Where you are see these log "Did not receive identification string"? F5 or Server?
Refer below URL for basic external monitor.
https://devcentral.f5.com/articles/ltm-external-monitors-the-basics
Please customize sample external monitor script.
remove IPv6/IPv4 compatibility prefix (LTM passes addresses in IPv6 format)
IP=`echo ${1} | sed 's/::ffff://'`
PORT=${2}
PIDFILE="/var/run/`basename ${0}`.${IP}_${PORT}.pid"
kill of the last instance of this monitor if hung and log current pid
if [ -f $PIDFILE ]
then
kill -9 `cat $PIDFILE` > /dev/null 2>&1
fi
echo "$$" > $PIDFILE
send request & check for expected response
curl -fNs -m ${P_TIMEOUT} http://${IP}:${PORT}${URI} | grep -i "${RECV}" 2>&1 > /dev/null
let bigd mark node UP if expected response was received
otherwise force the node DOWN immediately
if [ $? -eq 0 ]
then
echo "UP"
else
/bin/bigpipe pool ${POOL} member ${IP}:${PORT} down 2>&1 > /dev/null
fi
rm -f $PIDFILE
exit
Let us know if require any help to customize above script.