Friday, January 9, 2009

Using Snort Subscriber Rule Set Certified Shared Object Rules

In order to instantiate shared object rules, a rule stub file is required. These stub files are not distributed in the VRT Certified rule packs, however they can be generated using snort.

Here is an example showing the pertinent configuration options in snort.conf along with the command line option required to generate the stub files. In some installations, the files may well reside in /etc/, this example uses /usr/local/etc as the location for the configuration files. (I like man hier you should read it sometime)

In snort.conf

First set up some global variables:
var CONF_PATH /usr/local/etc/snort
var LIB_PATH /usr/local/lib

Dynamic preprocessor and dynamic engine information:
dynamicpreprocessor directory $LIB_PATH/snort_dynamicpreprocessor
dynamicengine $LIB_PATH/snort_dynamicengine/

Here is the configuration option that lists the location of the shared object files that snort is to use:
dynamicdetection directory $LIB_PATH/snort_dynamicrules

Dumping the rules

To dump the rule stub files into the required location the --dump-dynamic-rules option is used like so:
snort -c /usr/local/etc/snort/snort.conf --dump-dynamic-rules=/usr/local/etc/snort/so_rules

This command tells snort to use the snort.conf file where it will find the dynamic rule files (thanks to the configuration options above) and then use those files to generate the stub files and put them into /usr/local/etc/snort/so_rules/

After this is complete, the rule files appear in the directory.
# ls /usr/local/etc/snort/so_rules/
bad-traffic.rules  imap.rules        nntp.rules  web-client.rules
chat.rules         misc.rules        p2p.rules   web-misc.rules
dos.rules          multimedia.rules  smtp.rules
exploit.rules      netbios.rules     sql.rules

Using the rules

At the end of the snort.conf file are the locations of the stub files that can now be used:
include $SORULE_PATH/bad-traffic.rules
include $SORULE_PATH/chat.rules
include $SORULE_PATH/dos.rules
include $SORULE_PATH/exploit.rules
include $SORULE_PATH/imap.rules
include $SORULE_PATH/misc.rules
include $SORULE_PATH/multimedia.rules
include $SORULE_PATH/netbios.rules
include $SORULE_PATH/nntp.rules
include $SORULE_PATH/p2p.rules
include $SORULE_PATH/smtp.rules
include $SORULE_PATH/sql.rules
include $SORULE_PATH/web-client.rules
include $SORULE_PATH/web-misc.rules


  1. Can you explain the steps to compile the dynamic rules from the "snortrules-snapshot-CURRENT_s.tar.gz"? I can use the precompiled rules from the CURRENT sig set. When I attempt to compile from the so_rules/src/ dir I receive the following errors.

    ls: cannot access web-misc_*.c: No such file or directory
    ls: cannot access sql_*.c: No such file or directory
    ls: cannot access multimedia_*.c: No such file or directory
    building p2p ... ERROR: Dynamic detection lib ./ 1.0 isn't compatible with the current dynamic engine library ../..//src/dynamic-plugins/sf_engine/.libs/ 1.9.
    The dynamic detection lib is compiled with an older version of the dynamic engine.
    Fatal Error, Quitting..
    make: *** [p2p] Error 1

  2. I got stuff to compile by removing -Wno-strict-aliasing from the CFLAGS in the make file.

    I also removed* from the lib dir to get snort to load the rules without errors.

    This was on RHE 3...

  3. Enable shared object rules (SO RULE_PATH) excludes the rules defined in RULE_PATH?


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