Thursday, January 15, 2015

Snort++ Tips and Tricks

One of the goals of Snort++ is to make it easier to configure your sensor.  Here is a summary of tips and tricks you may find useful.

General Use

  • Snort tries hard not to error out too quickly.  It will report multiple semantic errors.
  • Snort always assumes the simplest mode of operation.  Eg, you can omit the -T option to validate the conf if you don't provide a packet source.
  • Warnings are not emitted unless --warn-* is specified.  --warn-all enables all warnings, and --pedantic makes such warnings fatal.
  • You can process multiple sources at one time by using the -z or --max-threads option.
  • To make it easy to find the important data, zero counts are not output at shutdown.

Lua Configuration

  • Configure the wizard and default bindings will be created based on configured inspectors.  No need to explicitly bind ports in this case.
  • You can override or add to your Lua conf with the --lua command line option.
  • The Lua conf is a live script that is executed when loaded.  You can add functions, grab environment variables, compute values, etc.
  • You can also rename symbols that you want to disable.  For example, changing normalizer to Xnormalizer will disable the normalizer.  This can be easier than commenting in some cases.
  • By default, symbols unknown to Snort++ are silently ignored.  You can generate warnings for them with --warn-unknown.  To ignore such symbols, export them in the environment variable SNORT_IGNORE.

Writing and Loading Rules

Snort++ rules allow arbitrary whitespace.  Multi-line rules make it easier to structure your rule for clarity.  There are multiple ways to add comments to your rules:
  • Like Snort, the # character starts a comment to end of line.  In addition, all lines between #begin and #end are comments.
  • The rem option allows you to write a comment that is conveyed with the rule.
  • C style multi-line comments are allowed, which means you can comment out portions of a rule while testing it out by putting the options between /* and */.
There are multiple ways to load rules too:
  • Set ips.rules or ips.include in your conf.
  • Snort2 include statements can be used in rules files.
  • Use -R to load a rules file.
  • Use --stdin-rules with command line redirection.
  • Use --lua to specify one or more rules as a command line argument.

Output Files

To make it simple to configure outputs when you run with multiple packet threads, output files are not explicitly configured.  Instead, you can use the options below to format the paths:
  • logdir is set with -l and defaults to ./
  • run_prefix is set with --run-prefix else not used
  • id# is the packet thread number that writes the file; with one packet thread, id# (zero) is omitted without --id-zero
  • x is / if you use --id-subdir, else _ if id# is used
  • name is based on module name that writes the file
  • all text mode outputs default to stdout

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