POP
====

-- Overview --
POP is an POP3 decoder for user applications. Given a data buffer,
POP will decode the buffer and find POP3 commands and responses.
It will also mark the command, data header data body sections and 
extract the POP3 attachments and decode it appropriately.

POP will handle stateful processing. It saves state between individual 
packets. However maintaining correct state is dependent on the resassembly
of the server side of the stream (ie, a loss of coherent stream data results
in a loss of state).

Stream5 should be turned on for POP. Please ensure that the POP ports are added
 to the stream5 ports for proper reassembly.


-- Configuration --

The configuration options are described below:

* ports { port [port] ... } *
This specifies on what ports to check for POP data.  Typically, this will
include 110. Default ports if none are specified are 110 .

* disabled *
Disables the POP preprocessor in a config. This is useful when specifying
the decoding depths such as b64_decode_depth, qp_decode_depth, uu_decode_depth, 
bitenc_decode_depth or the memcap used for decoding in default config 
without turning on the POP preprocessor.

* b64_decode_depth *
This config option is used to turn off/on or set the base64 decoding depth used to 
decode the base64 encoded MIME attachments. The value ranges from -1 to 65535. 
A value of -1 turns off the base64 decoding of MIME attachments. The value of 0 
sets the decoding of base64 encoded MIME attachments to unlimited. A value other 
than 0 or -1 restricts the decoding of base64 MIME attachments, and applies per 
attachment. A POP preprocessor alert with sid 4 is generated (if enabled) 
when the decoding fails.

Multiple MIME attachments/data in one packet are pipelined. When stateful inspection 
is turned on the base64 encoded MIME attachments/data across multiple packets are 
decoded too.

The decoded data is available for detection using the rule option file_data. 
See file_data rule option for more details.

It is recommended that user inputs a value that is a multiple of 4. When the value
specified is not a multiple of 4, the POP preprocessor will round it up to the next
multiple of 4.

In case of multiple configs, the value specified in the non-default config cannot 
exceed the value specified in the default config.

* qp_decode_depth *
This config option is used to turn off/on or set the Quoted-Printable decoding depth 
used to decode the Quoted-Printable(QP) encoded MIME attachments. The value ranges 
from -1 to 65535. A value of -1 turns off the QP decoding of MIME attachments. 
The value of 0 sets the decoding of QP encoded MIME attachments to unlimited. A 
value other than 0 or -1 restricts the decoding of QP MIME attachments, and applies 
per attachment. A POP preprocessor alert with sid 5 is generated (if enabled) when 
the decoding fails.

Multiple MIME attachments/data in one packet are pipelined. When stateful inspection 
is turned on the QP encoded MIME attachments/data across multiple packets are decoded too.

The decoded data is available for detection using the rule option file_data. 
See file_data rule option for more details.

In case of multiple configs, the value specified in the non-default config cannot exceed 
the value specified in the default config.

* bitenc_decode_depth *
This config option is used to turn off/on or set the non-encoded MIME extraction 
depth used to extract the non-encoded MIME attachments. The value ranges from -1 
to 65535. A value of -1 turns off the extraction of these MIME attachments. 
The value of 0 sets the extraction of these MIME attachments to unlimited. 
A value other than 0 or -1 restricts the extraction of these MIME attachments, and applies 
per attachment.


Multiple MIME attachments/data in one packet are pipelined. When stateful inspection 
is turned on the non-encoded MIME attachments/data across multiple packets are 
extracted too.

The extracted data is available for detection using the rule option file_data. 
See file_data rule option for more details.

In case of multiple configs, the value specified in the non-default config cannot exceed 
the value specified in the default config.

* uu_decode_depth *
This config option is used to turn off/on or set the Unix-to-Unix decoding depth 
used to decode the Unix-to-Unix(UU) encoded attachments. The value ranges 
from -1 to 65535. A value of -1 turns off the UU decoding of POP attachments. 
The value of 0 sets the decoding of UU encoded POP attachments to unlimited. A 
value other than 0 or -1 restricts the decoding of UU POP attachments, and applies 
per attachment. A POP preprocessor alert with sid 7 is generated (if enabled) when 
the decoding fails.

Multiple UU Encoded attachments/data in one packet are pipelined. When stateful inspection 
is turned on the UU encoded attachments/data across multiple packets are decoded too.

The decoded data is available for detection using the rule option file_data. 
See file_data rule option for more details.

In case of multiple configs, the value specified in the non-default config cannot exceed 
the value specified in the default config.

* memcap <int> *
This option determines (in bytes) the maximum amount of memory the POP preprocessor
will use for decoding base64 encoded/quoted-printable/non-encoded MIME attachments/data 
or Unix-to-Unix encoded attachments. This value can be set from 3276 bytes to 100MB. 

This option along with the maximum of the decoding depths will determine the POP 
sessions that will be decoded at any given instant. The default value for this option 
is 838860.

Note: It is suggested to set this value such that the max pop session calculated as
follows is atleast 1.

max pop session = memcap /(2 * max of (b64_decode_depth, uu_decode_depth, qp_decode_depth 
					or bitenc_decode_depth))

For example, if b64_decode_depth is 0 (indicates unlimited decoding) and qp_decode_depth is 100, then

max pop session = memcap/2*65535 (max value for b64_decode_depth)

In case of multiple configs, the memcap of the non-default configs will be overwritten by the
default config's value. Hence user needs to define it in the default config with the new keyword
disabled (used to disable POP preprocessor in a config).

When the memcap for decoding (memcap) is exceeded the POP preprocessor alert with sid 3 is 
generated (when enabled). 

Example:
preprocessor pop: \
  ports { 110 } \
  memcap 1310700 \
  qp_decode_depth -1 \
  b64_decode_depth 0 \
  bitenc_decode_depth 100
 

preprocessor pop: \
  memcap 1310700 \
  qp_decode_depth 0 \
  disabled

Default:
preprocessor pop: \
  ports { 110 } \
  b64_decode_depth 1460 \
  qp_decode_depth 1460 \
  bitenc_decode_depth 1460 \
  uu_decode_depth 1460

Events
================================================================================
The POP preprocessor uses GID 142 to register events.


SID   Description
--------------------------------------------------------------------------------
  1   Alert if POP encounters an invalid POP3 command.
  2   Alert if POP encounters an invalid POP3 response.
  3   If the decoding memory cap (memcap) is reached and the preprocessor is configured to alert,
       this alert will be created.
  4   If the decoding of a base64 MIME attachments fails or when the decoding stops due to exceeded
      b64_decode_depth.
  5   If the decoding of a Quoted-Printable MIME attachments fails or when the decoding stops due to exceeded
      qp_decode_depth.
  7   If the decoding of a Unix-to-Unix encoded attachments fails or when the decoding stops due to exceeded
      uu_decode_depth.
