create new firewall rule on pfSync interface to allow any traffic in that network](img/opnsense-vm-firewall-allow-pfsync.png)

#### Setup High Availability

Next, in `System` > `High Availability` > `Settings`.
- On the master (`poc-opnsense-1`), I configure both the `General Settings` and the `Synchronization Settings`.
- On the backup (`poc-opnsense-2`), only `General Settings` are needed, you don't want your backup overwrite the master config.
![OPNsense High Availability settings](img/opnsense-vm-high-availability-settings.png)

Once applied, I verify synchronization on the `Status` page:
![OPNsense High Availability status](img/opnsense-vm-high-availability-status.png)

#### Create Virtual IP Address

To provide a shared gateway for clients, I create a CARP Virtual IP (VIP) on the LAN interface. It is using the Common Address Redundancy Protocol. This IP is claimed by the active node and automatically fails over.

Navigate to `Interfaces` > `Virtual IPs` > `Settings`:
![Create CARP virtual IP in OPNsense](img/opnsense-vm-create-vip-carp.png)

To replicate the config, I go to `System > High Availability > Status` and click the button next to `Synchronize and reconfigure all`.

On the `Interfaces > Virtual IPs > Status` page, the master show the VIP as `MASTER`, while the backup report `BACKUP`.

#### Reconfigure DHCP

For HA, I need to adjust the DHCP setup. Since **Dnsmasq** does not support lease synchronization, both instances must serve leases independently.

On the master:
- `Services` > `Dnsmasq DNS & DHCP` > `General`: tick the `Disable HA sync` box. 
- `DHCP ranges`: also tick the `Disable HA sync` box
- `DHCP options`: add the option `router [3]` with the value `10.102.0.1` (LAN VIP)
- `DHCP options`: clone the rule for `router [6]` pointing to the same VIP.
![Edit DHCP options for Dnsmasq in OPNsense](img/opnsense-vm-dnsmasq-add-option.png)

On the backup:
- `Services` > `Dnsmasq DNS & DHCP` > `General`: also tick the `Disable HA sync` box
- Set `DHCP reply delay` to `5` seconds, to give master priority to answer.
- `DHCP ranges`: Use a different pool, smaller (`10.102.0.200` -> `220`)
- but I also set the value `5` to `DHCP reply delay`. This would give enough time to the master node to provide a DHCP lease before the backup node. In `DHCP ranges`, I edit the current one and give a smaller pool, different than the master's. Here I also tick the `Disable HA sync` box.

This way, only DHCP options sync between nodes, while lease ranges stay separate.

#### WAN Interface

My ISP modem only provides a single DHCP lease, I don't want my 2 VMs compete to claim it. To handle this:
1. In Proxmox, I copy the MAC of the `net0` (WAN) interface from `poc-opnsense-1` and applied it to `poc-opnsense-2`. This way, the DHCP lease could be shared among the nodes.
⚠️ If both VMs bring up the same MAC, it can cause ARP conflicts and break connectivity, only the MASTER should keep its WAN active.
2. CARP event hook provides the possibility to run scripts, I deployed this [Gist script](https://gist.github.com/spali/2da4f23e488219504b2ada12ac59a7dc#file-10-wancarp) in `/usr/local/etc/rc.syshook.d/carp/10-wan` on both nodes. This ensures the WAN is active only on the MASTER, avoiding conflicts.
```php
#!/usr/local/bin/php
<?php

require_once("config.inc");
require_once("interfaces.inc");
require_once("util.inc");
require_once("system.inc");

$subsystem = !empty($argv[1]) ? $argv[1] : '';
$type = !empty($argv[2]) ? $argv[2] : '';

if ($type != 'MASTER' && $type != 'BACKUP') {
    log_error("Carp '$type' event unknown from source '{$subsystem}'");
    exit(1);
}

if (!strstr($subsystem, '@')) {
    log_error("Carp '$type' event triggered from wrong source '{$subsystem}'");
    exit(1);
}

$ifkey = 'wan';

if ($type === "MASTER") {
    log_error("enable interface '$ifkey' due CARP event '$type'");
    $config['interfaces'][$ifkey]['enable'] = '1';
    write_config("enable interface '$ifkey' due CARP event '$type'", false);
    interface_configure(false, $ifkey, false, false);
} else {
    log_error("disable interface '$ifkey' due CARP event '$type'");
    unset($config['interfaces'][$ifkey]['enable']);
    write_config("disable interface '$ifkey' due CARP event '$type'", false);
    interface_configure(false, $ifkey, false, false);
}
```

### Test Failover

Time for the real test!

OPNsense provides a _CARP Maintenance Mode_. With the master active, WAN was enabled only on that node. Entering maintenance mode flipped the roles: the master became backup, its WAN disabled, while the backup enabled its WAN:
![Mode maintenance CARP dans OPNsense](img/opnsense-vm-carp-status.png)

While pinging outside the network, I observed zero packet loss during the failover.

Finally, I simulated a crash by powering off the master. The backup took over seamlessly, I saw only one dropped packet, and thanks to state synchronization, even my SSH session stayed alive. 🎉

## Conclusion

This proof of concept showed that running **OPNsense in high availability on Proxmox VE** is possible, even with a single WAN IP address. To achieve this, I needed these components:
- VLAN segmentation.
- Dedicated pfSync network.
- Shared virtual IP.
- Script to manage the WAN interface.

The setup behave exactly as expected, seamless failover, synchronized firewall states, and even live sessions surviving a node crash. The most delicate part was handling the WAN lease, since my ISP modem only provides one IP, but the CARP hook script solved that challenge.

🚀 The next milestone will be to prepare a new OPNsense HA cluster with the aim to completely replace my current physical box. Stay tuned!