
.. _bro-logging:

===========
Bro Logging
===========

.. contents::

Once Bro has been deployed in an environment and monitoring live
traffic, it will, in its default configuration, begin to produce
human-readable ASCII logs.  Each log file, produced by Bro's
:ref:`framework-logging`, is populated with organized, mostly
connection-oriented data.  As the standard log files are simple ASCII
data, working with the data contained in them can be done from a
command line terminal once you have been familiarized with the types
of data that can be found in each file. In the following, we work
through the logs general structure and then examine some standard ways
of working with them.

----------------------
Working with Log Files
----------------------

Generally, all of Bro's log files are produced by a corresponding
script that defines their individual structure. However, as each log
file flows through the Logging Framework, they share a set of
structural similarities. Without breaking into the scripting aspect of
Bro here, a bird's eye view of how the log files are produced
progresses as follows.  The script's author defines the kinds of data,
such as the originating IP address or the duration of a connection,
which will make up the fields (i.e., columns) of the log file.  The
author then decides what network activity should generate a single log
file entry (i.e., one line). For example, this could be a connection 
having been completed or an HTTP ``GET`` request being issued by an
originator. When these behaviors are observed during operation, the
data is passed to the Logging Framework which adds the entry
to the appropriate log file.

As the fields of the log entries can be further customized by the
user, the Logging Framework makes use of a header block to ensure that
it remains self-describing. This header entry can be see by running
the Unix utility ``head`` and outputting the first lines of the file:

.. btest:: using_bro
   
   @TEST-EXEC: btest-rst-cmd bro -r $TRACES/wikipedia.trace
   @TEST-EXEC: btest-rst-include -n 15 conn.log

As you can see, the header consists of lines prefixed by ``#`` and
includes information such as what separators are being used for
various types of data, what an empty field looks like and what an
unset field looks like.  In this example, the default TAB separator is
being used as the delimiter between fields (``\x09`` is the tab
character in hex).  It also lists the comma as the separator for set
data, the string ``(empty)`` as the indicator for an empty field and
the ``-`` character as the indicator for a field that hasn't been set.
The timestamp for when the file was created is included under
``#open``. The header then goes on to detail the fields being listed
in the file and the data types of those fields, in ``#fields`` and
``#types``, respectively. These two entries are often the two most
significant points of interest as they detail not only the field names
but the data types used. When navigating through the different log
files with tools like ``sed``, ``awk``, or ``grep``, having the field
definitions readily available saves the user some mental leg work. The
field names are also a key resource for using the :ref:`bro-cut
<bro-cut>` utility included with Bro, see below.

Next to the header follows the main content. In this example we see 7
connections with their key properties, such as originator and
responder IP addresses (note how Bro transparently handles both IPv4 and
IPv6), transport-layer ports, application-layer services ( - the
``service`` field is filled in as Bro determines a specific protocol to
be in use, independent of the connection's ports), payload size, and
more. See :bro:type:`Conn::Info` for a description of all fields.

In addition to ``conn.log``, Bro generates many further logs by
default, including:

``dpd.log``
    A summary of protocols encountered on non-standard ports.

``dns.log``
    All DNS activity.

``ftp.log``
    A log of FTP session-level activity.

``files.log``
    Summaries of files transferred over the network. This information
    is aggregated from different protocols, including HTTP, FTP, and
    SMTP.

``http.log``
    A summary of all HTTP requests with their replies.

``known_certs.log``
    SSL certificates seen in use.

``smtp.log``
    A summary of SMTP activity.

``ssl.log``
    A record of SSL sessions, including certificates being used.

``weird.log``
    A log of unexpected protocol-level activity. Whenever Bro's
    protocol analysis encounters a situation it would not expect
    (e.g., an RFC violation) it logs it in this file. Note that in
    practice, real-world networks tend to exhibit a large number of
    such "crud" that is usually not worth following up on.

As you can see, some log files are specific to a particular protocol,
while others aggregate information across different types of activity. 
For a complete list of log files and a description of its purpose, 
see :doc:`Log Files <../script-reference/log-files>`.

.. _bro-cut:

Using ``bro-cut``
-----------------

The ``bro-cut`` utility can be used in place of other tools to build
terminal commands that remain flexible and accurate independent of
possible changes to the log file itself.  It accomplishes this by parsing
the header in each file and allowing the user to refer to the specific
columnar data available (in contrast to tools like ``awk`` that
require the user to refer to fields referenced by their position).
For example, the following command extracts just the given columns
from a ``conn.log``:

.. btest:: using_bro

   @TEST-EXEC: btest-rst-cmd -n 10 "cat conn.log | bro-cut id.orig_h id.orig_p id.resp_h duration"

The corresponding ``awk`` command will look like this:

.. btest:: using_bro

   @TEST-EXEC: btest-rst-cmd -n 10 awk \'/^[^#]/ {print \$3, \$4, \$5, \$6, \$9}\' conn.log

While the output is similar, the advantages to using bro-cut over
``awk`` lay in that, while ``awk`` is flexible and powerful, ``bro-cut``
was specifically designed to work with Bro's log files.  Firstly, the
``bro-cut`` output includes only the log file entries, while the
``awk`` solution needs to skip the header manually. Secondly, since
``bro-cut`` uses the field descriptors to identify and extract data,
it allows for flexibility independent of the format and contents of
the log file.  It's not uncommon for a Bro configuration to add extra
fields to various log files as required by the environment.  In this
case, the fields in the ``awk`` command would have to be altered to
compensate for the new position whereas the ``bro-cut`` output would
not change.

.. note::

    The sequence of field names given to ``bro-cut`` determines the
    output order, which means you can also use ``bro-cut`` to reorder
    fields. That can be helpful when piping into, e.g., ``sort``.

As you may have noticed, the command for ``bro-cut`` uses the output
redirection through the ``cat`` command and ``|`` operator.  Whereas
tools like ``awk`` allow you to indicate the log file as a command
line option, bro-cut only takes input through redirection such as
``|`` and ``<``.  There are a couple of ways to direct log file data
into ``bro-cut``, each dependent upon the type of log file you're
processing.  A caveat of its use, however, is that all of the
header lines must be present.

.. note::

    ``bro-cut`` provides an option ``-c`` to include a corresponding
    format header into the output, which allows to chain multiple
    ``bro-cut`` instances or perform further post-processing that
    evaluates the header information.

In its default setup, Bro will rotate log files on an hourly basis,
moving the current log file into a directory with format
``YYYY-MM-DD`` and gzip compressing the file with a file format that
includes the log file type and time range of the file.  In the case of
processing a compressed log file you simply adjust your command line
tools to use the complementary ``z*`` versions of commands such as ``cat``
(``zcat``) or ``grep`` (``zgrep``).

Working with Timestamps
-----------------------

``bro-cut`` accepts the flag ``-d`` to convert the epoch time values
in the log files to human-readable format.  The following command
includes the human readable time stamp, the unique identifier, the
HTTP ``Host``, and HTTP ``URI`` as extracted from the ``http.log``
file:

.. btest:: using_bro

   @TEST-EXEC: btest-rst-cmd -n 5 "bro-cut -d ts uid host uri < http.log"

Often times log files from multiple sources are stored in UTC time to
allow easy correlation.  Converting the timestamp from a log file to
UTC can be accomplished with the ``-u`` option:

.. btest:: using_bro

   @TEST-EXEC: btest-rst-cmd -n 5 "bro-cut -u ts uid host uri < http.log"

The default time format when using the ``-d`` or ``-u`` is the
``strftime`` format string ``%Y-%m-%dT%H:%M:%S%z`` which results in a
string with year, month, day of month, followed by hour, minutes,
seconds and the timezone offset.  The default format can be altered by
using the ``-D`` and ``-U`` flags, using the standard ``strftime``
syntax. For example, to format the timestamp in the US-typical "Middle
Endian" you could use a format string of: ``%d-%m-%YT%H:%M:%S%z``

.. btest:: using_bro

   @TEST-EXEC: btest-rst-cmd -n 5 "bro-cut -D %d-%m-%YT%H:%M:%S%z ts uid host uri < http.log"

See ``man strfime`` for more options for the format string.

Using UIDs
----------

While Bro can do signature-based analysis, its primary focus is on
behavioral detection which alters the practice of log review from
"reactionary review" to a process a little more akin to a hunting
trip.  A common progression of review includes correlating a session
across multiple log files.  As a connection is processed by Bro, a
unique identifier is assigned to each session.  This unique identifier
is generally included in any log file entry associated with that
connection and can be used to cross-reference different log files.

A simple example would be to cross-reference a UID seen in a
``conn.log`` file.  Here, we're looking for the connection with the
largest number of bytes from the responder by redirecting the output
for ``cat conn.log`` into bro-cut to extract the UID and the
resp_bytes, then sorting that output by the resp_bytes field.

.. btest:: using_bro

   @TEST-EXEC: btest-rst-cmd "cat conn.log | bro-cut uid resp_bytes | sort -nrk2 | head -5"

Taking the UID of the first of the top responses, we can now
crossreference that with the UIDs in the ``http.log`` file.

.. btest:: using_bro

   @TEST-EXEC: btest-rst-cmd "cat http.log | bro-cut uid id.resp_h method status_code host uri | grep UM0KZ3MLUfNB0cl11"

As you can see there are two HTTP ``GET`` requests within the
session that Bro identified and logged.  Given that HTTP is a stream
protocol, it can have multiple ``GET``/``POST``/etc requests in a
stream and Bro is able to extract and track that information for you,
giving you an in-depth and structured view into HTTP traffic on your
network.
