How to confirm that the parameter INBOUND_CONNECT_TIMEOUT_listener_name is working

From the documentation:

Purpuose: To specify the time, in seconds, for the client to complete its connect request to the listener after the network connection had been established.

and

If the listener does not receive the client request in the time specified, then it terminates the connection. In addition, the listener logs the IP address of the client and an ORA-12525:TNS: listener has not received client's request in time allowed error message to the listener.log file.

My listener.ora file:

LISTENER =
  (DESCRIPTION_LIST =
    (DESCRIPTION =
      (ADDRESS = (PROTOCOL = TCP)(HOST = myserver.mydomain.com)(PORT = 1521))
      (ADDRESS = (PROTOCOL = IPC)(KEY = EXTPROC1521))
    )
  )

inbound_connect_timeout_listener=3

Verify that the parameter is set:

LSNRCTL> show inbound_connect_timeout
LISTENER parameter "inbound_connect_timeout" set to 3
LSNRCTL>

Verify that logging is set and the location of the logfile:

LSNRCTL> show log_file
Connecting to (DESCRIPTION=(ADDRESS=(PROTOCOL=TCP)(HOST=myserver.mydomain.com)(PORT=1521)))
LISTENER parameter "log_file" set to /u01/oracle/diag/tnslsnr/myserver/listener/alert/log.xml
The command completed successfully
LSNRCTL> show log_status
Connecting to (DESCRIPTION=(ADDRESS=(PROTOCOL=TCP)(HOST=myserver.mydomain.com)(PORT=1521)))
LISTENER parameter "log_status" set to ON
The command completed successfully

From a remote client, execute telnet against the listener port, in my case the default port 1521.

Use the "time" command in Linux to see how long the command executes:

[vk@myclient]$ time telnet myserver.mydomain.com 1521
Trying 192.168.0.122...
Connected to myserver.mydomain.com
Escape character is '^]'.
Connection closed by foreign host.

real    0m3.010s
user    0m0.001s
sys     0m0.001s

From the listener's log.xml file, we can see the message clearly:

<msg time='2017-01-23T11:09:12.118+01:00' org_id='oracle' comp_id='tnslsnr'
 type='UNKNOWN' level='16' host_id='myserver.mydomain.com'
 host_addr='192.168.0.122'>
 <txt>23-JAN-2017 11:09:12 * <unknown connect data> * (ADDRESS=(PROTOCOL=tcp)(HOST=192.168.0.25)(PORT=49264)) * establish * <unknown sid> * 12525
 </txt>
</msg>
<msg time='2017-01-23T11:09:12.119+01:00' org_id='oracle' comp_id='tnslsnr'
 type='UNKNOWN' level='16' host_id='myserver.mydomain.com'
 host_addr='192.168.0.122'>
 <txt>TNS-12525: TNS:listener has not received client's request in time allowed
 TNS-12535: TNS:operation timed out
  TNS-12606: TNS: Application timeout occurred
 </txt>
</msg>

From Oracle 10g and onwards, thet the default setting for INBOUND_CONNECT_TIMEOUT in listener.ora is 60 seconds
If set to zero, you disable the connect timeout functionality altogether. In such a situation, the telnet session above would simply "hang" and wait for its connection request to be completed.

Oracle recommends setting the INBOUND_CONNECTION_TIMEOUT in listener.ora in conjunction with INBOUND_CONNECT_TIMEOUT in sqlnet.ora.
Set the sqlnet.ora value slightly higher than the listener.ora value.

Sources:

http://docs.oracle.com/database/121/NETRF/sqlnet.htm#NETRF210
http://docs.oracle.com/database/121/NETRF/listener.htm#NETRF312