I have had to setup Apache to proxy splunk several times over the past 6 months and keep forgetting the splunk configuration to make this work.
Be sure to set the following in /opt/splunk/etc/system/local/web.conf:
enableSplunkWebSSL = 0 root_endpoint = /splunk tools.proxy.on = True
This assumes a proxy configuration of the following for apache 2.2:
ProxyPass /splunk http://localhost:8000/splunk ProxyPassReverse /splunk http://localhost:8000/splunk
Be sure to secure your proxy before enabling!
/opt/splunk/bin/splunk stop /opt/splunk/bin/splunk start