SMTP
====
Andrew Mullican <amullican@sourcefire.com>
Thanks to Dan Roelker <droelker@sourcefire.com>,
Marc Norton <mnorton@sourcefire.com>, and Steve Sturges
<ssturges@sourcefire.com> for their help with the design.

-- Overview --
SMTP is an SMTP decoder for user applications.  Given a data buffer,
SMTP will decode the buffer and find SMTP commands and responses.
It will also mark the command, data header data body sections, as well
as TLS data.

SMTP handles stateless and stateful processing.  It saves state
between individual packets.  However maintaining correct state is
dependent on the reassembly of the client side of the stream (ie, a
loss of coherent stream data results in a loss of state).

-- Configuration --
SMTP has the usual configuration items, such as port and inspection_type.
Also, SMTP command lines can be normalized to remove extraneous spaces.
TLS-encrypted traffic can be ignored, which improves performance.  In addition,
regular mail data can be ignored for an additional performance boost.  Since
so few (none in the current snort ruleset) exploits are against mail data,
this is relatively safe to do and can improve the performance of data
inspection.

The configuration options are described below:

* ports { port [port] ... } *
This specifies on what ports to check for SMTP data.  Typically, this will
include 25 and possibly 465, for encrypted SMTP.
Default ports if none are specified are 25, 587 (Message submission - see 
RFC 2476) and 691 (X-LINK2STATE).  If alerting on the X-LINK2STATE vulnerability
is disabled, port 691 will be removed from the default ports.
DEFAULT { 25 587 691 }

* inspection_type stateful|stateless
Indicate whether to operate in stateful or stateless mode.

* normalize all|none|cmds *
This turns on normalization.  Normalization checks for more than one space
character after a command.  Space characters are defined as space (ASCII 0x20)
or tab (ASCII 0x09).
all checks all commands
none turns off normalization for all commands.
cmds just checks commands listed with the "normalize_cmds" parameter.

* ignore_data *
Ignore data section of mail (except for mail headers) when processing rules.

* ignore_tls_data *
Ignore TLS-encrypted data when processing rules.

* max_command_line_len <int> *
Alert if an SMTP command line is longer than this value.  Absence of this
option or a "0" means never alert on command line length.
RFC 2821 recommends 512 as a maximum command line length.

* max_header_line_len <int> *
Alert if an SMTP DATA header line is longer than this value.  Absence of this
option or a "0" means never alert on data header line length.
RFC 2821 recommends 1024 as a maximum data header line length.

* max_response_line_len <int> *
Alert if an SMTP response line is longer than this value.  Absence of this
option or a "0" means never alert on response line length.
RFC 2821 recommends 512 as a maximum response line length.

* alt_max_command_line_len <int> { <cmd> [<cmd>] }
Overrides max_command_line_len for specific commands

* no_alerts *
Turn off all alerts for this preprocessor.

* invalid_cmds { <Space-delimited list of commands> } *
Alert if this command is sent from client side.
DEFAULT empty list

* valid_cmds { <Space-delimited list of commands> } *
List of valid commands.  We do not alert on commands in this list.
DEFAULT empty list, but preprocessor has this list hard-coded:
{ ATRN AUTH BDAT DATA DEBUG EHLO EMAL ESAM ESND ESOM ETRN EVFY EXPN }
{ HELO HELP IDENT MAIL NOOP QUIT RCPT RSET SAML SOML SEND ONEX QUEU }
{ STARTTLS TICK TIME TURN TURNME VERB VRFY X-EXPS X-LINK2STATE }
{ XADR XAUTH XCIR XEXCH50 XGEN XLICENSE XQUE XSTA XTRN XUSR  }

* alert_unknown_cmds *
Alert if we don't recognize command.
DEFAULT off

* normalize_cmds { <Space-delimited list of commands> } *
Normalize this list of commands
DEFAULT { RCPT VRFY EXPN }

* xlink2state { enable/disable [drop] }
See CVE-2005-0560 for a description of the vulnerability.
Enable/disable xlink2state alert
Drop if alerted
DEFAULT { enable }

* print_cmds *
List all commands understood by the preprocessor.  This not normally
printed out with the configuration because it prints so much data.

Example:
preprocessor SMTP: \
  ports { 25 } \
  inspection_type stateful \
  normalize cmds \
  normalize_cmds { EXPN VRFY RCPT } \
  ignore_data \
  ignore_tls_data \
  max_command_line_len  512 \
  max_header_line_len   1024 \
  max_response_line_len 512 \
  no_alerts \
  alt_max_command_line_len 300 { RCPT } \
  invalid_cmds { } \
  valid_cmds { } \
  xlink2state { disable } \
  print_cmds

Default:
preprocessor SMTP: \
  ports { 25 } \
  inspection_type stateful \
  normalize cmds \
  normalize_cmds { EXPN VRFY RCPT } \
  alt_max_command_line_len 260 { MAIL } \
  alt_max_command_line_len 300 { RCPT } \
  alt_max_command_line_len 500 { HELP HELO ETRN } \
  alt_max_command_line_len 255 { EXPN VRFY }

Notes:
"RCPT TO:" and "MAIL FROM:" are SMTP commands.  For the preprocessor
configuration, they are referred to as RCPT and MAIL, respectively.
Within the code, the preprocessor actually maps RCPT and MAIL to the
correct command name.