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<html><head><title>Output Channels - rsyslog.conf</title></head>
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<p>This is a part of the rsyslog.conf documentation.</p>
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<a href="rsyslog_conf.html">back</a>
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<h2>Output Channels</h2>
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<p>Output Channels are a new concept first introduced in rsyslog
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0.9.0. <b>As of this writing, it is most likely that they will
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be replaced by something different in the future.</b> So if you
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use them, be prepared to change you configuration file syntax when you
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upgrade to a later release.<br>
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The idea behind output channel definitions is that it shall provide an
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umbrella for any type of output that the user might want. In essence,<br>
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this is the "file" part of selector lines (and this is why we are not
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sure output channel syntax will stay after the next review). There is a<br>
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difference, though: selector channels both have filter conditions
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(currently facility and severity) as well as the output destination.
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they can only be used to write to files - not pipes, ttys or whatever
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Output channels define the output definition, only. As of this build,
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else. If we stick with output channels, this will change over time.</p>
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<p>In concept, an output channel includes everything needed to
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know about an output actions. In practice, the current implementation
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a filename, a maximum file size and a command to be issued when this
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file size is reached. More things might be present in future version,
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which might also change the syntax of the directive.</p>
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<p>Output channels are defined via an $outchannel directive. It's
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syntax is as follows:<br>
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$outchannel name,file-name,max-size,action-on-max-size<br>
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name is the name of the output channel (not the file), file-name is the
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file name to be written to, max-size the maximum allowed size and
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action-on-max-size a command to be issued when the max size is reached.
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This command always has exactly one parameter. The binary is that part
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of action-on-max-size before the first space, its parameter is
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everything behind that space.<br>
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Please note that max-size is queried BEFORE writing the log message to
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the file. So be sure to set this limit reasonably low so that any
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message might fit. For the current release, setting it 1k lower than
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you expected is helpful. The max-size must always be specified in bytes
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- there are no special symbols (like 1k, 1m,...) at this point of
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Keep in mind that $outchannel just defines a channel with "name". It
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does not activate it. To do so, you must use a selector line (see
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below). That selector line includes the channel name plus an $ sign in
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front of it. A sample might be:<br>
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In its current form, output channels primarily provide the ability to
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size-limit an output file. To do so, specify a maximum size. When this
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size is reached, rsyslogd will execute the action-on-max-size command
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and then reopen the file and retry. The command should be something
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like a <a href="log_rotation_fix_size.html">log rotation
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script</a> or a similar thing.</p>
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<p>If there is no action-on-max-size command or the command did
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not resolve the situation, the file is closed and never reopened by
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rsyslogd (except, of course, by huping it). This logic was integrated
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when we first experienced severe issues with files larger 2gb, which
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could lead to rsyslogd dumping core. In such cases, it is more
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appropriate to stop writing to a single file. Meanwhile, rsyslogd has
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been fixed to support files larger 2gb, but obviously only on file
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systems and operating system versions that do so. So it can still make
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sense to enforce a 2gb file size limit.</p>
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<p>[<a href="manual.html">manual index</a>]
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[<a href="rsyslog_conf.html">rsyslog.conf</a>]
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[<a href="http://www.rsyslog.com/">rsyslog site</a>]</p>
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<p><font size="2">This documentation is part of the
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<a href="http://www.rsyslog.com/">rsyslog</a> project.<br>
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Copyright © 2008 by <a href="http://www.gerhards.net/rainer">Rainer Gerhards</a> and
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<a href="http://www.adiscon.com/">Adiscon</a>. Released under the GNU GPL
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version 2 or higher.</font></p>
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