if you are familiar with C#, you might want to wrap server side OpenVPN authentication with openvpn-auth-radius
add the following line to server.conf:
auth-user-pass-verify '/usr/local/bin/mono /etc/openvpn/auth.exe' via-env
example auth.exe.config:
<?xml version="1.0" encoding="utf-8" ?>
<configuration>
<configSections>
<section name="MyConfig" type="auth.Config, auth" />
</configSections>
<MyConfig NAS_IDENTIFIER="OpenVpn">
<servers>
<server name="radius1.example" authport="1812" wait="1" retries="3" sharedsecret="secret1" />
<server name="radius2.example" authport="1812" wait="1" retries="2" sharedsecret="secret2" />
</servers>
</MyConfig>
</configuration>