I just set up a free Duo account and installed and configured the add-on without issues. The only other things I can suggest are 1) verify your Splunk instance's public egress address is in the Admin API application's "Networks for API access" list, 2) verify any intervening host or network firewalls or transparent proxies allow connectivity to your API hostname, and 3) verify your Splunk host can connect to your API hostname using openssl: $SPLUNK_HOME/bin/splunk cmd openssl s_client -connect api-xxx.duosecurity.com:443 The Duo Admin API Python client used by the add-on supports HTTP proxies, but Duo didn't include proxy support in the modular input. If you need this feature, you'll need to request it from Duo. ... View more