Probably you're using non-syslog sourcetype. In that case, try syslogSourceType attribute in outputs.conf. This should avoid adding the originated hostname.
syslogSourceType = <string>
* Specifies an additional rule for handling data, in addition to that
provided by the 'syslog' source type.
* This string is used as a substring match against the sourcetype key. For
example, if the string is set to "syslog", then all sourcetypes
containing the string 'syslog' receive this special treatment.
* To match a sourcetype explicitly, use the pattern
"sourcetype::sourcetype_name".
* Example: syslogSourceType = sourcetype::apache_common
* Data that is "syslog" or matches this setting is assumed to already be in
syslog format.
* Data that does not match the rules has a header, optionally a timestamp
(if defined in 'timestampformat'), and a hostname added to the front of
the event. This is how Splunk software causes arbitrary log data to match syslog expectations.
* No default.
For more detail,
Official Doc:
https://docs.splunk.com/Documentation/Splunk/latest/Data/HowSplunkEnterprisehandlessyslogdata
Community Wiki: (old)
https://wiki.splunk.com/Community:Test:How_Splunk_behaves_when_receiving_or_forwarding_udp_data
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