Monitoring Listener & Client

look at your /app/oracle/product/9.2.0/network/log/listener.log

lsnrctl
set current_listener LSNR_AOL
#this is the lowest to monitor connections
trace 4
#trace 16 

grep nttvlser lsnr_aol.trc
#if trace 16
trcasst lsnr_aol.trc > lsnr_aol.out

Append to your listener.ora:
TRACE_LEVEL_LISTENER = USER

To extract connection open $ORACLE_HOME/network/admin/trace/listener.trc
for each connection you find:
...
[30-AGO-2006 18:27:45:390] nttcnr: connected on ipaddr 127.0.0.1
...

Oracle certify as Ok db links when listener connection time is less than 40 ms

AL11 = (DESCRIPTION = (ADDRESS_LIST = (ADDRESS = (PROTOCOL = TCP)(HOST = 192.168.0.2)(PORT = 1521))) (CONNECT_DATA = (SERVICE_NAME = al11)))

#On client host
tnsping AL11 9999999 | grep -v 'OK (0 msec)' | grep -v 'OK (10 msec)' | grep -v 'OK (20 msec)' | grep -v 'OK (30 msec)' | grep -v 'OK (40 msec)'
ping -s -I 1 db001srv

#Show the number of connections On 192.168.0.2
netstat -a|grep 1525|wc -l

Launch using:

nohup sh Listener_Heartbeat_Monitor_Client.sh > Listener_Heartbeat_Monitor_Client.log &

#!/bin/ksh
echo '***************************************'
echo '* Listener Heartbeat Monitor - CLIENT *'
echo '***************************************'
echo  ' '
while true
do
  date
  tnsping srv_pro | cat -n | grep 12
  traceroute db001srv
  echo -----------------------------------------------------------------
  sleep 5
done

Launch using:

nohup sh Listener_Heartbeat_Monitor_Server.sh > Listener_Heartbeat_Monitor_Server.log &

#!/bin/ksh
echo '***************************************'
echo '* Listener Heartbeat Monitor - SERVER *'
echo '***************************************'
echo  ' '
while true
do
  date
  tnsping srv_pro | cat -n | grep 12
  traceroute db032ops
  echo 'netstat -a|grep 1525|wc -l, Show established TCP connections'
  netstat -a|grep 1525|wc -l
  echo -----------------------------------------------------------------
  sleep 5
done