Input Framework

Bro features a flexible input framework that allows users to import data into Bro. Data is either read into Bro tables or converted to events which can then be handled by scripts. This document gives an overview of how to use the input framework with some examples. For more complex scenarios it is worthwhile to take a look at the unit tests in testing/btest/scripts/base/frameworks/input/.

Reading Data into Tables

Probably the most interesting use-case of the input framework is to read data into a Bro table.

By default, the input framework reads the data in the same format as it is written by the logging framework in Bro - a tab-separated ASCII file.

We will show the ways to read files into Bro with a simple example. For this example we assume that we want to import data from a blacklist that contains server IP addresses as well as the timestamp and the reason for the block.

An example input file could look like this (note that all fields must be tab-separated):

#fields ip timestamp reason
192.168.17.1 1333252748 Malware host
192.168.27.2 1330235733 Botnet server
192.168.250.3 1333145108 Virus detected

To read a file into a Bro table, two record types have to be defined. One contains the types and names of the columns that should constitute the table keys and the second contains the types and names of the columns that should constitute the table values.

In our case, we want to be able to lookup IPs. Hence, our key record only contains the server IP. All other elements should be stored as the table content.

The two records are defined as:

type Idx: record {
        ip: addr;
};

type Val: record {
        timestamp: time;
        reason: string;
};

Note that the names of the fields in the record definitions must correspond to the column names listed in the ‘#fields’ line of the log file, in this case ‘ip’, ‘timestamp’, and ‘reason’. Also note that the ordering of the columns does not matter, because each column is identified by name.

The log file is read into the table with a simple call of the Input::add_table function:

global blacklist: table[addr] of Val = table();

event bro_init() {
    Input::add_table([$source="blacklist.file", $name="blacklist",
                      $idx=Idx, $val=Val, $destination=blacklist]);
    Input::remove("blacklist");
}

With these three lines we first create an empty table that should contain the blacklist data and then instruct the input framework to open an input stream named blacklist to read the data into the table. The third line removes the input stream again, because we do not need it any more after the data has been read.

Because some data files can - potentially - be rather big, the input framework works asynchronously. A new thread is created for each new input stream. This thread opens the input data file, converts the data into a Bro format and sends it back to the main Bro thread.

Because of this, the data is not immediately accessible. Depending on the size of the data source it might take from a few milliseconds up to a few seconds until all data is present in the table. Please note that this means that when Bro is running without an input source or on very short captured files, it might terminate before the data is present in the table (because Bro already handled all packets before the import thread finished).

Subsequent calls to an input source are queued until the previous action has been completed. Because of this, it is, for example, possible to call add_table and remove in two subsequent lines: the remove action will remain queued until the first read has been completed.

Once the input framework finishes reading from a data source, it fires the Input::end_of_data event. Once this event has been received all data from the input file is available in the table.

event Input::end_of_data(name: string, source: string) {
        # now all data is in the table
        print blacklist;
}

The table can be used while the data is still being read - it just might not contain all lines from the input file before the event has fired. After the table has been populated it can be used like any other Bro table and blacklist entries can easily be tested:

if ( 192.168.18.12 in blacklist )
        # take action

Re-reading and streaming data

For many data sources, like for many blacklists, the source data is continually changing. For these cases, the Bro input framework supports several ways to deal with changing data files.

The first, very basic method is an explicit refresh of an input stream. When an input stream is open (this means it has not yet been removed by a call to Input::remove), the function Input::force_update can be called. This will trigger a complete refresh of the table; any changed elements from the file will be updated. After the update is finished the Input::end_of_data event will be raised.

In our example the call would look like:

Input::force_update("blacklist");

Alternatively, the input framework can automatically refresh the table contents when it detects a change to the input file. To use this feature, you need to specify a non-default read mode by setting the mode option of the Input::add_table call. Valid values are Input::MANUAL (the default), Input::REREAD and Input::STREAM. For example, setting the value of the mode option in the previous example would look like this:

Input::add_table([$source="blacklist.file", $name="blacklist",
                  $idx=Idx, $val=Val, $destination=blacklist,
                  $mode=Input::REREAD]);

When using the reread mode (i.e., $mode=Input::REREAD), Bro continually checks if the input file has been changed. If the file has been changed, it is re-read and the data in the Bro table is updated to reflect the current state. Each time a change has been detected and all the new data has been read into the table, the end_of_data event is raised.

When using the streaming mode (i.e., $mode=Input::STREAM), Bro assumes that the source data file is an append-only file to which new data is continually appended. Bro continually checks for new data at the end of the file and will add the new data to the table. If newer lines in the file have the same index as previous lines, they will overwrite the values in the output table. Because of the nature of streaming reads (data is continually added to the table), the end_of_data event is never raised when using streaming reads.

Receiving change events

When re-reading files, it might be interesting to know exactly which lines in the source files have changed.

For this reason, the input framework can raise an event each time when a data item is added to, removed from, or changed in a table.

The event definition looks like this (note that you can change the name of this event in your own Bro script):

event entry(description: Input::TableDescription, tpe: Input::Event,
            left: Idx, right: Val) {
        # do something here...
        print fmt("%s = %s", left, right);
}

The event must be specified in $ev in the add_table call:

Input::add_table([$source="blacklist.file", $name="blacklist",
                  $idx=Idx, $val=Val, $destination=blacklist,
                  $mode=Input::REREAD, $ev=entry]);

The description argument of the event contains the arguments that were originally supplied to the add_table call. Hence, the name of the stream can, for example, be accessed with description$name. The tpe argument of the event is an enum containing the type of the change that occurred.

If a line that was not previously present in the table has been added, then the value of tpe will be Input::EVENT_NEW. In this case left contains the index of the added table entry and right contains the values of the added entry.

If a table entry that already was present is altered during the re-reading or streaming read of a file, then the value of tpe will be Input::EVENT_CHANGED. In this case left contains the index of the changed table entry and right contains the values of the entry before the change. The reason for this is that the table already has been updated when the event is raised. The current value in the table can be ascertained by looking up the current table value. Hence it is possible to compare the new and the old values of the table.

If a table element is removed because it was no longer present during a re-read, then the value of tpe will be Input::EVENT_REMOVED. In this case left contains the index and right the values of the removed element.

Filtering data during import

The input framework also allows a user to filter the data during the import. To this end, predicate functions are used. A predicate function is called before a new element is added/changed/removed from a table. The predicate can either accept or veto the change by returning true for an accepted change and false for a rejected change. Furthermore, it can alter the data before it is written to the table.

The following example filter will reject adding entries to the table when they were generated over a month ago. It will accept all changes and all removals of values that are already present in the table.

Input::add_table([$source="blacklist.file", $name="blacklist",
                  $idx=Idx, $val=Val, $destination=blacklist,
                  $mode=Input::REREAD,
                  $pred(typ: Input::Event, left: Idx, right: Val) = {
                    if ( typ != Input::EVENT_NEW ) {
                        return T;
                    }
                    return (current_time() - right$timestamp) < 30day;
                  }]);

To change elements while they are being imported, the predicate function can manipulate left and right. Note that predicate functions are called before the change is committed to the table. Hence, when a table element is changed (typ is Input::EVENT_CHANGED), left and right contain the new values, but the destination (blacklist in our example) still contains the old values. This allows predicate functions to examine the changes between the old and the new version before deciding if they should be allowed.

Different readers

The input framework supports different kinds of readers for different kinds of source data files. At the moment, the default reader reads ASCII files formatted in the Bro log file format (tab-separated values with a “#fields” header line). Several other readers are included in Bro.

The raw reader reads a file that is split by a specified record separator (newline by default). The contents are returned line-by-line as strings; it can, for example, be used to read configuration files and the like and is probably only useful in the event mode and not for reading data to tables.

The binary reader is intended to be used with file analysis input streams (and is the default type of reader for those streams).

The benchmark reader is being used to optimize the speed of the input framework. It can generate arbitrary amounts of semi-random data in all Bro data types supported by the input framework.

Currently, Bro supports the following readers in addition to the aforementioned ones:

Reading Data to Events

The second supported mode of the input framework is reading data to Bro events instead of reading them to a table.

Event streams work very similarly to table streams that were already discussed in much detail. To read the blacklist of the previous example into an event stream, the Input::add_event function is used. For example:

type Val: record {
        ip: addr;
        timestamp: time;
        reason: string;
};

event blacklistentry(description: Input::EventDescription,
                     t: Input::Event, data: Val) {
        # do something here...
        print "data:", data;
}

event bro_init() {
        Input::add_event([$source="blacklist.file", $name="blacklist",
                          $fields=Val, $ev=blacklistentry]);
}

The main difference in the declaration of the event stream is, that an event stream needs no separate index and value declarations – instead, all source data types are provided in a single record definition.

Apart from this, event streams work exactly the same as table streams and support most of the options that are also supported for table streams.

Copyright 2016, The Bro Project. Last updated on July 21, 2017. Created using Sphinx 1.4.8.