doc: making sense of alerts

doc/sphinx/index.rst

@@ -10,3 +10,4 @@ Suricata User Guide
    snort-compatibility
    rules/index.rst
    oinkmaster
+   make-sense-alerts

doc/sphinx/make-sense-alerts.rst

@@ -0,0 +1,77 @@
+Making sense out of Alerts
+==========================
+
+When alert happens it's important to figure out what it means. Is it
+serious? Relevant? A false positive?
+
+To find out more about the rule that fired, it's always a good idea to
+look at the actual rule.
+
+The first thing to look at in a rule is the description that follows
+the "msg" keyword. Lets consider an example:
+
+::
+
+  msg:"ET SCAN sipscan probe";
+
+The "ET" indicates the rule came from the Emerging Threats
+project. "SCAN" indicates the purpose of the rule is to match on some
+form of scanning. Following that a more or less detailed description
+is given.
+
+Most rules contain some pointers to more information in the form of
+the "reference" keyword.
+
+Consider the following example rule:
+
+::
+
+
+  alert tcp $HOME_NET any -> $EXTERNAL_NET $HTTP_PORTS \
+    (msg:"ET CURRENT_EVENTS Adobe 0day Shovelware"; \
+    flow:established,to_server; content:"GET "; nocase; depth:4; \
+    content:!"|0d 0a|Referer\:"; nocase; \
+    uricontent:"/ppp/listdir.php?dir="; \
+    pcre:"/\/[a-z]{2}\/[a-z]{4}01\/ppp\/listdir\.php\?dir=/U"; \
+    classtype:trojan-activity; \
+    reference:url,isc.sans.org/diary.html?storyid=7747; \
+    reference:url,doc.emergingthreats.net/2010496; \
+    reference:url,www.emergingthreats.net/cgi-bin/cvsweb.cgi/sigs/CURRENT_EVENTS/CURRENT_Adobe; \
+    sid:2010496; rev:2;)
+
+In this rule the reference keyword indicates 3 url's to visit for more
+information:
+
+::
+
+  isc.sans.org/diary.html?storyid=7747
+  doc.emergingthreats.net/2010496
+  www.emergingthreats.net/cgi-bin/cvsweb.cgi/sigs/CURRENT_EVENTS/CURRENT_Adobe
+
+Some rules contain a reference like: "reference:cve,2009-3958;" should
+allow you to find info about the specific CVE using your favourite
+search engine.
+
+It's not always straight forward and sometimes not all of that
+information is available publicly. Usually asking about it on the
+signature support lists helps a lot then.
+
+For the Emerging Threats list this is:
+http://lists.emergingthreats.net/mailman/listinfo/emerging-sigs
+
+For the VRT ruleset:
+https://lists.sourceforge.net/lists/listinfo/snort-sigs
+
+In many cases, looking at just the alert and the packet that triggered
+it won't be enough to be conclusive. When running an IDS engine like
+Suricata, it's always recommended to combine it with full packet
+capturing. Using tools like Sguil or Snorby, the full TCP session or
+UDP flow can be inspected.
+
+For example, if a rule fired that indicates your web application is
+attacked, looking at the full TCP session might reveal that the web
+application replied with 404 not found. This will usually mean the
+attack failed. Usually, not always.
+
+Obviously there is a lot more to Incidence Response, but this should
+get you started.