wpa_supplicant uses a separate code module for EAP peer implementation. This module was designed to use only a minimal set of direct function calls (mainly, to debug/event functions) in order for it to be usable in other programs. The design of the EAP implementation is based loosely on RFC 4137. The state machine is defined in this RFC and so is the interface between the peer state machine and methods. As such, this RFC provides useful information for understanding the EAP peer implementation in wpa_supplicant.
Some of the terminology used in EAP state machine is referring to EAPOL (IEEE 802.1X), but there is no strict requirement on the lower layer being IEEE 802.1X if EAP module is built for other programs than wpa_supplicant. These terms should be understood to refer to the lower layer as defined in RFC 4137.
Each EAP method is implemented as a separate module, usually as one C file named eap_<name of the method>.c, e.g., eap_md5.c. All EAP methods use the same interface between the peer state machine and method specific functions. This allows new EAP methods to be added without modifying the core EAP state machine implementation.
New EAP methods need to be registered by adding them into build (Makefile) and EAP method table in the beginning of eap.c. Each EAP method should use a build-time configuration option, e.g., EAP_TLS, in order to make it possible to select which of the methods are included in the build.
EAP methods must implement the interface defined in eap_i.h. struct eap_method defines the needed function pointers that each EAP method must provide. In addition, the EAP type and name are registered using this structure. This interface is based on section 4.4 of RFC 4137.
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