I recently installed the Oracle Instant Client 21c on a Linux host.

I then tried to access an Oracle 19c RAC database on another host:

connect soe/soe@racn1.rrcsc.pmax.local:1521/metrodb.rrcsc.pmax.local
ORA-12637: Packet receive failed

Numerous blog posts on this error, but in this case I needed to add the directive DISABLE_OOB=ON to my SQLNET.ORA.

Normally this would be in $ORACLE_HOME/network/admin, but on the Instant Client, it is located in:

[root@MYLINUX01 ~]# cat /lib/oracle/21/client64/lib/network/admin/sqlnet.ora

DISABLE_OOB=ON