To limit when a message is triggered, use the condition attribute and specify a filter expression: the action will be executed only if the condition is met. For example, the following action is executed only if the message was sent by the host called myhost.
<action condition="'${HOST}' == 'myhost'">
You can use the same operators in the condition that can be used in filters. For details, see Comparing macro values in filters.
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 ${DATE} </value>
</values>
</message>
</action>
</actions>
The following example triggers different actions based on the number of messages in the context. This way you can check if the context contains enough messages for the event to be complete, and execute a different action if it does not.
<actions>
<action condition='"$(context-length)" >= "4"'>
<message>
<values>
<value name="PROGRAM">event</value>
<value name="MESSAGE">Event complete</value>
</values>
</message>
</action>
<action condition='"$(context-length)" < "4"'>
<message>
<values>
<value name="PROGRAM">error</value>
<value name="MESSAGE">Error detected</value>
</values>
</message>
</action>
</actions>To perform an external action when a message is triggered, for example, to send the message in an e-mail, you have to route the generated messages to an external application using the program() destination.
The following sample configuration selects the triggered messages and sends them to an external script.
Set a field in the triggered message that is easy to identify and filter. For example:
<values>
<value name="MESSAGE">A log message from ${HOST} matched rule number $.classifier.rule_id</value>
<value name="TRIGGER">yes</value>
</values>
Create a destination that will process the triggered messages.
destination d_triggers {
program("/bin/myscript"; );
};
Create a filter that selects the triggered messages from the internal source.
filter f_triggers {
match("yes" value ("TRIGGER") type(string));
};
Create a logpath that selects the triggered messages from the internal source and sends them to the script:
log { source(s_local); filter(f_triggers); destination(d_triggers); };
Create a script that will actually process the generated messages, for example:
#!/usr/bin/perl
while (<>) {
# body of the script to send emails, snmp traps, and so on
}Certain features of generating messages can be used only if message correlation is used as well. For details on correlating messages, see 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 Referencing earlier messages of the context.
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'>
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">
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.
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