The Free Radius server is an open source Radius server available for all major Linux distributions, including Ubuntu: http://freeradius.org/doc/ You will need to configure the wireless access point to send authentication requests to the Radius server.
From the Cisco Document:
Configuring the Switch-to-RADIUS-Server Communication
RADIUS security servers are identified by their host name or IP
address, host name and specific UDP port numbers, or IP address and
specific UDP port numbers. The combination of the IP address and UDP
port number creates a unique identifier, which enables RADIUS requests
to be sent to multiple UDP ports on a server at the same IP address.
If two different host entries on the same RADIUS server are configured
for the same service—for example, authentication—the second host entry
configured acts as the fail-over backup to the first one. The RADIUS
host entries are tried in the order that they were configured.
Beginning in privileged EXEC mode, follow these steps to configure the
RADIUS server parameters on the switch. This procedure is required.
Command Purpose
Step 1
configure terminal
Enter global configuration mode.
Step 2
radius-server host {hostname | ip-address} auth-port port-number key string Configure the RADIUS server parameters on the switch. For
hostname | ip-address, specify the host name or IP address of the
remote RADIUS server.
For auth-port port-number, specify the UDP destination port for
authentication requests. The default is 1812.
For key string, specify the authentication and encryption key used
between the switch and the RADIUS daemon running on the RADIUS server.
The key is a text string that must match the encryption key used on
the RADIUS server.
Note Always configure the key as the last item in the radius-server
host command syntax because leading spaces are ignored, but spaces
within and at the end of the key are used. If you use spaces in the
key, do not enclose the key in quotation marks unless the quotation
marks are part of the key. This key must match the encryption used on
the RADIUS daemon.
If you want to use multiple RADIUS servers, re-enter this command.