Add threshold.config file.

This patch adds an example file and modify Makefile to have it
installed by 'make install-conf' command.

Makefile.am

@@ -20,6 +20,7 @@ install-conf:
 	@test -e "$(e_sysconfdir)/suricata.yaml" || install -m 600 "$(top_srcdir)/suricata.yaml" "$(e_sysconfdir)"
 	@test -e "$(e_sysconfdir)/classification.config" || install -m 600 "$(top_srcdir)/classification.config" "$(e_sysconfdir)"
 	@test -e "$(e_sysconfdir)/reference.config" || install -m 600 "$(top_srcdir)/reference.config" "$(e_sysconfdir)"
+	@test -e "$(e_sysconfdir)/threshold.config" || install -m 600 "$(top_srcdir)/threshold.config" "$(e_sysconfdir)"
 	install -d "$(e_logfilesdir)"
 
 install-rules:

threshold.config

@@ -0,0 +1,22 @@
+# Thresholding:
+#
+# This feature is used to reduce the number of logged alerts for noisy rules.
+# Thresholding commands limit the number of times a particular event is logged
+# during a specified time interval.
+#
+# Please note that thresholding can also be set inside signature.
+#
+# The syntax is the following:
+# (threshold|suppress) gen_id gen_id, sig_id sig_id, type (limit|threshold|both), track (by_src|by_dst), count n seconds
+
+# Limit to 10 alerts every 10 seconds for each source host
+#threshold gen_id 0, sig_id 0, type threshold, track by_src, count 10, seconds 10
+
+# Limit to 1 alert every 10 seconds for signature with sid 2404000
+#threshold gen_id 1, sig_id 2404000, type threshold, track by_dst, count 1, seconds 10
+
+# Avoid to alert on f-secure update
+# Example taken from http://blog.inliniac.net/2012/03/07/f-secure-av-updates-and-suricata-ips/
+#suppress gen_id 1, sig_id 2009557, track by_src, ip 217.110.97.128/25
+#suppress gen_id 1, sig_id 2012086, track by_src, ip 217.110.97.128/25
+#suppress gen_id 1, sig_id 2003614, track by_src, ip 217.110.97.128/25