| Symptom | Root Cause | Resolution | |---------|------------|------------| | ORA-12170: TNS timeout | Firewall blocking port 1521 | Verify iptables rules: iptables -L -n | | ORA-12545: Host not found | DNS resolution failure | Use IP addresses in tnsnames.ora or set /etc/hosts | | ORA-28040: No matching auth | Protocol version mismatch | Set SQLNET.ALLOWED_LOGON_VERSION=8 in sqlnet.ora | | Library link error | Missing libaio or libnsl | yum install libaio libnsl |

Mandate TLS 1.2+ via sqlnet.ora :

# Extract client libraries unzip instantclient-basic-linux.x64-21.13.0.0.0dbru.zip -d /opt/oracle echo "/opt/oracle/instantclient_21_13" > /etc/ld.so.conf.d/oracle.conf ldconfig Set environment variables export ORACLE_HOME=/opt/oracle/instantclient_21_13 export TNS_ADMIN=/etc/oracle export LD_LIBRARY_PATH=$ORACLE_HOME:$LD_LIBRARY_PATH