How to trace remote oracle connections, on the client side?

In order to trace oracle remote connections on the client side, first we need to add a few entries to the sqlnet.ora file; for example, to set trace at the highest level, for oracle support, we need level 16:

 box1:/u01/app/oracle/client_trace> cat /etc/sqlnet.ora
TRACE_LEVEL_CLIENT=16
TRACE_DIRECTORY_CLIENT=/u01/app/oracle/client_trace
LOG_DIRECTORY_CLIENT=/u01/app/oracle/client_trace
TRACE_TIMESTAMP_CLIENT=ON
DIAG_ADR_ENABLED=OFF

The last line, disabling diag is required starting with 11G.

In addition, there are two more steps to follow, to make sure that the trace/log file will be generated as expected:

1) Oracle will look for sqlnet.ora in $ORACLE_HOME/networking/admin, by default, so either update the file in this location or define TNS_ADMIN, in the environment where the connection to the DB is taking place.
2) Make sure the directory destination for trace/log files it has read/write permission for the owner of the process you want to trace; oracle will silently ignore your request and just it won't generate the trace file otherwise.