The problem doesn't seems to be the DB Connect, it seems to be your Postgres instance that is not configured to accept connections from your Splunk Heavy Fwd host.
Have a look on the pg_hba.conf file and some documentation, is quite straight forward to understand it, you basically define a source IP/netmask, db and the authentication method allowed. It works like a application firewall to prevent unauthorized access. You might need to restart/reload the Postgres after update the pg_hba.conf
... View more