Duo Security does not provide a native CAS integration. Unicon includes support for Duo in their third-party multifactor module for CAS 4.1.x. See Unicon's CAS-MFA project wiki for additional information.
CAS 5.0.x includes Duo multifactor support. See the CAS Documentation for configuration instructions.
This application communicates with Duo's service on TCP port 443. Firewall configurations that restrict outbound access to Duo's service with rules using destination IP addresses or IP address ranges aren't recommended, since these may change over time to maintain our service's high availability. If your organization requires IP-based rules, please review this Duo KB article.
The security of your Duo application is tied to the security of your secret key (skey). Secure it as you would any sensitive credential. Don't share it with unauthorized individuals or email it to anyone under any circumstances!