Description
OPTIONAL — An element containing custom keywords (tags) about the messages matching the patterns. The tags can be used to label specific events (for example, user logons). It is also possible to filter on these tags later (for details, see Tagging messages). Starting with syslog-ng Premium Edition 3.2, the list of tags assigned to a message can be referenced with the ${TAGS} macro.
Children
Example
<tags><tag>UserLogin</tag></tags>
The syslog-ng PE application can correlate log messages. Alternatively, you can also correlate log messages using pattern databases. For details, see Correlating log messages using pattern databases.
The syslog-ng PE application can correlate log messages that match a set of filters. This works similarly to SQL GROUP BY statements. Alternatively, you can also correlate log messages using pattern databases. For details, see Correlating log messages using pattern databases.
Log messages are supposed to describe events, but applications often separate information about a single event into different log messages. For example, the Postfix email server logs the sender and recipient addresses into separate log messages, or in case of an unsuccessful login attempt, the OpenSSH server sends a log message about the authentication failure, and the reason of the failure in the next message. Of course, messages that are not so directly related can be correlated as well, for example, login-logout messages, and so on.
To correlate log messages with syslog-ng PE, you can add messages into message-groups called contexts. A context consists of a series of log messages that are related to each other in some way, for example, the log messages of an SSH session can belong to the same context. As new messages come in, they may be added to a context. Also, when an incoming message is identified it can trigger actions to be performed, for example, generate a new message that contains all the important information that was stored previously in the context.
How the grouping-by() parser works
The grouping-by() parser has three options that determine if a message is added to a context: scope(), key(), and where().
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The scope() option acts as an early filter, selecting messages sent by the same process (${HOST}${PROGRAM}${PID} is identical), application (${HOST}${PROGRAM} is identical), or host.
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The key() identifies the context the message belongs to. (The value of the key must be the same for every message of the context.)
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To use a filter to further limit the messages that are added to the context, you can use the where() option.
The timeout() option determines how long a context is stored, that is, how long syslog-ng PE waits for related messages to arrive. If the group has a specific log message that ends the context (for example, a logout message), you can specify it using the trigger() option.
When the context is closed, and the messages match the filter set in the having() option (or the having() option is not set), syslog-ng PE generates and sends the message set in the aggregate() option.
NOTE: Message contexts are persistent and are not lost when syslog-ng PE is reloaded (SIGHUP), but are lost when syslog-ng PE is restarted.
Declaration
parser parser_name {
grouping-by(
key()
having()
aggregate()
timeout()
);
};
For the parser to work, you must set at least the following options: key(), aggregate(), and timeout().
Note the following points about timeout values:
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When a new message is added to a context, syslog-ng PE will restart the timeout using the context-timeout set for the new message.
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When calculating if the timeout has already expired or not, syslog-ng PE 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 syslog-ng PE 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), syslog-ng PE will replace its timestamp with the current system time.
Example: How syslog-ng PE 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 syslog-ng PE receives the messages within 1 sec, the timeout event will occur immediately, because the difference of the two timestamps (60 sec) is larger than the timeout value (10 sec).
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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.
When creating the aggregated message, or in the various parameters of the grouping-by() parser, 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, 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: 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. |
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):
aggregate(
value('value name="MESSAGE" An SSH session for ${SSH_USERNAME}@1 from ${SSH_CLIENT_ADDRESS}@2 closed. Session lasted from ${DATE}@2 to ${DATE}')
)
If you do not know 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.
