strongSwan

What strongSwan actually is

strongSwan is an open-source implementation of the IPsec protocol suite, with Internet Key Exchange as its center of gravity. It secures IP traffic in policy-based and route-based configurations, from a single site-to-site tunnel to large, complex VPN networks with thousands of peers. It was launched in 2005 as a fork of the discontinued FreeS/WAN project, folding in the X.509 patch its developers had contributed to FreeS/WAN since 2000, and it has been developed ever since out of the University of Applied Sciences Rapperswil (HSR) in Switzerland under Prof. Andreas Steffen. Two decades on, it is one of the reference IKE implementations in the industry.

The current release line fully implements IKEv2 as defined by RFC 7296 while still supporting IKEv1 for legacy peers. What distinguishes strongSwan from a simple VPN client is its depth in authentication and policy: X.509 certificates with CRL and OCSP checking, X.509 attribute certificates for group-based access control, private keys on smartcards through PKCS#11 and on TPM 2.0, a full set of EAP methods (TLS, TTLS, MSCHAPv2, GTC, MD5, AKA/SIM with USIM management), an EAP-RADIUS plugin for FreeRADIUS or Active Directory backends, RFC 4739 multiple authentication exchanges, RFC 5685 redirection, MOBIKE for roaming clients, and Trusted Network Connect interfaces for network access control. The IKEv2 daemon is multi-threaded and there is a high-availability mode where two gateways share load and take over each other's IKE and ESP state without rekeying.

It is also not the Linux-only tool people assume. The project describes itself as a multiplatform IPsec implementation: the charon daemon builds and runs on Linux, FreeBSD, macOS, and Windows (via the MinGW toolchain), there is a native Android VPN client with downloadable profiles, and the built-in IKEv2 clients of Windows 7 and newer and macOS interoperate against strongSwan gateways. For the platform list and history, see the project's own pages at strongswan.org and docs.strongswan.org — we have corrected our database row on this point, since it claimed no Windows or macOS support.

Pricing: what you really pay

The software is free. strongSwan is distributed under the GPLv2 license, source on GitHub, packages in the major distributions (Debian, Ubuntu, Fedora, Alpine and others all ship it), and no feature is held back behind a paid tier. There is no per-seat licensing, no subscription, and no usage metering — your cost is the servers you run it on and the people who operate it.

The project does offer commercial licensing (for cases where GPLv2 obligations are a problem in a product you ship) and commercial support options alongside the free community channels. That is the honest framing of the price of strongSwan: the license is zero, the operations are not. Running your own IPsec gateways means PKI design, configuration, monitoring, patching, and capacity planning, and those are real costs. Compared with a WireGuard-based overlay (simpler to run) or a commercial VPN service (someone else runs it), strongSwan is cheap in money and expensive in expertise.

One more practical note on cost: because strongSwan is standard IPsec, it interops with the VPN features of cloud providers and firewalls you may already pay for. A strongSwan gateway can terminate tunnels to a router, a firewall, or a cloud VPN gateway from another vendor, which is exactly the sort of avoid-reinventing-it value that shows up as saved budget rather than a line item. It also means your strongSwan investment survives vendor changes elsewhere in the stack — the configuration you write today still terminates tunnels against next year's firewall.

Performance & infrastructure

strongSwan runs in the kernel you give it. IPsec data plane performance is essentially your server's crypto throughput: AES-NI and related CPU extensions, the crypto library you select at build time (the plugin system supports multiple backends), MTU and path characteristics, and whether you use tunnel or transport mode. The IKE daemon itself is multi-threaded (16 threads by default) and the plugin architecture keeps optional features out of the hot path when you do not enable them. For hardware-assisted setups there is support for crypto acceleration including smartcard and TPM-based key storage. The plugin model also shapes deployment: you compile in exactly the features you run — RADIUS integration, TNC, specific EAP methods, alternative crypto libraries — which keeps the attack surface and memory footprint small on embedded hardware, one reason the codebase shows up in routers and industrial gateways as often as on data-center servers.

The networking feature set is where infrastructure teams will spend their attention. Policy-based and route-based VPNs are both supported; virtual IP pools can be assigned from RAM, SQLite, or MySQL with configurable lease times; MOBIKE lets mobile or multi-homed peers change address without rekeying; the HA cluster mode gives two-node active load-sharing with state takeover. IPv6 is a first-class citizen — the project states fully tested support for IPv6 IPsec tunnel and transport mode, with worked configuration examples in the docs.

We have not benchmarked VPN throughput for strongSwan and will not quote numbers we did not measure. Real-world throughput is dominated by the host hardware and kernel anyway; for the server platforms we have measured with sysbench, fio, and iperf3, see our benchmarks and pick a machine with the crypto headroom your tunnel count needs.

Where it falls short

Configuration is the barrier to entry, and it is not a small one. Modern strongSwan leans on swanctl and a daemon architecture that assumes you understand IKE semantics, proposals, identity matching, and IPsec policy. There is no friendly control plane, no hosted dashboard, no packaged appliance — Arch wiki pages and the project's how-tos take you most of the way, but this is a component you integrate, not a product you switch on. Teams without network security experience on staff should look at managed alternatives.

Support is the second gap. Free support is community support: documentation, the mailing list, and bug reports. Commercial support and licensing exist but are not the same as a vendor SLA with an escalation path and a response-time guarantee — if your gateway is the single point of connectivity for a production network, you are signing up to be your own support organization (or contracting one of the third-party integrators).

Finally, the paradigm itself. For simple point-to-point or mesh encryption between your own servers, WireGuard is smaller, faster to configure, and built into modern kernels; strongSwan earns its complexity when you need IKEv2 features (EAP authentication, dynamic address assignment, interoperability with non-Linux peers, MOBIKE, certificate-based access control, TNC). If you do not need those, the complexity is pure overhead. And as with any widely deployed security component, patching is on you: the project releases regularly (the current line is version 6.x, with 6.1.0 shipping in September 2026), but distribution packages lag and nobody updates your gateways for you.

Who should pick this

Our pick for infrastructure teams that need standards-compliant IPsec with real authentication options: site-to-site VPNs between heterogeneous vendors, remote-access gateways with certificate or EAP-based auth, SD-WAN and embedded deployments where a modular C codebase beats a heavyweight daemon, and any environment where smartcards, TPM, or network access control are part of the requirement. It is the tool we would reach for when a customer hands us a specification written in RFC numbers. Within our private networking category it is the standards-and-depth choice as opposed to the simplicity choice.

We would not pick it for a two-server mesh (use WireGuard), for teams with no network engineering capacity (use a commercial VPN or a managed gateway), or for anyone who needs a support contract with teeth before signing (evaluate the commercial support options first, or pick a vendor-backed product).

Key takeaways

• strongSwan is a GPLv2 open-source IPsec/IKE implementation, launched in 2005 from the FreeS/WAN fork and developed at HSR Rapperswil (Switzerland); current release line is 6.x.
• Full IKEv2 (RFC 7296) with IKEv1 legacy support, deep authentication (X.509 attribute certificates, PKCS#11 smartcards, TPM 2.0, EAP/RADIUS), MOBIKE, TNC, and two-node HA with state takeover.
• Multiplatform: Linux, FreeBSD, macOS, Windows (MinGW), plus a native Android VPN client — interoperable with the built-in Windows and macOS IKEv2 clients.
• The software is free; the cost is operations. Community support is free, commercial licensing and support options exist, but there is no vendor SLA.
• Best for standards-driven site-to-site and remote-access IPsec; overkill for a simple server mesh where WireGuard is the better tool.
• IPv6 tunnel and transport mode are fully supported; performance depends on your host's crypto capability — no benchmark claims from us.

Sources

• https://strongswan.org/about.html
• https://strongswan.org/
• https://www.strongswan.org/license.html
• https://mail.strongswan.org/
• https://mail.strongswan.org/support.html
• https://en.wikipedia.org/wiki/StrongSwan
• https://docs.strongswan.org/docs/latest/os/windows.html
• https://docs.strongswan.org/docs/latest/os/macos.html
• https://docs.strongswan.org/docs/latest/config/IPv6.html
• https://docs.strongswan.org/docs/5.9/_attachments/LinuxKongress_Dresden_2009.pdf