there is different conf files you can set this depending if you are looking at setting the actual hostname or the instance name or custom client name:
inputs.conf for splunk actual hostname
host = <string>
* Sets the host key/field to a static value for this stanza.
* Primarily used to control the host field, which will be used for events coming in
via this input stanza.
* Detail: Sets the host key's initial value. The key is used during parsing/indexing,
in particular to set the host field. It is also the host field used at search time.
* As a convenience, the chosen string is prepended with 'host::'.
* WARNING: Do not quote the <string> value: host=foo, not host="foo".
* If set to '$decideOnStartup', will be interpreted as hostname of executing machine;
such interpretation will occur on each splunkd startup. This is the default.
server.conf for splunk instance name :
serverName = <ASCII string>
* The name used to identify this Splunk instance for features such as distributed search.
* Defaults to <hostname>-<user running splunk>.
* May not be an empty string
* May contain environment variables
* After any environment variables have been expanded, the server name (if not an IPv6
address) can only contain letters, numbers, underscores, dots, and dashes; and
it must start with a letter, number, or an underscore.
deploymentclient.conf for custom client name
clientName = deploymentClient
* Defaults to deploymentClient.
* A name that the deployment server can filter on.
* Takes precedence over DNS names.
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