Mail Output Module (ommail)

Module Name: ommail

Author: Rainer Gerhards <rgerhards@adiscon.com>

Description:

This module supports sending syslog messages via mail. Each syslog message is sent via its own mail. Obviously, you will want to apply rigorous filtering, otherwise your mailbox (and mail server) will be heavily spammed. The ommail plugin is primarily meant for alerting users. As such, it is assume that mails will only be sent in an extremely limited number of cases.

Please note that ommail is especially well-suited to work in tandem with imfile to watch files for the occurence of specific things to be alerted on. So its scope is far broader than forwarding syslog messages to mail recipients.

Ommail uses two templates, one for the mail body and one for the subject line. If neither is provided, a quite meaningless subject line is used and the mail body will be a syslog message just as if it were written to a file. It is expected that the users customizes both messages. In an effort to support cell phones (including SMS gateways), there is an option to turn off the body part at all. This is considered to be useful to send a short alert to a pager-like device.

It is highly recommended to use the "$ActionExecOnlyOnceEveryInterval <seconds>" directive to limit the amount of mails that potentially be generated. With it, mails are sent at most in a <seconds> interval. This may be your life safer. And remember that an hour has 3,600 seconds, so if you would like to receive mails at most once every two hours, include a "$ActionExecOnlyOnceEveryInterval 7200" immediately before the ommail action. Messages sent more frequently are simpy discarded.

Configuration Directives:

$ActionMailSMTPServer
Name or IP address of the SMTP server to be used. Must currently be set. The default is 127.0.0.1, the SMTP server on the local machine. Obviously it is not good to expect one to be present on each machine, so this value should be specified.
$ActionMailSMTPPort
Port number or name of the SMTP port to be used. The default is 25, the standard SMTP port.
$ActionMailFrom
The email address used as the senders address. There is no default.
$ActionMailTo
The recipients email address. There is no default.
$ActionMailSubject
The name of the template to be used as the mail subject. If this is not specified, a more or less meaningless mail subject is generated (we don't tell you the exact text because that can change - if you want to have something specific, configure it!).
$ActionMailEnableBody
Setting this to "off" permits to exclude the actual message body. This may be useful for pager-like devices or cell phone SMS messages. The default is "on", which is appropriate for allmost all cases. Turn it off only if you know exactly what you do!

Caveats/Known Bugs:

The current ommail implementation supports SMTP-direct mode only. In that mode, the plugin talks to the mail server via SMTP protocol. No other process is involved. This mode offers best reliability as it is not depending on any external entity except the mail server. Mail server downtime is acceptable if the action is put onto its own action queue, so that it may wait for the SMTP server to come back online. However, the module implements only the bare SMTP essentials. Most importantly, it does not provide any authentication capabilities. So your mail server must be configured to accept incoming mail from ommail without any authentication needs (this may be change in the future as need arises, but you may also be referred to sendmail-mode).

In theory, ommail should also offer a mode where it uses the sendmail utility to send its mail (sendmail-mode). This is somewhat less reliable (because we depend on an entity we do not have close control over - sendmail). It also requires dramatically more system ressources, as we need to load the external process (but that should be no problem given the expected infrequent number of calls into this plugin). The big advantage of sendmail mode is that it supports all the bells and whistles of a full-blown SMTP implementation and may even work for local delivery without a SMTP server being present. Sendmail mode will be implemented as need arises. So if you need it, please drop us a line (I nobody does, sendmail mode will probably never be implemented).

Sample:

The following sample alerts the operator if the string "hard disk fatal failure" is present inside a syslog message. The mail server at mail.example.net is used and the subject shall be "disk problem on <hostname>". Note how \r\n is included inside the body text to create line breaks. A message is sent at most once every 6 hours, any other messages are silently discarded (or, to be precise, not being forwarded - they are still being processed by the rest of the configuration file).

A more advanced example plus a discussion on using the email feature inside a reliable system can be found in Rainer's blogpost "Why is native email capability an advantage for a syslogd?"

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