This section describes how to use pattern databases in syslog-ng OSE.

To classify messages using a pattern database, include a db-parser() statement in your syslog-ng OSE configuration file using the following syntax:

Declaration

parser <identifier> {
    db-parser(file("<database_filename>"));
};

Note that using the parser in a log statement only performs the classification, but does not automatically do anything with the results of the classification.

Example: Defining pattern databases

The following statement uses the database located at /opt/syslog-ng/var/db/patterndb.xml.

parser pattern_db {
    db-parser(
        file("/opt/syslog-ng/var/db/patterndb.xml")
    );
};

To apply the patterns on the incoming messages, include the parser in a log statement:

log {
    source(s_all);
    parser(pattern_db);
    destination( di_messages_class);
};

By default, syslog-ng OSE tries to apply the patterns to the body of the incoming messages, that is, to the value of the ${MESSAGE} macro. If you want to apply patterns to a specific field, or to an expression created from the log message (for example, using template functions or other parsers), use the message-template() option.
For example:

parser pattern_db {
    db-parser(
        file("/opt/syslog-ng/var/db/patterndb.xml")
        message-template("${MY-CUSTOM-FIELD-TO-PROCESS}")
    );
};

By default, syslog-ng OSE uses the name of the application (content of the ${PROGRAM} macro) to select which rules to apply to the message. If the content of the ${PROGRAM} macro is not the proper name of the application, you can use the program-template() option to specify it.
For example:

parser pattern_db {
    db-parser(
        file("/opt/syslog-ng/var/db/patterndb.xml")
        program-template("${MY-CUSTOM-FIELD-TO-SELECT-RULES}")
    );
};

Note that the program-template() option is available in syslog-ng OSE version 3.21 and later.

The default location of the pattern database file is /opt/syslog-ng/var/patterndb.xml (the exact path depends on the --prefix used at build time). The file option of the db-parser() statement can be used to specify a different file, thus different db-parser statements can use different pattern databases.

Example: Using classification results

The following destination separates the log messages into different files based on the class assigned to the pattern that matches the message (for example, Violation and Security type messages are stored in a separate file), and also adds the ID of the matching rule to the message:

destination di_messages_class {
    file(
        "/var/log/messages-${.classifier.class}"
        template("${.classifier.rule_id};${S_UNIXTIME};${SOURCEIP};${HOST};${PROGRAM};${PID};${MESSAGE}\n")
        template-escape(no)
    );
};

Note that if you chain pattern databases, that is, use multiple databases in the same log path, the class assigned to the message (the value of ${.classifier.class}) will be the one assigned by the last pattern database. As a result, a message might be classified as unknown even if a previous parser successfully classified it.
For example, consider the following configuration:

log {
    ...
    parser(db_parser1);
    parser(db_parser2);
    ...
};

Even if db_parser1 matches the message, db_parser2 might set ${.classifier.class} to unknown. To avoid this problem, you can use an if statement to apply the second parser only if the first parser could not classify the message:

log {
    ...
    parser{ db-parser(file("db_parser1.xml")); };
    if (match("^unknown$" value(".classifier.class"))) {
        parser { db-parser(file("db_parser2.xml")); };
    };
    ...
};

For details on how to create your own pattern databases see The syslog-ng OSE pattern database format.

Drop unmatched messages

If you want to automatically drop unmatched messages (that is, discard every message that does not match a pattern in the pattern database), use the drop-unmatched() option in the definition of the pattern database:

parser pattern_db {
    db-parser(
        file("/opt/syslog-ng/var/db/patterndb.xml")
        drop-unmatched(yes)
    );
};

Note that the drop-unmatched() option is available in syslog-ng OSE version 3.11 and later.

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