FreeRADIUS InkBridge

EAP-TLS: Certificate-based authentication

This documentation is a work in progress. This page is taken from the version 3 documentation, and has not yet been updated for version 4.

The general approach of this page is correct, but you will have to update the syntax if you are using version 4.

Contact InkBridge Networks for more details.

Goal: To configure the server to use the EAP-TLS authentication protocol and to send and receive test packets.

Time: 20-35 minutes.

Files:

  • mods-available/eap

  • scripts/certs.sh

  • certs/

EAP-TLS is an authentication protocol that uses a TLS session, along with client and server certificates, to authenticate a user. You should now verify that the rlm_eap_tls module was built and installed by executing:

$ ls -l usr/lib64/freeradius/rlm_eap*

Use the directory where the FreeRADIUS libraries were installed.

You should see not only files like rlm_eap.so, but also rlm_eap_md5.so and rlm_eap_tls.so. If you do not see those files, you will have to install them now. If necessary, you may have to re-build the server, as follows:

$ ./configure --prefix=$HOME/freeradius/ \
              --with-openssl-include-dir=/usr/local/ssl/include \
              --with-openssl-lib-dir=/usr/local/ssl/lib
$ make
$ make install

This exercise has insufficient room to describe how to debug any configuration, build, or installation problems with the rlm_eap_tls module. For the remainder of this exercise, we will assume that the driver is installed in the appropriate library directory.

By default, the server creates a set of test certificates inside of the raddb directory, called certs. This directory contains sample server and client certificates, along with a demonstration certificate authority. These certificates may be used for this exercise, but should not be used in any live deployment of the server.

You now edit the mods-available/eap file to enable the tls. You should also set the configuration entry default_eap_type to tls.

Start the server and verify that the tls module was loaded and that the server is Ready to process requests.

This exercise does not cover how to configure EAP-TLS on the wireless client, nor how to set up a wireless access point to perform EAP-TLS. We suggest that the you consult the documentation for your wireless client software for details on this process.

The wireless client will require the client certificate from the certs directory.

Once the wireless client has been configured to enable EAP-TLS, you should perform a test authentication to the server. If all goes well, the server, AP, and wireless client should exchange multiple RADIUS Access-Request and Access-Challenge packets. This process should take a few seconds, and you should wait until it is done. If all goes well, the final packet from the server should be an Access-Accept and should contain the MS-MPPE-Recv-Key and MS-MPPE-Send-Key attributes.

Verify that the authentication succeeded by using the ping command to see if the wireless client now has network access.

Further considerations

EAP-TLS is an authentication protocol and nothing more. As such, the wireless client will still need to obtain an IP address. This address may be configured statically, or it may be obtained through DHCP.

A benefit of EAP-TLS over EAP-MD5 is that it supports the creation of dynamic WEP keys. These keys are more secure than statically configured WEP keys and do not require administrator interaction to configure them. However, they are not perfectly secure, and we recommend that the server be configured to send a Session-Timeout attribute in the RADIUS Access-Accept packet in order to force the user to re-authenticate periodically, which will create new dynamic WEP keys.

An issue with EAP-TLS is that it requires a client certificate. For secure authentication that does not require client certificates, see the EAP-TTLS and EAP-PEAP protocols.

	Configuring the wireless client (usually a laptop or notebook
computer) to perform EAP-TLS may be difficult. Windows (since Windows XP)
ships with an EAP-PEAP client, which may influence any decision about
which wireless authentication protocol to use.

Questions

  1. What other benefits does EAP-TLS have over EAP-MD5?

  2. What happens to the user’s password when they use EAP-TLS?

  3. Would you use EAP-TLS in a large deployment? If so, why? If not, why not?

  4. What is the purpose of the MS-MPPE-Recv-Key and MS-MPPE-Send-Key attributes in the final Access-Accept packet?