TCP.VALIDNODE_CHECKING

Well, this is embarassing :)

I unwittingly learnt how these parameters work:

In my SQLNET.ORA, I had

TCP.VALIDNODE_CHECKING=YES
TCP.EXCLUDED_NODES=SQLSERVER1

Looks like this file gets read by the LISTENER and NOT the database! Argh!

When I restarted the listener, the customer's SQL Server host was blocked from accessing any Oracle databases on my db host. Had to troubleshoot using listener tracing, and figured out that the listener was the culprit.

So, I simply removed those lines and restarted the listener - problem gone away.

Note that the error message on the client-side is misleading, since it says that the service is not found:

ORA-12514: TNS:listener does not currently know of service requested in connect descriptor

Off course, this is totally misleading.