Flow Keywords
=============

Flowbits
~~~~~~~~

Flowbits consists of two parts. The first part describes the action it
is going to perform, the second part is the name of the flowbit.

There are multiple packets that belong to one flow. Suricata keeps
those flows in memory. For more information see
:ref:`suricata-yaml-flow-settings`.  Flowbits can make sure an alert
will be generated when for example two different packets match.  An
alert will only be generated when both packets match. So, when the
second packet matches, Suricata has to know if the first packet was a
match too. Flowbits marks the flow if a packet matches so Suricata
'knows' it should generate an alert when the second packet matches as
well.

Flowbits have different actions. These are:

::

  flowbits: set, name                Will set the condition/'name', if present, in the flow.
  flowbits: isset, name              Can be used in the rule to make sure it generates an alert
                                     when the rule matches and the condition is set in the flow.
  flowbits: toggle, name             Reverses the present setting. So for example if a condition is set,
                                     it will be unset and vice-versa.
  flowbits: unset, name              Can be used to unset the condition in the flow.
  flowbits: isnotset, name           Can be used in the rule to make sure it generates an alert
                                     when it matches and the condition is not set in the flow.
  flowbits: noalert                  No alert will be generated by this rule.

Example:

.. image:: flow-keywords/Flowbit_3.png

When you take a look at the first rule you will notice it would
generate an alert if it would match, if it were not for the 'flowbits:
noalert' at the end of that rule. The purpose of this rule is to check
for a match on 'userlogin' and mark that in the flow. So, there is no
need for generating an alert.  The second rule has no effect without
the first rule. If the first rule matches, the flowbits sets that
specific condition to be present in the flow. Now with the second rule
there can be checked whether or not the previous packet fulfills the
first condition. If at that point the second rule matches, an alert
will be generated.

It is possible to use flowbits several times in a rule and combine the
different functions.

Flow
~~~~

The flow keyword can be used to match on direction of the flow, so to/from
client or to/from server. It can also match if the flow is established or not.
The flow keyword can also be use to say the signature has to match on stream
only (only_stream) or on packet only (no_stream).

So with the flow keyword you can match on:

::

  to_client                       established           only_stream
  from_client                     stateless             no_stream
  to_server
  from_server

**NOTE:** *from_server* and *to_client* are the same, and so are
 *to_server* and _from_client_. This comes from the original Snort
 language and we support it for compatibility reasons.

These options can be combined. You can use
up to three options. For example:

::

  flow:to_client, established;
  flow:from_client, established, only_stream;

There are different ways in which can be checked whether the
connection is established or not. With tcp-traffic a connection starts
with the three-way-handshake. In the flow is a part in which the state
is set. There will be checked in which state the connection is.  In
other cases there will be just checked if traffic comes from two
sides.

Example:

.. image:: flow-keywords/Flow1.png

.. image:: flow-keywords/Flow2.png

Flowint
~~~~~~~

For information, read the information on the :doc:`flowint` page.

stream_size
~~~~~~~~~~~

The stream size option matches on traffic according to the registered
amount of bytes by the sequence numbers.  There are several modifiers
to this keyword:

::

  >      greater than
  <      less than
  =      equal
  !=     not equal
  >=    greater than or equal
  <=    less than or equal

Format

::

  stream_size:<server|client|both|either>, <modifier>, <number>;

Example of the stream-size keyword in a rule:
