How to configure snort to detect portscan??

[mickey12]

hey guys.........i have some problem out here........i am using snort on my fedora machine.......the problem is .......whenver i run snort and also run a portscan using Nmap from some other machine.........it doesnt generate any alert which says dat there has been a portscan......or a simple scan...........all i get is alerts which say dat icmp destination unreachable........something like this.....

sc activity] [Priority: 3] {ICMP} 192.168.45.23 -> 192.168.45.118
04/25-00:54:14.536423 [**] [1:486:5] ICMP Destination Unreachable Communication with Destination Host is Administratively Prohibited [**] [Classification: Misc activity] [Priority: 3] {ICMP} 192.168.45.23 -> 192.168.45.11
ted [**] [Classification: Misc activity] [Priority: 3] {ICMP} 192.168.45.23 -> 192.168.45.11

04/25-00:54:16.889515 [**] [1:1418:13] SNMP request tcp [**] [Classification: Attempted Information Leak] [Priority: 2] {TCP} 192.168.45.11:48628 -> 192.168.45.23:161
04/25-00:54:17.545495 [**] [1:486:5] ICMP Destination Unreachable Communication with Destination Host is Administratively Prohibited [**] [Classification: Misc activity] [Priority: 3] {ICMP} 192.168.45.23 -> 192.168.45.11
04/25-00:54:17.578158 [**] [1:1421:13] SNMP AgentX/tcp request [**] [Classification: Attempted Information Leak] [Priority: 2] {TCP} 192.168.45.11:48627 -> 192.168.45.23:705
04/25-00:54:17.687237 [**] [1:1421:13] SNMP AgentX/tcp request [**] [Classification: Attempted

# sfPortscan
# ----------
# Portscan detection module. Detects various types of portscans and
# portsweeps. For more information on detection philosophy, alert types,
# and detailed portscan information, please refer to the README.sfportscan.
#
# -configuration options-
# proto { tcp udp icmp ip all }
#
#
preprocessor sfportscan: proto { all } \
memcap { 10000000 } \
sense_level { medium }


################################################## ##################


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pls help me out guys.......i have to complete the proj in a day.......pls send ur replies .........thanking u in advance..........bye..tc......God bless..