I am running splunk server(indexer) 4.2.3 on a RHEL machine and appr. 200 universal forwarders(all running 4.2.3) are sending logs to this server. My question is, I want to upgrade my server(indexer) from 4.2.3 to 4.3. So after the indexer upgrade, does the universal forwarders need to be upgraded to 4.3? Please let me know about this. Thanks in Advance!!!
It's not required, but there were a couple security fixes in 4.2.5 so you should at least review the change notes for 4.2.5 to see if they apply to you.
It is not required. "The universal forwarder is both backwards compatible with older Splunk indexers and forward compatible with newer ones. You can forward data to any Splunk indexer that is version 3.4.14 or above" (from the Universal forwarder deployment overview in the Distributed Deployment Manual). If you do want to upgrade your universal forwarders, see the upgrading information in the Installation Manual.