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Correlating log messages using pattern databases

The AxoSyslog application can correlate log messages identified using pattern databases. Alternatively, you can also correlate log messages using the grouping-by() parser. For details, see Correlating messages using the grouping-by() parser.

Log messages are supposed to describe events, but applications often separate information about a single event into different log messages. For example, the Postfix email server logs the sender and recipient addresses into separate log messages, or in case of an unsuccessful login attempt, the OpenSSH server sends a log message about the authentication failure, and the reason of the failure in the next message. Of course, messages that are not so directly related can be correlated as well, for example, login-logout messages, and so on.

To correlate log messages with AxoSyslog, you can add messages into message-groups called contexts. A context consists of a series of log messages that are related to each other in some way, for example, the log messages of an SSH session can belong to the same context. As new messages come in, they may be added to a context. Also, when an incoming message is identified it can trigger actions to be performed, for example, generate a new message that contains all the important information that was stored previously in the context.

(For details on triggering actions and generating messages, see Triggering actions for identified messages.)

There are two attributes for pattern database rules that determine if a message matching the rule is added to a context: context-scope and context-id. The context-scope attribute acts as an early filter, selecting messages sent by the same process (${HOST}${PROGRAM}${PID} is identical), application (${HOST}${PROGRAM} is identical), or host, while the context-id actually adds the message to the context specified in the id. The context-id can be a simple string, or can contain macros or values extracted from the log messages for further filtering. Starting with AxoSyslog version 3.5, if a message is added to a context, AxoSyslog automatically adds the identifier of the context to the .classifier.context_id macro of the message.

Another parameter of a rule is the context-timeout attribute, which determines how long a context is stored, that is, how long AxoSyslog waits for related messages to arrive.

Note the following points about timeout values:

  • When a new message is added to a context, AxoSyslog will restart the timeout using the context-timeout set for the new message.

  • When calculating if the timeout has already expired or not, AxoSyslog uses the timestamps of the incoming messages, not system time elapsed between receiving the two messages (unless the messages do not include a timestamp, or the keep-timestamp(no) option is set). That way AxoSyslog can be used to process and correlate already existing log messages offline. However, the timestamps of the messages must be in chronological order (that is, a new message cannot be older than the one already processed), and if a message is newer than the current system time (that is, it seems to be coming from the future), AxoSyslog will replace its timestamp with the current system time.

    Example: How AxoSyslog calculates context-timeout

    Consider the following two messages:

        <38>1990-01-01T14:45:25 customhostname program6[1234]: program6 testmessage
    <38>1990-01-01T14:46:25 customhostname program6[1234]: program6 testmessage

    If the context-timeout is 10 seconds and AxoSyslog receives the messages within 1 second, the timeout event will occour immediately, because the difference of the two timestamp (60 seconds) is larger than the timeout value (10 seconds).

  • Avoid using unnecessarily long timeout values on high-traffic systems, as storing the contexts for many messages can require considerable memory. For example, if two related messages usually arrive within seconds, it is not needed to set the timeout to several hours.

Example: Using message correlation

   <rule xml:id="..." context-id="ssh-session" context-timeout="86400" context-scope="process">
        <patterns>
            <pattern>Accepted @ESTRING:usracct.authmethod: @for @ESTRING:usracct.username: @from @ESTRING:usracct.device: @port @ESTRING:: @@ANYSTRING:usracct.service@</pattern>
        </patterns>
    ...
    </rule>

For details on configuring message correlation, see the context-id, context-timeout, and context-scope attributes of pattern database rules.

1 - 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}.

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, the context-lookup, or the context-values template functions.

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.