Referencing earlier messages of the context
When using the <value> element in pattern database rules 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 example, if there are three log messages in a context, and you are creating a generated message for the third log message, the ${HOST}@1 expression refers to the host field of the current (third) message in the context, the ${HOST}@2 expression refers to the host field of the previous (second) message in the context, ${PID}@3 to the PID of the first message, and so on. For example, the following message can be created from SSH login/logout messages (for details on generating new messages, see Triggering actions for identified messages): An SSH session for ${SSH_USERNAME}@1 from ${SSH_CLIENT_ADDRESS}@2 closed. Session lasted from ${DATE}@2 to ${DATE}.
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Caution:
When referencing an earlier message of the context, always enclose the field name between braces, for example, ${PID}@3. The reference will not work if you omit the braces. |
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NOTE:
To use a literal @ character in a template, use @@. |
Example: 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>If you do not know in which message of the context contains the information you need, you can use the grep template function. For details, see grep.
Example: Using the grep template function
The following example selects the message of the context that has a username name-value pair with the root value, and returns the value of the auth_method name-value pair.
$(grep ("${username}" == "root") ${auth_method})To perform calculations on fields that have numerical values, see Numerical operations.