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211 lines
5.9 KiB
ReStructuredText
Global-Thresholds
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=================
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Thresholds can be configured in the rules themselves, see
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:doc:`../rules/thresholding`. They are often set by rule writers based on
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their intelligence for creating a rule combined with a judgement on how often
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a rule will alert.
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Threshold Config
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----------------
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Next to rule thresholding more thresholding can be configured on the sensor
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using the threshold.config.
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threshold/event_filter
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~~~~~~~~~~~~~~~~~~~~~~
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Syntax:
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::
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threshold gen_id <gid>, sig_id <sid>, type <threshold|limit|both>, \
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track <by_src|by_dst|by_rule|by_both>, count <N>, seconds <T>
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rate_filter
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~~~~~~~~~~~
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Rate filters allow changing of a rule action when a rule matches.
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Syntax::
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rate_filter: rate_filter gen_id <gid>, sig_id <sid>, track <tracker>, \
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count <c>, seconds <s>, new_action <action>, timeout <timeout>
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Example::
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rate_filter gen_id 1, sig_id 1000, track by_rule, count 100, seconds 60, \
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new_action alert, timeout 30
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gen_id
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^^^^^^
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Generator id. Normally 1, but if a rule uses the ``gid`` keyword to set
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another value it has to be matched in the ``gen_id``.
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sig_id
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^^^^^^
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Rule/signature id as set by the rule ``sid`` keyword.
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track
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^^^^^
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Where to track the rule matches. When using by_src/by_dst the tracking is
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done per IP-address. The Host table is used for storage. When using by_rule
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it's done globally for the rule.
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Option by_both used to track per IP pair of source and destination. Packets
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going to opposite directions between same addresses tracked as the same pair.
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count
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^^^^^
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Number of rule hits before the ``rate_filter`` is activated.
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seconds
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^^^^^^^
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Time period within which the ``count`` needs to be reached to activate
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the ``rate_filter``
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new_action
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^^^^^^^^^^
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New action that is applied to matching traffic when the ``rate_filter``
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is in place.
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Values::
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<alert|drop|pass|reject>
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Note: 'sdrop' and 'log' are supported by the parser but not implemented otherwise.
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timeout
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^^^^^^^
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Time in seconds during which the ``rate_filter`` will remain active.
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Example
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^^^^^^^
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Lets say we want to limit incoming connections to our SSH server. The rule
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``888`` below simply alerts on SYN packets to the SSH port of our SSH server.
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If an IP-address triggers this more than 10 or more with a minute, the
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drop ``rate_filter`` is set with a timeout of 5 minutes.
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Rule::
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alert tcp any any -> $MY_SSH_SERVER 22 (msg:"Connection to SSH server"; \
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flow:to_server; flags:S,12; sid:888;)
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Rate filter::
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rate_filter gen_id 1, sig_id 888, track by_src, count 10, seconds 60, \
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new_action drop, timeout 300
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suppress
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~~~~~~~~
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Suppressions can be used to suppress alerts for a rule or a
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host/network. Actions performed when a rule matches, such as setting a
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flowbit, are still performed.
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Syntax:
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::
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suppress gen_id <gid>, sig_id <sid>
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suppress gen_id <gid>, sig_id <sid>, track <by_src|by_dst|by_either>, ip <ip|subnet|addressvar>
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Examples:
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::
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suppress gen_id 1, sig_id 2002087, track by_src, ip 209.132.180.67
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This will make sure the signature 2002087 will never match for src
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host 209.132.180.67.
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Other possibilities/examples::
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suppress gen_id 1, sig_id 2003614, track by_src, ip 217.110.97.128/25
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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]
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suppress gen_id 1, sig_id 2003614, track by_src, ip $HOME_NET
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suppress gen_id 1, sig_id 2003614, track by_either, ip 217.110.97.128/25
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In the last example above, the ``by_either`` tracking means that if either
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the ``source ip`` or ``destination ip`` matches ``217.110.97.128/25`` the
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rule with sid 2003614 is suppressed.
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.. _global-thresholds-vs-rule-thresholds:
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Global thresholds vs rule thresholds
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------------------------------------
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**Note: this section applies to 1.4+ In 1.3 and before mixing rule and
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global thresholds is not supported.**
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When a rule has a threshold/detection_filter set a rule can still be
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affected by the global threshold file.
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The rule below will only fire if 10 or more emails are being
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delivered/sent from a host within 60 seconds.
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::
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alert tcp any any -> any 25 (msg:"ET POLICY Inbound Frequent Emails - Possible Spambot Inbound"; \
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flow:established; content:"mail from|3a|"; nocase; \
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threshold: type threshold, track by_src, count 10, seconds 60; \
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reference:url,doc.emergingthreats.net/2002087; classtype:misc-activity; sid:2002087; rev:10;)
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Next, we'll see how global settings affect this rule.
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Suppress
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~~~~~~~~
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Suppressions can be combined with rules with
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thresholds/detection_filters with no exceptions.
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::
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suppress gen_id 1, sig_id 2002087, track by_src, ip 209.132.180.67
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suppress gen_id 0, sig_id 0, track by_src, ip 209.132.180.67
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suppress gen_id 1, sig_id 0, track by_src, ip 209.132.180.67
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Each of the rules above will make sure 2002087 doesn't alert when the
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source of the emails is 209.132.180.67. It **will** alert for all other
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hosts.
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::
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suppress gen_id 1, sig_id 2002087
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This suppression will simply convert the rule to "noalert", meaning it
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will never alert in any case. If the rule sets a flowbit, that will
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still happen.
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Threshold/event_filter
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~~~~~~~~~~~~~~~~~~~~~~
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When applied to a specific signature, thresholds and event_filters
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(threshold from now on) will override the signature setting. This can
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be useful for when the default in a signature doesn't suit your
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environment.
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::
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threshold gen_id 1, sig_id 2002087, type both, track by_src, count 3, seconds 5
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threshold gen_id 1, sig_id 2002087, type threshold, track by_src, count 10, seconds 60
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threshold gen_id 1, sig_id 2002087, type limit, track by_src, count 1, seconds 15
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Each of these will replace the threshold setting for 2002087 by the
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new threshold setting.
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**Note:** overriding all gids or sids (by using gen_id 0 or sig_id 0)
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is not supported. Bug https://redmine.openinfosecfoundation.org/issues/425.
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Rate_filter
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~~~~~~~~~~~
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see https://redmine.openinfosecfoundation.org/issues/425.
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