Here is the way to change the default splunk forwarder management port:
In /opt/splunkforwarder/etc/system/local, add the following file and contents:
web.conf with
[settings]
location of splunkd; don't include http[s]:// in this anymore.
mgmtHostPort = 127.0.0.1:8090
or whatever port that is not used, and restart splunk.
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