Are you using "sslCommonNameToCheck"; this name should be defined in your cert installed on DS for validating the connections from Forwarders. below for your reference... sslCommonNameToCheck = <comma-separated list>
* One or more X.509 standard Common Names of the server certificate which splunkd,
as a client, checks against when it connects to a server using TLS.
* The Common Name (CN) is an X.509 standard field in a certificate that identifies the
host name that is associated with the certificate.
* The CN can be a short host name or a fully qualified domain name. For example,
the CN can be one of "example", "www.example.com", or "example.com".
* If the client cannot match the CN in the certificate that the server presents,
then the client cannot authenticate the server, and terminates the session
negotiation immediately.
* For this setting to have any affect, the 'sslVerifyServerCert' setting must have
a value of "true".
* This setting is optional.
* No default (no common name checking).
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