Shared memory support

In order to minimize memory consumption when multiple Snort instances are running concurrently, we introduce the support of shared memory. After configured, all the snort instances share the same IP tables in shared memory.

System requirement

This feature is supported only in Linux.

Build configuration

A new option, -enable-shared-rep is introduced to ./configure command. This option enables the support for shared memory.

Configuration

shared_mem

If the build supports shared memory, this configuration will enable shared memory. If this option isn't set, standard memory is used. This option must specify a path or directory where IP lists will be loaded in shared memory. One snort instance will create and maintain the shared IP lists. We use instance ID 1, specified in the snort -G option to be the master snort. All the other snort instances are clients (readers).

Syntax

    shared_mem: path

Examples

          
    shared_mem  /user/reputation/iplists

shared_refresh

This option changes the period of checking new shared memory segment, in the unit of second. By default, the refresh rate is $60$ seconds.

Syntax

    
    shared_refresh <period>
    period = "1 - 4294967295"

Examples

            
    shared_refresh 60

Steps to configure shared memory

• When building Snort, add option -enable-shared-rep to ./configure
  For example:

         ./configure --enable-gre --enable-sourcefire --enable-flexresp3 
         --enable-dynamicplugin --enable-pthread --enable-linux-smp-stats 
         --enable-targetbased --enable-shared-rep --enable-control-socket
  

• Put your IP list file into a directory, where snort has full access.
  For example:

         /user/reputation/iplists
  

  In order to separate whitelist with blacklist, you need to specify whitelist with .wlf extension and blacklist with .blf extension.

• In snort config file, specify shared memory support with the path to IP files.
  For example:

         shared_mem  /user/reputation/iplists
  

  If you want to change the period of checking new IP lists, add refresh period.
  For example:

         shared_refresh 300
  

• Start shared memory master(writer) with -G 0 option. Note: only one master should be enabled.
• Start shared memory clients (readers) with -G 1 or other IDs. Note: for one ID, only one snort instance should be enabled.
• You will see the IP lists got loaded and shared across snort instances!

Reload IP lists using control socket

• Run snort using command line with option -cs-dir <path> or configure snort with:

   
        config cs_dir:<path>
  

• (Optional) you can create a version file named ``IPRVersion.dat'' in the IP list directory. This file helps managing reloading IP lists, by specifying a version. When the version isn't changed, IP lists will not be reloaded if they are already in shared memory. The version number should be a 32 bit number.
  For example:

        VERSION=1
  

• In the <snort root>/src/tools/control directory, you will find snort_control command if built with -enable-control-socket option.
• Type the following command to reload IP lists. Before typing this command, make sure to update version file if you are using version file. The <path> is the same path in first step.
  

        <snort root>/src/tools/control/snort_control  <path> 1361
  

Using manifest file to manage loading (optional)

Using manifest file, you can control the file loading sequence, action taken, and support zone based detection. You can create a manifest file named ``zone.info'' in the IP list directory.

When Snort is signaled to load new lists, a manifest file is read first to determine which zones the IPs in each list are applicable to and what action to take per list (Block, White, Monitor).

Files listed in manifest are loaded from top to bottom. You should put files that have higher priority first. In manifest file, you can put up to 255 files. Without manifest file, files will be loaded in alphabet order.

Here's the format of the manifest file. Each line of the file has the following format:

 
     <filename>, <list id>,<action>[, <zone>]+
  
     <list id> ::= 32 bit integer
     <action> ::= "monitor"|"block"|"white"
     <zone>  ::= [0-1051]

Using manifest file, you can specify a new action called ``monitor'', which indicates a packet needs to be inspected, but does not disable detection. This is different from ``block'' action, which disables further detection. This new action helps users evaluate their IP lists before applying it.

An exmple manifest file:

 
     #ipreputation manifest file
     white.wlf, 111 ,white, 
     black1.blf, 1112, black,  3, 12
     black2.blf, 1113, black,  3, 12
     monitor.blf,2222, monitor, 0, 2, 8