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Extensible Authentication Protocol (EAP)

Introduction

Extensible Authentication Protocol(EAP), RFC 3748, is an authentication framework and data link layer protocol that allows network access points to support multiple authentication methods. Each EAP Type indicates a specific authentication mechanism. The 802.1X standard authenticates both wireless and wired LAN users/devices trying to access Enterprise networks.

RADIUS attribute used for EAP is EAP-Message, 79(rfc2869). RADIUS communicates all EAP messages by embedding them in this attribute.

General Terminology Authenticator/NAS/Access Point(AP) - A network device providing users Supplicant/EAP Client - The software on the end-user/client machine (e.g. machine with the wireless card or connected to an 802.1X switch port). with a point of entry into the network. EAPOL - EAP over LAN as defined in 802.1X standard. EAPOW - EAP over Wireless as defined in the 802.11 standard.

   +----------+        +----------+        +----------+
   |          |  EAPOL |          | RADIUS |          |
   |  EAP     |<------>|  Access  |<------>|  RADIUS  |
   |  Client  |  EAPOW |  Point   |  (EAP) |  Server  |
   |          |        |          |        |          |
   +----------+        +----------+        +----------+

The sequence of events, for EAP-MD5, runs as follows:

  1. The end-user associates with the Access Point(AP).

  2. The supplicant specifies AP to use EAP by sending EAP-Start.

  3. AP requests the supplicant to Identify itself (EAP-Identity).

  4. Supplicant then sends its Identity (username) to the AP.

  5. AP forwards this EAP-response AS-IS to the RADIUS server. (The supplicant and the RADIUS server mutually authenticate via AP. AP just acts as a passthru till authentication is finished.)

  6. The server sends a challenge to the supplicant.

  7. The supplicant carries out a hash on the password and sends this hashed password to the RADIUS server as its response.

  8. The RADIUS server performs a hash on the password for that supplicant in its user database and compares the two hashed values and authenticates the client if the two values match(EAP-Success/EAP-Failure)

  9. AP now opens a port to accept data from the end-user.

Currently, EAP is widely used in wireless networks than in wired networks. In 802.11/wireless based networking, following sequence of events happen in addition to the above EAP events.

  1. RADIUS server and the supplicant agree to a specific WEP key.

  2. The supplicant loads the key ready for logging on.

  3. The RADIUS server sends the key for this session (Session key) to the AP.

  4. The AP encrypts its Broadcast key with the Session key

  5. The AP sends the encypted key to the supplicant

  6. The supplicant decrypts the Broadcast key with the Session key and the session continues using the Broadcast and Session keys until the session ends.

References:

The Implementation of EAP over RADIUS is based on the following RFCs

  • rfc2869 - RADIUS Extensions

  • rfc2284 - PPP Extensible Authentication Protocol (EAP)

  • rfc2716 - PPP EAP TLS Authentication Protocol

Following links help to understand HOW EAP works http://www.ieee802.org/1/mirror/8021/docs2000/ieee_plenary.PDF

EAP Code Organization

EAP is implemented as a module in freeradius and the code is placed in src/modules/rlm_eap. All EAP-Types are organized as subdirectories in rlm_eap/types/.

Each EAP-Type, like types/rlm_eap_md5, contains a chunk of code that knows how to deal with a particular kind of authentication mechanism.

To add a new EAP-Type then a new directory should be created as rlm_eap/types/rlm_eap_XXXX, where XXXX is EAP-Type name ie for EAP-Type like ONE TIME PASSWORD (OTP) it would be rlm_eap_otp

Path Description

src/modules/rlm_eap

Contains the basic EAP and generalized interfaces to all the EAP-Types.

rlm_eap/types

Contains all the supported EAP-Types

rlm_eap/types/rlm_eap_md5

EAP-MD5 authentication.

rlm_eap/types/rlm_eap_tls

EAP-TLS based authentication.

rlm_eap/types/rlm_eap_ttls

TTLS based authentication.

rlm_eap/types/rlm_eap_peap

Windows PEAP based authentication.

rlm_eap/types/rlm_eap_sim

EAP-SIM (GSM) based authentication

Configuration

Add the eap configuration stanza to the modules section in radiusd.conf to load and control rlm_eap and all the supported EAP-Types:

For example:

modules {
    ...
    eap {
        default_eap_type = md5

        md5 {
        }
        ...
    }
    ...
}
You cannot have empty eap stanza. At least one EAP-Type sub-stanza should be defined as above, otherwise the server will not know what type of eap authentication mechanism to be used and the server will exit with error.

All the various options and their associated default values for each EAP-Type are documented in the sample radiusd.conf that is provided with the distribution.

Since the EAP requests may not contain a requested EAP type, the default_eap_type configuration options is used by the EAP module to determine which EAP type to choose for authentication.

EAP cannot authorize a user. It can only authenticate. Other Freeradius modules authorize the user.

EAP SIM server

To configure EAP-SIM authentication, the following attributes must be set in the server. This can be done in the users file, but in many cases will be taken from a database server, via one of the SQL interface.

If one has SIM cards that one controls (i.e. whose share secret you know), one should be able to write a module to generate these attributes (the triplets) in the server.

If one has access to the SS7 based settlement network, then a module to fetch appropriate triplets could be written. This module would act as an authorization only module.

The attributes are:

Attribute Size

EAP-Sim-Rand1

16 bytes

EAP-Sim-SRES1

4 bytes

EAP-Sim-KC1

8 bytes

EAP-Sim-Rand2

16 bytes

EAP-Sim-SRES2

4 bytes

EAP-Sim-KC2

8 bytes

EAP-Sim-Rand3

16 bytes

EAP-Sim-SRES3

4 bytes

EAP-Sim-KC3

8 bytes
EAP-SIM will send WEP attributes to the resquestor.

EAP Clients

The main EAP client is https://w1.fi/wpa_supplicant/

Audit

An Open Source audit tool is available at:

https://github.com/ANSSI-FR/audit-radius

FAQ & Examples

How do i use it?

  1. How can I enable EAP-MD5 authentication ?

In radiusd.conf

  modules {
    ...
    eap {
        default_eap_type = md5
        md5 {
        }
        ...
    }
    ...
  }

  # eap sets the authenticate type as EAP
  recv Access-Request {
    ...
    eap
  }

  # eap authentication takes place.
  process Access-Request {
    eap
  }

  1. My Userbase is in LDAP and I want to use EAP-MD5 authentication

In radiusd.conf

  modules {
    ...
    eap {
        default_eap_type = md5
        md5 {
        }
        ...
    }
    ...
  }

  # ldap gets the Configured password.
  # eap sets the authenticate type as EAP
  recv Access-Request {
    ...
    ldap
    eap
    ...
  }

  # eap authentication takes place.
  process Access-Request {
    ...
    eap
    ...
  }

  1. How can I Proxy EAP messages, with/without User-Name attribute in the Access-Request packets

With User-Name attribute in Access-Request packet, EAP-proxying is just same as RADIUS-proxying.

If User-Name attribute is not present in Access-Request packet, Freeradius can proxy the request with the following configuration in radiusd.conf

#  eap module should be configured as the First module in
#  the authorize stanza

recv Access-Request {
  eap
  ...  other modules.
}

With this configuration, eap_authorize creates User-Name attribute from EAP-Identity response, if it is not present. Once User-Name attribute is created, RADIUS proxying takes care of EAP proxying.

  1. How Freeradius can handle EAP-START messages ?

In most of the cases this is handled by the Authenticator.

Only if it is required then, in radiusd.conf

recv Access-Request {
    eap
    ...  other modules.
}

With the above configuration, RADIUS server immediately responds with EAP-Identity request.

EAP does not check for any Identity or maintains any state in case of EAP-START. It blindly responds with EAP-Identity request. Proxying is handled only after EAP-Identity response is received.

  1. I want to enable multiple EAP-Types, how can I configure ?

In radiusd.conf

modules {
    ...
    eap {
        default_eap_type = tls
        md5 {
        }
        tls {
            ...
        }
        ...
    }
    ...
}

The above configuration will let the server load all the EAP-Types, but the server can have only one default EAP-Type, as above.

Once EAP-Identity response is received by the server, based on the default_eap_type, the server will send a new request (MD5-Challenge request incase of md5, TLS-START request incase of tls) to the supplicant. If the supplicant is rfc2284 compliant and does not support the EAP-Type sent by the server then it sends EAP-Acknowledge with the supported EAP-Type. If this EAP-Type is supported by the server then it will send the respective EAP-request.

Example: If the supplicant supports only EAP-MD5 but the server default_eap_type is configured as EAP-TLS, as above, then the server will send TLS-STAR after EAP-Identity is received. Supplicant will respond with EAP-Acknowledge (EAP-MD5). Server now responds with MD5-Challenge.

Installation

EAP, EAP-MD5, and EAP-MSCHAPv2 do not require any additional packages. Freeradius contains all the required packages.

For EAP-TLS, EAP-TTLS, and PEAP, OPENSSL, http://www.openssl.org/, is required to be installed. Any version from 0.9.7, should fairly work with this module.

EAP-SIM should not require any additional packages.

Implementation (For Developers)

The rlm_eap module only deals with EAP specific authentication mechanism and the generic interface to interact with all the EAP-Types.

Currently, these are the existing interfaces,

int attach(CONF_SECTION *conf, void **type_arg);
int initiate(void *type_arg, EAP_HANDLER *handler);
int authenticate(void *type_arg, EAP_HANDLER *handler);
int detach(void **type_arg);

attach() and detach() functions allocate and deallocate all the required resources.

initiate() function begins the conversation when EAP-Identity response is received. Incase of EAP-MD5, initiate() function sends the challenge.

authenticate() function uses specific EAP-Type authentication mechanism to authenticate the user. During authentication many EAP-Requests and EAP-Responses takes place for each authentication. Hence authenticate() function may be called many times. EAP_HANDLER contains the complete state information required.

How EAP works

as posted to the list, by John Lindsay jlindsay@internode.com.au

To make it clear for everyone, the supplicant is the software on the client (machine with the wireless card).

The EAP process doesn’t start until the client has associated with the Access Point using Open authentication. If this process isn’t crystal clear you need to go away and gain understanding.

Once the association is made the AP blocks all traffic that is not 802.1X so although associated the connection only has value for EAP. Any EAP traffic is passed to the radius server and any radius traffic is passed back to the client.

So, after the client has associated to the Access Point, the supplicant starts the process for using EAP over LAN by asking the user for their logon and password.

Using 802.1X and EAP the supplicant sends the username and a one-way hash of the password to the AP.

The AP encapsulates the request and sends it to the RADIUS server.

The radius server needs a plaintext password so that it can perform the same one-way hash to determine that the password is correct. If it is, the radius server issues an access challenge which goes back via to the AP to the client. (my study guide says client but my brain says supplicant)

The client sends the EAP response to the challenge via the AP to the RADIUS server.

If the response is valid the RADIUS server sends a success message and the session WEP key (EAP over wireless) to the client via the AP. The same session WEP key is also sent to the AP in the success packet.

The client and the AP then begin using session WEP keys. The WEP key used for multicasts is then sent from the AP to the client. It is encrypted using the session WEP key.

Acknowledgements