Preprocessor Documentation



                              Frag3 -------------------------------------------------------------------------------- Author: Martin Roesch <>


The frag3 preprocessor is a target-based IP defragmentation module for Snort. Frag3 is designed with the following goals:

1) Faster execution with less complex data management.

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

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 occur and when this happens different 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

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

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.


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 - Maximum simultaneous fragments to track, default is 8192 memcap - Memory cap for self preservation, default is 4MB prealloc_memcap - alternate memory management mode, use preallocated fragment nodes based on a memory cap (faster in some situations) prealloc_frags - alternate memory management mode, use preallocated fragment nodes based on a static number (faster in some situations) disabled - This optional keyword is allowed with any policy to avoid packet processing. This option disables the preprocessor for this config, but not for other instances of multiple configurations. Use the disable keyword in the base configuration to specify values for the options memcap, prealloc_memcap, and prealloc_frags without having the preprocessor inspect traffic for traffic applying to the base configuration. The other options are parsed but not used. Any valid configuration may have "disabled" added to it.

Engine Configuration
- Preprocessor name: frag3_engine - Available Options NOTE: Engine configuration options are space separated. timeout - Timeout for fragments, fragments in the engine for longer than this period will be automatically dropped. Default is 60 seconds. min_ttl - Minimum acceptable TTL value for a fragment packet. Default is 1. The accepted range for this option is 1 - 255. detect_anomalies - Detect fragment anomalies bind_to - 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 - Limits the number of overlapping fragments per packet. The default is "0" (unlimited). This config option takes values equal to or greater than zero. This is an optional parameter. detect_anomalies option must be configured for this option to take effect. min_fragment_length - 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). This config option takes values equal to or greater than zero. This is an optional parameter. detect_anomalies option must be configured for this option to take effect. policy - 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
                 fragmented 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 preprocessor frag3_engine: policy first bind_to [,] 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