The latest version of the Splunk App for AWS version 4.0 (https://splunkbase.splunk.com/app/1274/) has simplified the process of adding one or more AWS accounts. All configurations for additional AWS accounts and data sources (CloudTrail, Config, VPC Flow Logs, CloudWatch, etc.) are now done all within the Splunk App for AWS. Here are the instructions on how to configure additional AWS accounts for the Splunk App for AWS.
https://docs.splunk.com/Documentation/AWS/4.0.0/Installation/Setuptheapp
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