# $Id$
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                                  Frag3
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Author: Martin Roesch <roesch@sourcefire.com>

Overview
--------
The frag3 preprocessor is a target-based IP defragmentation module for Snort.
Frag3 is intended as a replacement for the frag2 defragmentation module and 
was designed with the following goals:

1) Faster execution that frag2 with less complex data management.

2) Target-based host modeling anti-evasion techniques.

The frag2 preprocessor used splay trees extensively for managing the data 
structures associated with defragmenting packets.  Splay trees are excellent 
data structures to use when you have some assurance of locality of reference
for the data that you are handling but in high speed, heavily fragmented 
environments the nature of the splay trees worked against the system and 
actually hindered performance.  Frag3 uses the sfxhash data structure and 
linked lists for data handling internally which allows it to have much more
predictable and deterministic performance in any environment which should 
aid us in managing heavily fragmented environments.

Target-based analysis is a relatively new concept in network-based intrusion
detection.  The idea of a target-based system is to model the actual targets
on the network instead of merely modeling the protocols and looking for 
attacks within them.  When IP stacks are written for different operating 
systems, they are usually implemented by people who read the RFCs and then
their interpretation of what the RFC outlines into code.  Unfortunately, there
are ambiguities in the way that the RFCs define some of the edge conditions 
that may occurr and when this happens differnt people implement certain aspects
of their IP stacks differently.  For an IDS this is a big problem.

In an environment where the attacker can determine what style of IP 
defragmentation being used on a particular target, the attacker can try to
fragment packets such that the target will put them back together in a 
specific manner while any passive systems trying to model the host traffic 
have to guess which way the target OS is going to handle the overlaps and 
retransmits.  As I like to say, if the attacker has more information about the
targets on a network than the IDS does, it is possible to evade the IDS.  This
is where the idea for "target-based IDS" came from.  For more detail on this
issue and how it affects IDSes, check out the famous Ptacek & Newsham paper at

http://www.snort.org/docs/idspaper/

The basic idea behind target-based IDS is that we tell the IDS information 
about hosts on the network so that it can avoid Ptacek & Newsham style evasion
attacks based on information about how an individual target IP stack operates.
Vern Paxson and Umesh Shankar did a great paper on this very topic in 2003 that 
detailed mapping the hosts on a network and determining how their various IP 
stack implementations handled the types of problems seen in IP defragmentation 
and TCP stream reassembly.  Check it out at

http://www.icir.org/vern/papers/activemap-oak03.pdf

We can also present the IDS with topology information to avoid TTL-based 
evasions and a variety of other issues, but that's a topic for another day.  
Once we have this information we can start to really change the game for these 
complex modeling problems.

Frag3 was implemented to showcase and prototype a target-based module within
Snort to test this idea.

Configuration
-------------

Frag3 configuration is somewhat more complex than frag2.  There are at least
two preprocessor directives required to activate frag3, a global configuration
directive and an engine instantiation.  There can be an arbitrary number of
engines defined at startup with their own configuration, but only one global
configuration.

Global configuration
 - Preprocessor name: frag3_global
 - Available Options
   NOTE: Global configuration options are comma separated.
     max_frags <number> - Maximum simultaneous fragments to track, default 
                          is 8192
     memcap <bytes> - Memory cap for self preservation, default is 4MB
     prealloc_memcap <bytes> - alternate memory management mode, use
                               preallocated fragment nodes based on a
                               memory cap (faster in some situations)
     prealloc_frags <number> - alternate memory management mode, use
                               preallocated fragment nodes based on a
                               static number (faster in some situations)

Engine Configuration                          
 - Preprocessor name: frag3_engine
 - Available Options
   NOTE: Engine configuration options are space separated.
     timeout <seconds> - Timeout for fragments, fragments in the engine for 
                         longer than this period will be automatically dropped.
                         Default is 60 seconds.
     min_ttl <value> - Minimum acceptable TTL value for a fragment packet.  
                       Default is 1.
     detect_anomalies - Detect fragment anomalies 
     bind_to <ip_list> - IP List to bind this engine to.  This engine will only
                         run for packets with destination addresses contained
                         within the IP List.  Default value is "all".
     overlap_limit <number> - Limits the number of overlapping fragments per packet.  The default 
                       is "0" (unlimited), the minimum is "0", and the maximum is "255". This is an 
                       optional parameter. detect_anomalies option must be configured for this option 
                       to take effect.
     min_fragment_length <number> - Defines smallest fragment size (payload size) that should be considered valid.
                    Fragments smaller than or equal to this limit are considered malicious and an event is raised, 
                    if detect_anomalies is also configured.  The default is "0" (check is disabled), the minimum is "0", 
                    and the maximum is "255". 
                    This is an optional parameter. detect_anomalies option must be configured for this option to take effect.
     policy <type> - Select a target-based defragmentation mode.  Available 
                     types are first, last, bsd, bsd-right, linux, windows
                     and solaris.  Default type is bsd.

                     The Paxson Active Mapping paper introduced the terminology
                     frag3 is using to describe policy types.  It has been
                     extended to address differences between a true "first"
                     policy and how Windows and Solaris platforms handle
                     fagmented traffic.  The known mappings are as follows.

                     Anyone who develops more mappings and would like to add
                     to this list please feel free to send us an email!
                     Platform | Type
                     ---------------
                        AIX 2  | BSD 
                AIX 4.3 8.9.3  | BSD 
                    Cisco IOS  | Last 
                      FreeBSD  | BSD 
       HP JetDirect (printer)  | BSD-right 
                HP-UX B.10.20  | BSD 
                  HP-UX 11.00  | First 
                  IRIX 4.0.5F  | BSD 
                     IRIX 6.2  | BSD 
                     IRIX 6.3  | BSD 
                   IRIX64 6.4  | BSD 
                 Linux 2.2.10  | linux 
             Linux 2.2.14-5.0  | linux 
               Linux 2.2.16-3  | linux 
       Linux 2.2.19-6.2.10smp  | linux 
               Linux 2.4.7-10  | linux 
   Linux 2.4.9-31SGI 1.0.2smp  | linux 
   Linux 2.4 (RedHat 7.1-7.3)  | linux 
      MacOS (version unknown)  | First 
             NCD Thin Clients  | BSD 
    OpenBSD (version unknown)  | linux 
    OpenBSD (version unknown)  | linux 
                  OpenVMS 7.1  | BSD 
       OS/2 (version unknown)  | BSD 
                    OSF1 V3.0  | BSD 
                    OSF1 V3.2  | BSD 
            OSF1 V4.0,5.0,5.1  | BSD 
                  SunOS 4.1.4  | BSD 
      SunOS 5.5.1,5.6,5.7,5.8  | First 
      Solaris 9, Solaris 10    | Solaris
        Tru64 Unix V5.0A,V5.1  | BSD 
                      Vax/VMS  | BSD 
   Windows (95/98/NT4/W2K/XP)  | Windows

Example configuration (Basic)
preprocessor frag3_global
preprocessor frag3_engine

Example configuration (Advanced)
preprocessor frag3_global: prealloc_frags 8192 
preprocessor frag3_engine: policy linux, bind_to 192.168.1.0/24
preprocessor frag3_engine: policy first, bind_to [10.1.47.0/24,172.16.8.0/24]
preprocessor frag3_engine: policy last, detect_anomalies

Note in the advanced example, there are three engines specified running with 
linux, first and last policies assigned.  The first two engines are bound to
specific IP address ranges and the last one applies to all other traffic, 
packets that don't fall within the address requirements of the first two engines
automatically fall through to the third one.

Alert Output
------------

Frag3 uses generator ID 123 for generating the following alerts:

SID   Description
---   -----------
1     IP Options on fragmented packet
2     Teardrop attack
3     Short fragment, possible DoS attempt
4     Fragment packet ends after defragmented packet
5     Zero-byte fragment
6     Bad fragment size, packet size is negative
7     Bad fragment size, packet size is greater than 65536
8     Fragmentation overlap
9     IPv6 BSD mbufs remote kernel buffer overflow
10    Bogus fragmentation packet. Possible BSD attack
11    TTL value less than configured minimum, not using for reassembly
12    Number of overlapping fragments exceed configured limit
13    Fragments smaller than configured min_fragment_length
