Introduction

A more secure way than using pre-shared keys (WPA2) is to use EAP-TLS and use separate certificates for each device. In the previous tutorial Linux Router with VPN on a Raspberry Pi I mentioned I'd be doing this with a (Ubiquiti UniFi AP). I have tested this with two phones running CyanogenMod 11 (Android 4.4.4).

Installation

Install freeradius and haveged. You'll need haveged to increase randomness of /dev/random Entropy and randomness. When feature 3465 is resolved if you have a Raspberry Pi you could use it's own hardware random number generator (bcm2708-rng).

apk add freeradius freeradius-eap haveged

Haveged requires no configuration:

service haveged start

rc-update add haveged default

Certificates

You will want to create your certificates. The easiest way to do that is to use the scripts provided by FreeRadius. The scripts allow you to easily create a CA (certificate authority), Server certificate, and Client certificates. Remember to increase the expiry time from 60 days if that doesn't suit you and fill in the other information in the .cnf files like the README says.

The readme for that script is in /etc/raddb/certs/README or can be found here.

Certificate Revocation List

The CRL is not created by the script, you have to do that one manually.

I created a file called crl.cnf:

[ ca ]
default_ca		= CA_default

[ CA_default ]
dir			= ./
certs			= $dir
crl_dir			= $dir/crl
database		= $dir/index.txt
new_certs_dir		= $dir
certificate		= $dir/ca.pem
serial			= $dir/serial
crl			= $dir/crl.pem
private_key		= $dir/ca.key
RANDFILE		= $dir/.rand
name_opt		= ca_default
cert_opt		= ca_default
default_days		= 730
default_crl_days	= 730
default_md		= sha256
preserve		= no
policy			= policy_match
crlDistributionPoints	= URI:http://www.example.com/example_ca.crl

[ policy_match ]
countryName		= match
stateOrProvinceName	= match
organizationName	= match
organizationalUnitName	= optional
commonName		= supplied
emailAddress		= optional

[ policy_anything ]
countryName		= optional
stateOrProvinceName	= optional
localityName		= optional
organizationName	= optional
organizationalUnitName	= optional
commonName		= supplied
emailAddress		= optional

[ req ]
prompt			= no
distinguished_name	= cacrl
default_bits		= 2048
input_password		= <password1>
output_password		= <password2>
x509_extensions		= v3_ca

[certificate_authority]
countryName		= <COUNTRY_CODE>
stateOrProvinceName	= Radius
localityName		= <REGION>
organizationName	= FreeRadius
emailAddress		= freeradius@localhost 
commonName		= "FreeRadius Certificate Authority"

[v3_ca]
subjectKeyIdentifier	= hash
authorityKeyIdentifier	= keyid:always,issuer:always
basicConstraints	= CA:true
crlDistributionPoints	= URI:http://www.example.com/example_ca.crl

Create the revocation list:

openssl ca -gencrl -keyfile ca.key -cert ca.pem -out crl.pem -config crl.cnf

Finally, create new file which will hold both CA and revoked certificates:

cat ca.pem crl.pem > cacrl.pem

Create the Diffie-Hellman file

openssl dhparam -check -text -5 1024 -out /etc/raddb/certs/dh

Or you can use a larger one, eg (this can take a while if you're unlucky!).

openssl dhparam -check -text -5 4096 -out /etc/raddb/certs/dh

Configuration

/etc/raddb/clients.conf

First we're going to add a client, this is your WiFi AP:

client home {
    ipaddr = 192.168.1.10
    proto = *
    secret = <PASSWORD USED BY YOUR AP TO AUTHENTICATE WITH THIS RADIUS SERVER>
    shortname = <YOUR_SSID>
    require_message_authenticator = no
    nas_type = other

    limit {
        max_connections = 16
        lifetime = 0
        idle_timeout = 30
    }
}

/etc/raddb/mods-enabled/eap

Next we configure eap. Note the + and - represent lines removed and added, don't include them in your config!

You're going to want to make these changes:

- default_eap_type = md5
+ default_eap_type = tls

-   private_key_password = whatever
+   private_key_password = <Password you set output_password in server.cnf>
    private_key_file = ${certdir}/server.pem

-   ca_file = ${cadir}/ca.pem
+   ca_file = ${cadir}/cacrl.pem

-   random_file = /dev/urandom
+   random_file = /dev/random

- # check_crl = yes                 
+   check_crl = yes

Reduce cipher list from DEFAULT to HIGH, or even a specific list:

-   cipher_list = "DEFAULT"
+   cipher_list = "HIGH"

Or a shorter list if you decide (might cause some device compatibility issues)

+   cipher_list = "ECDHE-RSA-AES128-GCM-SHA256:ECDHE-ECDSA-AES128-GCM-SHA256:ECDHE-RSA-AES128-SHA256:ECDHE-ECDSA-AES128-SHA256:ECDHE-RSA-AES128-SHA:ECDHE-ECDSA-AES128-SHA:DHE-RSA-AES128-GCM-SHA256:DHE-RSA-AES128-SHA256:DHE-RSA-AES128-SHA:ECDHE-RSA-AES256-GCM-SHA384:ECDHE-ECDSA-AES256-GCM-SHA384:ECDHE-RSA-AES256-SHA384:ECDHE-ECDSA-AES256-SHA384:ECDHE-RSA-AES256-SHA:ECDHE-ECDSA-AES256-SHA:DHE-RSA-AES256-GCM-SHA384:DHE-RSA-AES256-SHA256:DHE-RSA-AES256-SHA:DHE-RSA-CAMELLIA256-SHA:DHE-RSA-CAMELLIA128-SHA"

Change ecdh curve to something stronger:

-   ecdh_curve = "prime256v1"
+   ecdh_curve = "secp384r1"

You can also increase the curve to a higher bit (521), but this may cause compatibility problems.

+   ecdh_curve = "secp521r1"

These all worked with Android 4.4.4, but if you have older stuff you may need to set the list to HIGH or DEFAULT.

Couple of other things to change:

-     #name = "EAP module"
+     name = "EAP-TLS"

-     #persist_dir = "${logdir}/tlscache"
+     persist_dir = "${logdir}/tlscache"

/etc/raddb/sites-enabled/default

Change the listening port to what suits you

- ipaddr = *
+ ipv4addr = 192.168.1.1

Disable chap

#  The chap module will set 'Auth-Type := CHAP' if we are
#  handling a CHAP request and Auth-Type has not already been set
- chap
+ # chap

Disable mschap

#  the MS-CHAP-Challenge attribute, and add 'Auth-Type := MS-CHAP'
#  to the request, which will cause the server to then use
#  the mschap module for authentication.
- mschap
+ # mschap

Disable pap

#  This module should be listed last, so that the other modules
#  get a chance to set Auth-Type for themselves.

- pap
+ #pap

Disable the auth types we're not using

- Auth-Type PAP {
-   pap
- }
+ #Auth-Type PAP {
+ # pap
+ #}

- Auth-Type CHAP {
-   chap
- }
+ #Auth-Type CHAP {
+ # chap
+ #}

- Auth-Type MS-CHAP {
-   mschap
- }
+ #Auth-Type MS-CHAP {
+ # mschap
+ #}

Enable eap

-#  eap
+ eap

/etc/raddb/sites-available/tls

tls {
-   private_key_password = whatever
+   private_key_password = <Password you set input_password in server.cnf>
    private_key_file = ${certdir}/server.pem

IPtables rules

Next up you're going to want some iptables rules. Using the rules from the previous tutorial add them somewhere near the top:

#Accept incoming connections from client FreeRadius
iptables -A INPUT -i ${INT_IF} -p tcp -s 192.168.1.10 --dport 1812 -j ACCEPT
iptables -A OUTPUT -o ${INT_IF} -p tcp --sport 1812 -j ACCEPT
iptables -A INPUT -i ${INT_IF} -p udp -s 192.168.1.10 --dport 1812 -j ACCEPT
iptables -A OUTPUT -o ${INT_IF} -p udp --sport 1812 -j ACCEPT

Configure AP

You're going to want to configure your access point to talk to your new Radius server.

Using the secret and shortname from clients.conf enter them into your access point administration panel.

Start Radius

service radiusd start

Add to default run level.

rc-update add radiusd default

You can debug it with radiusd -X from the console, or check /var/log/radius/radius.log if that didn't work.

Configure a device

On Android I go into "Settings > Security > Install from Storage" and select ca.pem

I then do "Settings > Security > Install from Storage" and select client.p12"

After putting in the correct passwords it should work. On Android you may see a warning such as "Network May Be Monitored by an Unknown Third Party". You can fix this by moving the CA from /data/misc/keychain/cacerts-added to /system/etc/security/cacerts make sure the user and group are root and that the permissions are set to 644, ie readable by everyone, only root has permissions to write to the files. Keep it in /sdcard/ so you can move it back if you re-flash the phone with a newer ROM.

Revoke a certificate

If in the future you want to revoke the certificates of a particular user you can do this by:

openssl ca -revoke user@example.com.pem -keyfile ca.key -cert ca.pem -config ca.cnf

Now, take a moment and open index.txt and you should see "R" next to cert index number. If you ever need to make this cert valid again, you would edit line with "R" to match other certs format.

Now you need to create crl list again, just like it was done at the beginning of tutorial: