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Old 04-26-2004, 05:53 PM   #1
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Question Snort doesn't seem to catch anything I throw at it?

I have been trying to get this phase of my IDS implementation set up for weeks, and Snort is not cooperating. I printed and have read the manual, surfed the mailing lists, and beat my head against a wall, but nothing seems to work. What I am looking for is for my Squid Proxy (the sensor) to be able to detect port scans and other nefarious activity originating from it or directed at it. After starting Snort with -D -c /usr/local/Snort-2.1.2/etc/snort.conf, and verifying it is running in ps, I blast away on the network with every type of nmap scan I know, and nothing. Even when I direct my scans at the machines it is supposed to be watching, it misses every port scan. Occasionally I get a useless portscan alert in the logs, but it is always from my Squid proxy's external ip, and never coincides with an actual scan I am running.

Here is my conf file. Can anyone see what is wrong? I have stream4 and flow/flow-portscan set-up. Why is it failing to detect my scans??
Snort is running on my Squid Proxy/firewall (0.8 internal, 10.10 external)

var SQL_SERVERS [,,]

var HTTP_PORTS 80:8080
var RULE_PATH ../rules

config disable_decode_alerts

preprocessor flow: stats_interval 0 hash 2
preprocessor frag2
preprocessor stream4: detect_scans,disable_evasion_alerts,ttl_limit 0
preprocessor stream4_reassemble

preprocessor http_inspect: global \
iis_unicode_map 1252

preprocessor http_inspect_server: server \
ports { 80 3128 8080 } \
flow_depth 0 \
ascii no \
double_decode yes \
non_rfc_char { 0x00 } \
chunk_length 500000 \
non_strict \
oversize_dir_length 500 \

preprocessor rpc_decode: 111 32771
preprocessor bo
preprocessor telnet_decode

preprocessor flow-portscan: \
talker-sliding-scale-factor 0.50 \
talker-fixed-threshold 30 \
talker-sliding-threshold 30 \
talker-sliding-window 20 \
talker-fixed-window 30 \
scoreboard-rows-talker 30000 \
server-watchnet $HOME_NET \
server-ignore-limit 200 \
server-rows 65535 \
server-learning-time 3200 \
server-scanner-limit 4 \
scanner-sliding-window 20 \
scanner-sliding-scale-factor 0.50 \
scanner-fixed-threshold 15 \
scanner-sliding-threshold 40 \
scanner-fixed-window 15 \
scoreboard-rows-scanner 30000 \
# src-ignore-net [] \
# dst-ignore-net [] \
alert-mode once \
output-mode msg \
tcp-penalties on

output alert_syslog: LOG_AUTH LOG_ALERT
output alert_full: alert

include classification.config
include $RULE_PATH/local.rules
include $RULE_PATH/bad-traffic.rules
include $RULE_PATH/exploit.rules
include $RULE_PATH/scan.rules
include ...............(all the other rules files as well).....

If someone could point me in the right direction here you would be my hero.

Old 04-28-2004, 01:04 PM   #2
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A closer inspection of the messages log showed that by default Snort was binding to eth0 (the wrong NIC). I added -i eth1 to the command line start up so it read

snort -D -i eth1 -c /etc/snort/snort.conf

and all is well. :-)
Old 04-28-2004, 03:54 PM   #3
Registered: May 2001
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Oh cool, you solved your own problem :-]


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