Teams evaluating an open firewall usually start by comparing feature grids against their incumbent NGFW, and that is exactly where the decision goes wrong. An open firewall will match the packet-filtering, NAT, VPN, and signature-IPS lines cleanly, but the columns that sold you the commercial box, application identification, cloud-delivered threat intelligence, curated URL categories, integrated SD-WAN, rarely have a one-for-one open equivalent. The real question is not which product has more features; it is how much you depend on the proprietary ones. This guide is a rubric for answering that before you shortlist anything, and for setting the expectation early: there is generally no automated policy importer, so the rulebase is rebuilt by hand.
Start with the decision, not the appliance
Before you look at OPNsense or pfSense, list the NGFW features you actually use in policy, not the ones merely licensed and switched on. Separate the segments that lean on app-ID, sandboxing, or vendor URL categories from the segments doing straightforward stateful filtering, NAT, and VPN. Note your throughput needs, your VPN types and peers, whether you run multi-site central management, and how comfortable your team is operating a BSD or Linux firewall from the CLI as well as the GUI. Those inputs decide the answer; the feature matrices do not.
The factors that matter here
- Proprietary NGFW dependence. This is the deciding factor. App-ID-style application identification, cloud sandboxing of unknown files, and vendor-curated URL and app categories do not have clean open replacements. Suricata covers signature IPS and proxy plugins cover some filtering, but neither reproduces app-identification policy. Where a segment genuinely depends on those, plan to keep the incumbent there rather than force a swap.
- Throughput, sized honestly. Size on NGFW threat-protection throughput, not raw firewall throughput, and account for IPsec VPN throughput, concurrent sessions, and connections per second. TLS inspection cuts effective throughput sharply. Under-sizing is the classic open-firewall mistake.
- VPN types. Inventory your tunnels: IPsec site-to-site, SSL-VPN remote access, and WireGuard. IPsec and WireGuard rebuild cleanly; a proprietary SSL-VPN client with no open equivalent means moving those users to OpenVPN or WireGuard, which is a client-side change to plan, not a config port.
- Multi-site central management. A single edge is straightforward; a fleet managed centrally from a vendor console is not. Be honest about whether you are replacing one appliance or the management plane that ties dozens together, because the second is a much larger undertaking.
- Team comfort with a BSD/Linux firewall. OPNsense and pfSense are operated, not just licensed. If your team lives entirely in a vendor GUI today, factor in the ramp to CLI-level troubleshooting, alias-based object modeling, and Suricata tuning.
Which segments an open firewall fits
Score your segments first, then choose. Edge, branch, and internal-segmentation firewalls doing stateful filtering, NAT, IPsec and WireGuard VPN, and Suricata-based IDS/IPS are the sweet spot for OPNsense and pfSense, and either fits most of that range; the choice between them usually comes down to interface preference, plugin ecosystem, and support model rather than raw capability. The commercial sources you are leaving, Palo Alto, Fortinet, Check Point, SonicWall, are not replaced feature-for-feature; you keep the incumbent where a segment truly needs app-ID, cloud threat intel, or integrated SD-WAN, and move the segments that do not. That split, driven by your dependency scores, is the whole decision.
The traps to avoid
The first and most expensive mistake is assuming feature parity, expecting an open firewall to reproduce app-ID or cloud sandboxing, and discovering the gap after cutover. The second is sizing on raw throughput and ignoring the hit from threat inspection and TLS decryption, so the box is undersized on day one. The third is expecting to import the rulebase; there is no reliable importer, so the policy is rebuilt by hand, which is slower but is also where you finally delete the stale rules nobody dared touch. The fourth is enabling Suricata in blocking mode from the start instead of running alert-only, tuning out false positives on real traffic, and only then switching to block.
Prove it at one site first
Turn your top candidate into a pilot at a low-risk site before any rollout. Deploy in HA, rebuild rules, aliases, and NAT by hand in the original evaluation order, recreate IPsec and remote-access VPN with phase parameters matched to each peer, and wire logging to your SIEM. Validate a real allow-and-deny matrix live, confirm VPN connectivity site-to-site and remote, run IDS and throughput tests, and exercise HA failover so the failover path is proven, not assumed. Only then roll out site by site with rollback ready, keeping the source firewall available to re-point traffic to during hypercare. Model the per-firewall three-year cost against a real renewal quote from your incumbent, and use the TCO calculator to ground the comparison, treating the figures as illustrative until a vendor quotes your configuration.