File: README.alert_order

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ALERT ORDERING
--------------

The Snort 2.0 detection engine changes how the ordering of rules
affect which alerts fire.  Before Snort 2.0, knowing which alerts would fire
first was determined by the position of the rule during initialization.
If the rule was read before another rule, then the rule that was read first
would be the alert that was logged.

This has all changed with the 2.0 detection engine.  There are now two stages
that determine which alerts will fire for a packet.  Ideally, Snort would have
the ability to log all alerts in a packet, but the current output modules do
not allow for this.

The first stage in the 2.0 detection engine is rule set selection.  Depending
on the rule set that is selected, different alerts may be generated.  The
rule sets are select first by transport protocol and then by characteristics
within the specific transport protocol:

  * TCP/UDP:  selection based on source and destination ports
  * ICMP:     selection based on ICMP type
  * IP:       selection based on IP transport protocol (if not TCP/UDP/ICMP)

Each protocol also has a generic rule set associated with it.  This provides
for the case where a packet does not match any unique properties of the
transport protocol.

It is important to note that every packet matches against a generic rule set,
since every unique rule set includes the generic rule set.  For example, if
a packet with a destination port of 80 is inspected, the rule set that
contains destination port 80 rules is selected, not the generic rule set.

The rule set selected is important.  In the second stage of the 2.0
detection engine, which rules get matched are determined by the rule set that
is selected.

Once a rule set is selected, two general types of rules are matched against.
These rules are content and non-content rules.  The content rules have
higher rule ordering priority over non-content rules, so if a content rule
matches a packet and a non-content rule matches a packet, the content rule
will always win.  If no content rules match, then the non-content rule that
is first in the file (the old snort way) will win.  This doesn't
apply when a unique rule set has been selected because the unique non-content
rules are first in the inspection order.  For example,  if an ICMP packet of
type 8 is inspected, two rules will match the packet.  One of the rules
is a generic ICMP Echo Request with no type indicated, and the other rule is
an ICMP Echo Request with a itype:8.  The itype:8 rule will always fire 
regardless of it's position in the rule file because it is the more unique
rule (since it has an itype:8).

-- Examples --

Which rule fires when there are two identical rules:

alert tcp any any -> any any ( msg:"foo1"; content: "foo"; )
alert tcp any any -> any any ( msg:"foo2"; content: "foo"; )

foo1 fires because it is first in the rules file.  This applies for all rules
(uricontent, content, no-content) that are exactly the same.  The first rule
in the rule files alerts.

Which rule fires when there are two rule with the same content, but one rule
has any any ports and the other has a specific port?

alert tcp any any -> any any ( msg:"foo1"; content: "foo"; )
alert tcp any any -> any 80 ( msg:"foo2"; content: "foo"; )

foo2 fires because it is considered a unique rule because it specifies a port
and gets put in the unique rule group for port 80.  foo1 is considered a 
generic rule because it has no specific port characteristics.

Which rule fires when a uricontent rule and a content rule both match a
packet?

alert tcp any any -> any 80 ( msg:"foo1"; content: "foo"; )
alert tcp any any -> any 80 ( msg:"foo2"; uricontent: "foo"; )

foo2 fires (as long as http_inspect is on) because uricontent rules are
matched against the packet first, and if there is a uricontent match then
this rule takes priority over content and no-content rules.

Which rule fires when a content rule and a no-content rule both match a
packet?

alert tcp any any -> any any ( msg:"foo1"; content: "foo"; )
alert tcp any any -> any any ( msg:"foo2"; )

foo1 fires because content rules are matched against a packet first, and if
there is a content match, then any no-content rules that match the packet
also will take a lower priority than any content rule that matches a packet.

Which rule fires when two content rules match a packet?

alert tcp any any -> any any ( msg:"foo1"; content: "foo"; )
alert tcp any any -> any any ( msg:"foo2"; content: "foobar";)

foo2 fires because the content rule with the longer content string takes
the higher priority.

Which rule fires when two ICMP rules match a packet?

alert icmp any any -> any any ( msg:"ICMP-No-iType"; dsize:>800; )
alert icmp any any -> any any ( msg:"ICMP-iType"; itype:8; dsize:>800;)

ICMP-iType fires because it has an 'itype' parameter, which specifies the
ICMP rule as unique ('itype' is the only parameter for ICMP rules that
specify uniqueness, otherwise it's considered generic).