By links to reports do you mean alert emails? For alert emails you can set the hostname option in alert_actions.conf.
alert_actions.conf.spec
hostname = <string>
* Sets the hostname used in the web link (url) sent in alerts.
* This value accepts two forms.
* hostname
examples: splunkserver, splunkserver.example.com
* protocol://hostname:port
examples: http://splunkserver:8000, https://splunkserver.example.com:443
* When this value is a simple hostname, the protocol and port which
are configured within splunk are used to construct the base of
the url.
* When this value begins with 'http://', it is used verbatim.
NOTE: This means the correct port must be specified if it is not
the default port for http or https.
* This is useful in cases when the Splunk server is not aware of
how to construct an externally referencable url, such as SSO
environments, other proxies, or when the Splunk server hostname
is not generally resolvable.
* Defaults to current hostname provided by the operating system, or if that fails "localhost".
* When set to empty, default behavior is used.
HTH
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