@emph{NOTE: Bro Signatures mechanism is still under development}

Signatures in Bro are quite different than standard packet matching signatures such as those used in
@uref{http://www.snort.org', Snort}. A Bro signature, or @emph{Rule}, is a @emph{contextual signature}
that can include connection-level information. Hence Bro signatures generate @strong{far} fewer
false positives.

However, Bro's contextual signatures are fairly CPU and memory intensive,
and still generate more false positives than we'd like,
so for now they are turned off by default. See the next section for information on how to turn them on.

For example, an packet-level signature of a HTTP attack only looks at the attack packet, where
the Bro contextual signature also looks for the HTTP reply, and only generates an alarm if the attack was 
successful.

In this section we explain how to customize signatures for your site,
and how to import new signatures from Snort and bro-ids.org. More
information on the details of Bro signatures are in
@uref{http://www.bro-ids.org/Bro-reference-manual/Signature-language.html, the
signature section of the reference manual}.

The following files are used to control and customize Bro signatures.

@itemize
@item @code{$BROHOME/site/signatures.sig}: Bro version of snort signatures
@item @code{$BROHOME/policy/sig-addendum.sig}: Bro supplied signatures
@item @code{$BROHOME/policy/sig-action.bro}: policy file to control signature notification type
@end itemize

Files in @code{$BROHOME/policy} contain the default Bro signatures, and should not be edited.
Files in @code{$BROHOME/site} contain files you will use to customize signatures for your site.
New signatures that you write go here too. All files ending in @code{.sig} in this directory
will be loaded into the signature engine. In fact, all .sig files in any
directory in @code{$BROPATH} (set in @code{$BROHOME/etc/bro.cfg}) will be loaded.

@menu
* Turning Signatures ON/OFF ::
* Add a New Signature ::
* Editing Existing Signatures ::
* Importing Snort Signatures ::
* Checking for new Signatures from bro-ids.org ::
@end menu


@node Turning Signatures ON/OFF 
@subsection Turning Signatures ON/OFF
@cindex Turning Signatures ON/OFF

Signature matching is off by default. To use a small set of
known, high quality signatures, add the following to your site policy file:
@smallexample
@@load brolite-sigs
@end smallexample

To use the full set of converted snort signatures,
add both of these lines:
@smallexample
@@load brolite-sigs
redef signature_files += "signatures";
@end smallexample

If signatures are turned on, then you can control the 
signature "action" levels through the file 
@code{$BROHOME/site/sigaction.bro}.
You can set the signature action to the one of the following:

@verbatim
    SIG_IGNORE          # ignore this sig. completely 
    SIG_FILE            # write to signatures and notice files
    SIG_ALARM           # alarm and write to notice and alarm files
    SIG_ALARM_PER_ORIG  # alarm once per originator
    SIG_ALARM_ONCE      # alarm once and then never again
@end verbatim

All signatures default to action = @code{SIG_ALARM}. To lower the alarm level of the signature,
add an entry to the file @code{$BROHOME/site/sigaction.bro}.  The Bro distribution
contains a default sigaction.bro file that lowers the level of a number of signatures from ALARM
to FILE (notice) .

To permanently remove a signature you can delete it from the @code{.sig} file.


@node  Add a New Signature
@subsection  Add a New Signature
@cindex  Add a New Signature

To add a new signature to a running Bro, add the signature to the file
@code{$BROHOME/site/site.sig} (or create a new @code{.sig} file in @code{$BROHOME/site}), 
and then restart Bro using "@code{$BROHOME/etc/bro.rc checkpoint}".

A sample signature looks like this:

@verbatim
signature formmail-cve-1999-0172 {
       ip-proto == tcp
       dst-ip == 1.2.0.0/16
       dst-port = 80
       http /.*formmail.*\?.*recipient=[^&]*[;|]/
       event "formmail shell command"
       }
@end verbatim

For more details, see the
@uref{http://www.bro-ids.org/Bro-reference-manual/Signature-language.html, 
reference manual}.

@node   Editing Existing Signatures
@subsection   Editing Existing Signatures
@cindex   Editing Existing Signatures

Bro supplied signatures are in $BROHOME/sigs. You should not edit these, as they will
get overwritten when you update Bro. Instead, make your modifications in $BROHOME/site.
If you use the same signature ID as an existing signature, the site sig will take precedence.

@node Importing Snort Signatures
@subsection Importing Snort Signatures
@cindex Importing Snort Signatures

New snort signatures come out almost every week. To add these to Bro, do the following:

(XXX section not done!)

Add instructions for incorporating new sigs from Snort.

@node Checking for new Signatures from bro-ids.org
@subsection Checking for new Signatures from bro-ids.org
@cindex download new Signatures 

note: this functionality is currently under development, and does
not yet exist

The Bro team will be constantly updating our set of default signatures and posting
them on the Bro web site. To download the latest signatures and incorporate
them into your Bro setup, run the script:
@example
$BROHOME/scripts/update-sigs
@end example
This script uses the @code{wget} command to download the latest signatures 
and puts them into
the required Bro files, and then restarts Bro to load the new signatures..