strongSwan

Its comprehensive implementation of the Internet Key Exchange protocols supports policy-based and route-based IPsec deployments, from straightforward connections to complex network configurations. The software implements IKEv2 and also supports IKEv1, with features including IPv6, NAT traversal, mobility for changing IP addresses, and management of virtual IP address pools.

A flexible plugin system lets organizations extend the solution with cryptographic algorithms, authentication methods, credentials, and network integrations. Authentication options include certificates, pre-shared keys, and a range of EAP methods, with support for technologies such as smartcards and TPM 2.0. strongSwan runs on Linux and has also been ported to Android, FreeBSD, macOS, iOS, and Windows. Users can integrate it with Linux desktops through a NetworkManager plugin, and an Android app is available. The project provides extensive documentation and offers both free and commercial support. Its source code is licensed under GPLv2, with commercial licensing options also available.