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This documentation is no longer maintained and exists for historical purposes. The current documentation is located at http://suricata.readthedocs.io/.


Global-Thresholds

Thresholds can be configured in the rules themselves, see Rule-Thresholding. They are often set by rule writers based on their intel for creating a rule combined with a judgement on how often a rule will alert.

Next to these settings, thresholding can be configured on the sensor using the threshold.config.

threshold/event_filter

Syntax:

threshold gen_id <gid>, sig_id <sid>, type <threshold|limit|both>, track <by_src|by_dst>, count <N>, seconds <T>

rate_filter

TODO

suppress

Suppressions can be used to suppress alerts for a rule or a host/network. Actions performed when a rule matches, such as setting a flowbit, are still performed.

Syntax:

suppress gen_id <gid>, sig_id <sid>
suppress gen_id <gid>, sig_id <sid>, track <by_src|by_dst>, ip <ip|subnet>

Examples:

suppress gen_id 1, sig_id 2002087, track by_src, ip 209.132.180.67

This will make sure the signature 2002087 will never match for src host 209.132.180.67.

Other possibilities/examples:

suppress gen_id 1, sig_id 2003614, track by_src, ip 217.110.97.128/25
suppress gen_id 1, sig_id 2003614, track by_src, ip [192.168.0.0/16,10.0.0.0/8,172.16.0.0/12]
suppress gen_id 1, sig_id 2003614, track by_src, ip $HOME_NET

Global thresholds vs rule thresholds

Note: this section applies to 1.4+ In 1.3 and before mixing rule and global thresholds is not supported.

When a rule has a threshold/detection_filter set a rule can still be affected by the global threshold file.

The rule below will only fire if 10 or more emails are being delivered/sent from a host within 60 seconds.

alert tcp any any -> any 25 (msg:"ET POLICY Inbound Frequent Emails - Possible Spambot Inbound"; \
     flow:established; content:"mail from|3a|"; nocase;                                          \
     threshold: type threshold, track by_src, count 10, seconds 60;                              \
     reference:url,doc.emergingthreats.net/2002087; classtype:misc-activity; sid:2002087; rev:10;)

Next, we'll see how global settings affect this rule.

Suppress

Suppressions can be combined with rules with thresholds/detection_filters with no exceptions.

suppress gen_id 1, sig_id 2002087, track by_src, ip 209.132.180.67
suppress gen_id 0, sig_id 0, track by_src, ip 209.132.180.67
suppress gen_id 1, sig_id 0, track by_src, ip 209.132.180.67

Each of the rules above will make sure 2002087 doesn't alert when the source of the emails is 209.132.180.67. It will alert for all other hosts.

suppress gen_id 1, sig_id 2002087

This suppression will simply convert the rule to "noalert", meaning it will never alert in any case. If the rule sets a flowbit, that will still happen.

Threshold/event_filter

When applied to a specific signature, thresholds and event_filters (threshold from now on) will override the signature setting. This can be useful for when the default in a signature doesn't suit your evironment.

threshold gen_id 1, sig_id 2002087, type both, track by_src, count 3, seconds 5
threshold gen_id 1, sig_id 2002087, type threshold, track by_src, count 10, seconds 60
threshold gen_id 1, sig_id 2002087, type limit, track by_src, count 1, seconds 15

Each of these will replace the threshold setting for 2002087 by the new threshold setting.

Note: overriding all gids or sids (by using gen_id 0 or sig_id 0) is not supported. Bug #425.

Rate_filter

TODO