In a previous post I connected a Nucleo board with an Ethernet shield, with the SPI wired between the STM32 and the Wiznet W5100 chip. I am now in the process of writing some C tests for it on a GitHub repository, and in particular a socket library that tries to conform to POSIX specifications. At this time the TCP connections are working, both in server mode and in client mode. Many functionalities and details are currently overlooked or not implemented, but the basics are there and the code is getting better little by little.
One of my goals is to put together a system that is able to perform secure connections such as with OpenSSL, as a baseline for security in the Internet of Things.