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~ A REALLY smart preprocessor module for Snort ~
by BlackLight <blacklight@autistici.org>, http://0x00.ath.cx
============================================================================
This document describes the AI preprocessor module for Snort.
It also describes how to get it, install it, configure it and use it correctly.
Table of contents:
1. What's Snort AI preprocessor
2. How to get Snort AI preprocessor
3. Installation
3.1 Dependancies
3.2 Configure options
4. Basic configuration
5. Correlation rules
6. Additional documentation
===============================
1. What's Snort AI preprocessor
===============================
Snort AI preprocessor is a preprocessor module for Snort whose purpose is making
the reading of Snort's alerts more comfortable, clustering false positive alarms
emphasizing their root cause in order to reduce log pollution, clustering
similar alerts in function of the type and hierarchies over IP addresses and
ports that can be decided by the user, depending on the kind of traffic and
topology of the network, and constructing the flows of a multi-step attack in
function of correlation rules between hyperalerts provided by the developer
itself, by third parts or created by the user itself, again, in function of the
scenario of the network. It will furthermore possible, in a close future, to
correlate the hyperalerts automatically, by self-learning on the base of the
acquired alerts.
===================================
2. How to get Snort AI preprocessor
===================================
It it strongly suggested to get the latest and always-fresh release of Snort AI
preprocessor from GitHub -> http://github.com/BlackLight/Snort_AIPreproc
git clone git://github.com/BlackLight/Snort_AIPreproc.git
If git is not available on the machine or cannot be used, from the same page you
can also choose "download source" and download the source code in tar.gz format.
===============
3. Installation
===============
The installation procedure is the usual one:
$ ./configure
$ make
$ make install
If you did not install Snort in /usr directory you may need to use the --prefix
option with configure for selecting the directory where you installed Snort (for
example ./configure --prefix=$HOME/local/snort). If the prefix was
specified correctly, and it actually points to the location where Snort was
installed, the module binaries should be placed in
$SNORT_DIR/lib/snort_dynamicpreprocessor after the installation, and
automatically loaded by Snort at the next start. Moreover, a new directory
named corr_rules will be created, in /etc/snort if the prefix was /usr or in
$SNORT_DIR/etc otherwise, containing XML files describing default correlation
rules provided by the developer. This set can be enriched in any moment with new
XML files, provided by third parts or created by the user itself, describing
more hyperalerts.
================
3.1 Dependancies
================
Dependancies required for a correct compilation and configuration:
- pthread (REQUIRED), used for running multiple threads inside of the module. On
a Debian-based system, install libpthread-dev if you don't already have it.
- libxml2 (REQUIRED), used for parsing XML files from corr_rules directory. On a
Debian-based system, install libxml2-dev if you don't already have it.
- libgraphviz (RECOMMANDED), used for generating PNG (and in future PS too)
files representing hyperalert correlation graphs from .dot files
generated from the software. You can remove this dependancy from the
compilation process by specifying --without-graphviz to ./configure, but in
this case you will have .dot files, not easily understandable by a human,
for representing correlation graphs, and you may need an external graph
rendering software for converting them in a more easily readable format. On
a Debian system, install libgraphviz-dev if you don't already have it.
- libmysqlclient (OPTIONAL), used if you want to read alerts information saved
on MySQL DBMS, or enable MySQL support in the module. This option is disabled by
default (if not specified otherwise, the module will read the alerts from Snort
plain log files), and can be enabled by specifying the option
--with-mysql to ./configure. On a Debian-based system you may need to install
libmysqlclient-dev.
- libpq (OPTIONAL), used if you want to read alerts information saved on
PostgreSQL DBMS, or enable PostgreSQL support in the module. This option is
disabled by the default, and can be enabled by specifying the option
--with-postgresql to ./configure. On a Debian-based system you may need to
install libpq-dev.
=====================
3.2 Configure options
=====================
You can pass the following options to ./configure script before compiling:
--with-mysql - Enables MySQL DBMS support into the module (it requires
libmysqlclient)
--with-pq - Enables PostgreSQL DBMS support into the module (it requires libpq)
--without-graphviz - Disables Graphviz support from the module, avoiding the
generation of PNG or PS files representing hyperalerts correlation as well
======================
4. Basic configuration
======================
After installing the module in Snort installation directory a configuration for
this is required in snort.conf. A sample configuration may appear like the
following:
preprocessor ai: \
hashtable_cleanup_interval 300 \
tcp_stream_expire_interval 300 \
alertfile "/your/snort/dir/log/alert" \
alert_history_file "/your/snort/dir/log/alert_history" \
alert_serialization_interval 3600 \
alert_bufsize 30 \
alert_clustering_interval 300 \
bayesian_correlation_interval 1200 \
bayesian_correlation_cache_validity 600 \
correlation_graph_interval 300 \
correlation_rules_dir "/your/snort/dir/etc/corr_rules" \
correlated_alerts_dir "/your/snort/dir/log/correlated_alerts" \
correlation_threshold_coefficient 0.5 \
database ( type="dbtype", name="snort", user="snortusr", password="snortpass", host="dbhost" ) \
output_database ( type="dbtype", name="snort", user="snortusr", password="snortpass", host="dbhost" ) \
database_parsing_interval 30 \
cluster_max_alert_interval 14400 \
clusterfile "/your/snort/dir/log/clustered_alerts" \
cluster ( class="dst_port", name="privileged_ports", range="1-1023" ) \
cluster ( class="dst_port", name="unprivileged_ports", range="1024-65535" ) \
cluster ( class="src_addr", name="local_net", range="192.168.1.0/24" ) \
cluster ( class="src_addr", name="dmz_net", range="155.185.0.0/16" ) \
cluster ( class="src_addr", name="vpn_net", range="10.8.0.0/24" ) \
cluster ( class="dst_addr", name="local_net", range="192.168.1.0/24" ) \
cluster ( class="dst_addr", name="dmz_net", range="155.185.0.0/16" ) \
cluster ( class="dst_addr", name="vpn_net", range="10.8.0.0/24" )
The options are the following:
- alertfile: The file where Snort saves its alerts, if they are saved to a file
and not to a database (default if not specified: /var/log/snort/alert)
- alert_history_file: The file keeping track of the history, in binary format,
of all the alerts received by the IDS, so that the module can build some
statistical correlation inferences over the past
- alert_serialization_interval: The interval that should occur from a
serialization of a buffer of alerts on the history file and the next one
(default if not specified: 1 hour, as it is a quite expensive operation in terms
of resources if the system received many alerts)
- alert_bufsize: Size of the buffer containing the alerts to be sent, in group,
to the serializer thread. The buffer is sent when full and made empty even
when the alert_serialization_interval parameter is not expired yet, for
avoiding overflows, other memory problems or deadlocks (default value if
not specified: 30)
- alert_clustering_interval: The interval that should occur from the clustering
of the alerts in the log according to the provided clustering hierarchies and
the next one (default if not specified: 300 seconds)
- bayesian_correlation_interval: Interval, in seconds, that should occur between
two alerts in the history for considering them as, more or less strongly,
correlated (default: 1200 seconds). NOTE: A value of 0 will disable the bayesian
correlation. This setting is strongly suggested when your alert log is still
"learning", i.e. when you don't have enough alerts yet. After this period, you
can set the correlation interval to any value.
- bayesian_correlation_cache_validity: interval, in seconds, for which an entry
in the bayesian correlation hash table (i.e. a pair of alerts with the
associated historical bayesian correlation) is considered as valid
before being updated (default: 600 seconds)
- correlation_graph_interval: The interval that should occur from the building
of the correlation graph between the clustered alerts and the next one (default
if not specified: 300 seconds)
- correlation_rules_dir: Directory where the correlation rules are saved, as XML
files (default if not specified: /etc/snort/corr_rules)
- correlated_alerts_dir: Directory where the information between correlated
alerts will be saved, as .dot files ready to be rendered as graphs and, if
libgraphviz support is enabled, as .png and .ps files as well (default if not
specified: /var/log/snort/clustered_alerts)
- correlation_threshold_coefficient: The threshold the software uses for stating
two alerts are correlated is avg(correlation coefficient) + k *
std_deviation(correlation_coefficient). The value of k is specified through this
option, whose value is 0.5 by default, and is dependant on how "sensible" you
want the correlation algorithm. A value of k=0 means "consider all the couples
of alerts whose correlation coefficient is greater than the average one as
correlated" (negative values of k are allowed as well, but unless you know what
you're doing they're unrecommended, as some correlation constraints may appear
where no correlation exists). When the value of k raises also the threshold for
two alerts for being considered as correlated raises. A high value of k may just
lead to an empty correlation graph
- clusterfile: File where the clustered alerts will be saved by the module
(default if not specified: /var/log/snort/clustered_alerts)
- cluster_max_alert_interval: Maximum time interval, in seconds, occurred
between two alerts for considering them as part of the same cluster (default:
14400 seconds, i.e. 4 hours). Specify 0 for this option if you want to
cluster alerts regardlessly of how much time occurred between them
- cluster: Clustering hierarchy or list of hierarchies to be applied for
grouping similar alerts. This option needs to specify:
-- class: Class of the cluster node. It may be src_addr, dst_addr, src_port
or dst_port
-- name: Name for the clustering node
-- range: Range of the clustering node. It can include a single port or IP
address, an IP range (specified as subnet x.x.x.x/x), or a port
range (specified as xxx-xxx)
- database: If Snort saves its alerts to a database and the module was compiled
with database support (e.g. --with-mysql) this option specifies the
information for accessing that database. The fields in side are
-- type: DBMS to be used (so far MySQL and PostgreSQL are supported)
-- name: Database name
-- user: Username for accessing the database
-- password: Password for accessing the database
-- host: Host holding the database
- database_parsing_interval: The interval that should occur between a read of
the alerts from database and the next one (default if not specified: 30 seconds)
- hashtable_cleanup_interval: The interval that should occur from the cleanup of
the hashtable of TCP streams and the next one (default if not specified: 300
seconds)
- output_database: Specify this option if you want to save the outputs from the
module (correlated alerts, clustered alerts, alerts information and their
associated packets streams, and so on) to a relational database as
well (by default the module only saves the alerts on static plain files). The
options here are the same specified for the 'database' option.
The structure of this database can be seen in the files schemas/*.sql (replace
to * the name of your DBMS). If you want to initialize the tables needed by the
module, just give the right file to your database, e.g. for MySQL
$ mysql -uusername -ppassword dbname < schemas/mysql.sql
- tcp_stream_expire_interval: The interval that should occur for marking a TCP
stream as "expired", if no more packets are received inside of that and it's not
"marked" as suspicious (default if not specified: 300 seconds)
====================
5. Correlation rules
====================
The hyperalert correlation rules are specified in $SNORT_DIR/etc/corr_rules
directory through a very simple XML syntax, and any user can add some new ones.
The files in there must be named like the Snort alert ID they want to model. For
example, if we want to model a TCP portscan alert (Snort ID: 122.1.0) as a
hyperalert, i.e. as an alert with pre-conditions and post-conditions to be
correlated to other alerts, then we need to create a file named 122-1-0.xml
inside $SNORT_DIR/etc/corr_rules with a content like the following:
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE hyperalert PUBLIC "-//blacklighth//DTD HYPERALERT SNORT MODEL//EN" "http://0x00.ath.cx/hyperalert.dtd">
<hyperalert>
<snort-id>122.1.0</snort-id>
<desc>(portscan) TCP Portscan</desc>
<pre>HostExists(+DST_ADDR+)</pre>
<post>HasService(+DST_ADDR+, +ANY_PORT+)</post>
</hyperalert>
The <desc> tag is optional, same for <pre> and <post> if an alert has no
pre-conditions and/or post-conditions, while the <snort-id> tag is mandatory for
identifying the hyperalert. In this case we say that the pre-condition for a TCP
portscan for being successful is that the host +DST_ADDR+ exists (the macro
+DST_ADDR+ will automatically be expanded at runtime and substituted
with the target address of the portscan). The post-condition of a
portscan consists in the attacker knowing that +DST_ADDR+ runs a service on
+ANY_PORT+ (+ANY_PORT+ is another macro that will be expanded on runtime). The
hyperalerts model in corr_rules are the knowledge base used for correlating
alerts triggered by Snort, the more information it has inside, the more accurate
and complete the correlation will be. The macros recognized and automatically
expanded from these XML files are
- +SRC_ADDR+: The IP address triggering the alert
- +DST_ADDR+: The target IP address in the alert
- +SRC_PORT+: The port from which the alert was triggered
- +DST_PORT+: The port on which the alert was triggered
- +ANY_ADDR+: Identifies any IP address
- +ANY_PORT+: Identifies any port
The correlation between two alerts A and B is made comparing the post-conditions
of A with the pre-conditions of B. If the correlaton coefficient computed in
this way is greater than a certain threshold (see "Basic configuration ->
correlation_threshold_coefficient") then the alerts are marked as
correlated, i.e. the alert A determines the alert B. This supports an elementary
reasoning algorithm for doing inferences on the conditions. For example, if A
has the post-condition "HasService(+DST_ADDR+, +ANY_PORT+)" and B has the
pre-condition "HasService(+DST_ADDR, 22)", a match between A and B is triggered.
Same if A has "HostExists(10.8.0.0/24)" as post-condition and B has
"HostExists(10.8.0.1)" as pre-condition.
There is no fixed semantics for the the predicates in pre-conditions and
post-conditions, any predicates can be used. The only constraint is that the
same predicates have the same semantic and prototype in all of the hyperalerts.
For example, if HasService has a prototype like "HasService(ip_addr, port)",
then all the hyperalerts should follow this prototype, otherwise the
matching would fail. Any new predicates can be defined as well in
hyperalerts, provided that it respects this constraint.
===========================
6. Additional documentation
===========================
The additional documentation over the code, functions and data structures can
be automatically generated by Doxygen by typing `make doc', and installed in
SNORT_DIR/doc then typing `make doc-install'.
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