EAP server implementation

Extensible Authentication Protocol (EAP) is an authentication framework defined in RFC 3748. hostapd uses a separate code module for EAP server 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 server state machine and methods. As such, this RFC provides useful information for understanding the EAP server implementation in hostapd.

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.

Adding EAP methods

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 server 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 the build (Makefile) and the EAP method registration list in the eap_server_register_methods() function of eap_methods.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.

It is recommended that the EAP methods would use generic helper functions, eap_msg_alloc() and eap_hdr_validate() when processing messages. This allows code sharing and can avoid missing some of the needed validation steps for received packets. In addition, these functions make it easier to change between expanded and legacy EAP header, if needed.

When adding an EAP method that uses a vendor specific EAP type (Expanded Type as defined in RFC 3748, Chapter 5.7), the new method must be registered by passing vendor id instead of EAP_VENDOR_IETF to eap_server_method_alloc(). These methods must not try to emulate expanded types by registering a legacy EAP method for type 254. See eap_vendor_test.c for an example of an EAP method implementation that is implemented as an expanded type.

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