13.4.3. Actions and message correlation

Certain features of generating messages can be used only if message correlation is used as well. For details on correlating messages, see Section 13.3, Correlating log messages using pattern databases.

  • The syslog-ng OSE application automatically fills the fields for the generated message based on the scope of the context, for example, the HOST and PROGRAM fields if the context-scope is program.

  • When used together with message correlation, you can also refer to fields and values of earlier messages of the context by adding the @<distance-of-referenced-message-from-the-current> suffix to the macro. For details, see Section 13.3.1, Referencing earlier messages of the context.

    Example 13.15. Referencing values from an earlier message

    The following action can be used to log the length of an SSH session (the time difference between a login and a logout message in the context):

    <actions>
        <action>
            <message>
                <values>
                    <value name="MESSAGE">An SSH session for ${SSH_USERNAME}@1 from ${SSH_CLIENT_ADDRESS}@2 closed. Session lasted from ${DATE}@2 to ${DATE} </value>
                </values>
            </message>
        </action>
    </actions>
  • You can use the name-value pairs of other messages of the context. If you set the inherit-properties attribute of the generated message to context, syslog-ng OSE collects every name-value pair from each message stored in the context, and includes them in the generated message. This means that you can refer to a name-value pair without having to know which message of the context included it. If a name-value pair appears in multiple messages of the context, the value in the latest message will be used. To refer to an earlier value, use the @<distance-of-referenced-message-from-the-current> suffix format.

    <action>
        <message inherit-properties='context'>
    Example 13.16. Using the inherit-properties option

    For example, if inherit-properties is set to context, and you have a rule that collects SSH login and logout messages to the same context, you can use the following value to generate a message collecting the most important information form both messages, including the beginning and end date.

    <value name="MESSAGE">An SSH session for ${SSH_USERNAME} from ${SSH_CLIENT_ADDRESS} closed. Session lasted from ${DATE}@2 to $DATE pid: $PID.</value>

    The following is a detailed rule for this purpose.

    <patterndb version='4' pub_date='2015-04-13'>
        <ruleset name='sshd' id='12345678'>
            <pattern>sshd</pattern>
                <rules>
                    <!-- The pattern database rule for the first log message -->
                    <rule provider='me' id='12347598' class='system'
                        context-id="ssh-login-logout" context-timeout="86400"
                        context-scope="process">
                    <!-- Note the context-id that groups together the
                    relevant messages, and the context-timeout value that
                    determines how long a new message can be added to the
                    context  -->
                        <patterns>
                            <pattern>Accepted @ESTRING:SSH.AUTH_METHOD: @for @ESTRING:SSH_USERNAME: @from @ESTRING:SSH_CLIENT_ADDRESS: @port @ESTRING:: @@ANYSTRING:SSH_SERVICE@</pattern>
                            <!-- This is the actual pattern used to identify
                            the log message. The segments between the @
                            characters are parsers that recognize the variable
                            parts of the message - they can also be used as
                            macros.  -->
                        </patterns>
                    </rule>
                    <!-- The pattern database rule for the fourth log message -->
                    <rule provider='me' id='12347599' class='system' context-id="ssh-login-logout" context-scope="process">
                        <patterns>
                             <pattern>pam_unix(sshd:session): session closed for user @ANYSTRING:SSH_USERNAME@</pattern>
                        </patterns>
                        <actions>
                            <action>
                                <message inherit-properties='context'>
                                    <values>
                                        <value name="MESSAGE">An SSH session for ${SSH_USERNAME} from ${SSH_CLIENT_ADDRESS} closed. Session lasted from ${DATE}@2 to $DATE pid: $PID.</value>
                                        <value name="TRIGGER">yes</value>
                                        <!-- This is the new log message
                                        that is generated when the logout
                                        message is received. The macros ending
                                        with @2 reference values of the
                                        previous message from the context. -->
                                    </values>
                                </message>
                            </action>
                        </actions>
                    </rule>
                </rules>
        </ruleset>
    </patterndb>
  • It is possible to generate a message when the context-timeout of the original message expires and no new message is added to the context during this time. To accomplish this, include the trigger="timeout" attribute in the action element:

    <action trigger="timeout">
    Example 13.17. Sending alert when a client disappears

    The following example shows how to combine various features of syslog-ng OSE to send an e-mail alert if a client stops sending messages.

    • Configure your clients to send MARK messages periodically. It is enough to configure MARK messages for the destination that forwards your log messages to your syslog-ng OSE server (mark-mode(periodical)).

    • On your syslog-ng OSE server, create a pattern database rule that matches on the incoming MARK messages. In the rule, set the context-scope attribute to host, and the context-timeout attribute to a value that is higher than the mark-freq value set on your clients (by default, mark-freq is 1200 seconds, so set context-timeout at least to 1500 seconds, but you might want to use a higher value, depending on your environment).

    • Add an action to this rule that sends you an e-mail alert if the context-timeout expires, and the server does not receive a new MARK message (<action trigger="timeout">).

    • On your syslog-ng OSE server, use the pattern database in the log path that handles incoming log messages.