fastd

"A fast and secure tunneling daemon.

The features:

• Very small binary (about 100KB on OpenWRT in the default configuration, including all dependencies besides libc)
• Exchangable crypto methods
• Transport over UDP for simple usage behind NAT
• Can run in 1:1 and 1:n scenarios
• There are no server and client roles defined by the protocol, this is just defined by the usage.

fastd supports various combinations of ciphers and authentication schemes using different method providers. All ciphers, message authentication codes (MACs) and method providers can be disabled during compilation to reduce the binary size."

https://projects.universe-factory.net/projects/fastd/wiki

http://fastd.readthedocs.io/en/v18/

https://news.ycombinator.com/item?id=9025792