We've noticed that our splunk server was phoning home to an external IP over port 443. What's the purpose of this traffic?
This is most likely splunk phoning home to apps.splunk.com to check for updates. You can verifiy if this is the case by looking up the destination IP:
dig +short apps.splunk.com
dig +short -x 22.214.171.124
You should see a correlating event in the splunkd.log, but I believe only if there is an error:
WARN HTTPClient - Connect to=apps.splunk.com:443 timed out; exceeded 10sec
ERROR ApplicationUpdater - Error checking for update, URL=/api/apps:resolve/checkforupgrade: Connect to=https://apps.splunk.com timed out; exceeded 10sec
ERROR HTTPClient - Cannot resolve IP of host=apps.splunk.com: No such host is known.
If you would like to disable this behavior, check this answer: