Actions

Legacy Description

Templates can be used with many actions. If used, the specified template is used to generate the message content (instead of the default template). To specify a template, write a semicolon after the action value immediately followed by the template name. Beware: templates MUST be defined BEFORE they are used. It is OK to define some templates, then use them in selector lines, define more templates and use use them in the following selector lines. But it is NOT permitted to use a template in a selector line that is above its definition. If you do this, the action will be ignored.

You can have multiple actions for a single selector (or more precisely a single filter of such a selector line). Each action must be on its own line and the line must start with an ampersand (’&’) character and have no filters. An example would be

*.=crit :omusrmsg:rger
& root
& /var/log/critmsgs

These three lines send critical messages to the user rger and root and also store them in /var/log/critmsgs. Using multiple actions per selector is convenient and also offers a performance benefit. As the filter needs to be evaluated only once, there is less computation required to process the directive compared to the otherwise-equal config directives below:

*.=crit :omusrmsg:rger
*.=crit root
*.=crit /var/log/critmsgs

Regular File

Typically messages are logged to real files. The file usually is specified by full pathname, beginning with a slash “/”. Starting with version 4.6.2 and 5.4.1 (previous v5 version do NOT support this) relative file names can also be specified. To do so, these must begin with a dot. For example, use “./file-in-current-dir.log” to specify a file in the current directory. Please note that rsyslogd usually changes its working directory to the root, so relative file names must be tested with care (they were introduced primarily as a debugging vehicle, but may have useful other applications as well). You may prefix each entry with the minus “-’’ sign to omit syncing the file after every logging. Note that you might lose information if the system crashes right behind a write attempt. Nevertheless this might give you back some performance, especially if you run programs that use logging in a very verbose manner.

If your system is connected to a reliable UPS and you receive lots of log data (e.g. firewall logs), it might be a very good idea to turn of syncing by specifying the “-” in front of the file name.

The filename can be either static(always the same) or dynamic (different based on message received). The later is useful if you would automatically split messages into different files based on some message criteria. For example, dynamic file name selectors allow you to split messages into different files based on the host that sent them. With dynamic file names, everything is automatic and you do not need any filters.

It works via the template system. First, you define a template for the file name. An example can be seen above in the description of template. We will use the “DynFile” template defined there. Dynamic filenames are indicated by specifying a questions mark “?” instead of a slash, followed by the template name. Thus, the selector line for our dynamic file name would look as follows:

*.* ?DynFile

That’s all you need to do. Rsyslog will now automatically generate file names for you and store the right messages into the right files. Please note that the minus sign also works with dynamic file name selectors. Thus, to avoid syncing, you may use

*.* -?DynFile

And of course you can use templates to specify the output format:

*.* ?DynFile;MyTemplate

A word of caution: rsyslog creates files as needed. So if a new host is using your syslog server, rsyslog will automatically create a new file for it.

Creating directories is also supported. For example you can use the hostname as directory and the program name as file name:

$template DynFile,"/var/log/%HOSTNAME%/%programname%.log"

Named Pipes

This version of rsyslogd(8) has support for logging output to named pipes (fifos). A fifo or named pipe can be used as a destination for log messages by prepending a pipe symbol (”|’’) to the name of the file. This is handy for debugging. Note that the fifo must be created with the mkfifo(1) command before rsyslogd(8) is started.

Terminal and Console

If the file you specified is a tty, special tty-handling is done, same with /dev/console.

Remote Machine

Rsyslogd provides full remote logging, i.e. is able to send messages to a remote host running rsyslogd(8) and to receive messages from remote hosts. Using this feature you’re able to control all syslog messages on one host, if all other machines will log remotely to that. This tears down administration needs.

To forward messages to another host, prepend the hostname with the at sign (“@”). A single at sign means that messages will be forwarded via UDP protocol (the standard for syslog). If you prepend two at signs (“@@”), the messages will be transmitted via TCP. Please note that plain TCP based syslog is not officially standardized, but most major syslogds support it (e.g. syslog-ng or WinSyslog). The forwarding action indicator (at-sign) can be followed by one or more options. If they are given, they must be immediately (without a space) following the final at sign and be enclosed in parenthesis. The individual options must be separated by commas. The following options are right now defined:

z<number>

Enable zlib-compression for the message. The <number> is the compression level. It can be 1 (lowest gain, lowest CPU overhead) to 9 (maximum compression, highest CPU overhead). The level can also be 0, which means “no compression”. If given, the “z” option is ignored. So this does not make an awful lot of sense. There is hardly a difference between level 1 and 9 for typical syslog messages. You can expect a compression gain between 0% and 30% for typical messages. Very chatty messages may compress up to 50%, but this is seldom seen with typically traffic. Please note that rsyslogd checks the compression gain. Messages with 60 bytes or less will never be compressed. This is because compression gain is pretty unlikely and we prefer to save CPU cycles. Messages over that size are always compressed. However, it is checked if there is a gain in compression and only if there is, the compressed message is transmitted. Otherwise, the uncompressed messages is transmitted. This saves the receiver CPU cycles for decompression. It also prevents small message to actually become larger in compressed form.

Please note that when a TCP transport is used, compression will also turn on syslog-transport-tls framing. See the “o” option for important information on the implications.

Compressed messages are automatically detected and decompressed by the receiver. There is nothing that needs to be configured on the receiver side.

o

This option is experimental. Use at your own risk and only if you know why you need it! If in doubt, do NOT turn it on.

This option is only valid for plain TCP based transports. It selects a different framing based on IETF internet draft syslog-transport-tls-06. This framing offers some benefits over traditional LF-based framing. However, the standardization effort is not yet complete. There may be changes in upcoming versions of this standard. Rsyslog will be kept in line with the standard. There is some chance that upcoming changes will be incompatible to the current specification. In this case, all systems using -transport-tls framing must be upgraded. There will be no effort made to retain compatibility between different versions of rsyslog. The primary reason for that is that it seems technically impossible to provide compatibility between some of those changes. So you should take this note very serious. It is not something we do not *like* to do (and may change our mind if enough people beg…), it is something we most probably *can not* do for technical reasons (aka: you can beg as much as you like, it won’t change anything…).

The most important implication is that compressed syslog messages via TCP must be considered with care. Unfortunately, it is technically impossible to transfer compressed records over traditional syslog plain tcp transports, so you are left with two evil choices…

The hostname may be followed by a colon and the destination port.

The following is an example selector line with forwarding:

*.*    @@(o,z9)192.168.0.1:1470

In this example, messages are forwarded via plain TCP with experimental framing and maximum compression to the host 192.168.0.1 at port 1470.

*.* @192.168.0.1

In the example above, messages are forwarded via UDP to the machine 192.168.0.1, the destination port defaults to 514. Messages will not be compressed.

Note that IPv6 addresses contain colons. So if an IPv6 address is specified in the hostname part, rsyslogd could not detect where the IP address ends and where the port starts. There is a syntax extension to support this: put square brackets around the address (e.g. “[2001::1]”). Square brackets also work with real host names and IPv4 addresses, too.

A valid sample to send messages to the IPv6 host 2001::1 at port 515 is as follows:

*.* @[2001::1]:515

This works with TCP, too.

Note to sysklogd users: sysklogd does not support RFC 3164 format, which is the default forwarding template in rsyslog. As such, you will experience duplicate hostnames if rsyslog is the sender and sysklogd is the receiver. The fix is simple: you need to use a different template. Use that one:

$template sysklogd,”<%PRI%>%TIMESTAMP% %syslogtag%%msg%\””

*.* @192.168.0.1;sysklogd

List of Users

Usually critical messages are also directed to “root’’ on that machine. You can specify a list of users that shall get the message by simply writing “:omusrmsg: followed by the login name. For example, the send messages to root, use “:omusrmsg:root”. You may specify more than one user by separating them with commas (“,’’). Do not repeat the “:omusrmsg:” prefix in this case. For example, to send data to users root and rger, use “:omusrmsg:root,rger” (do not use “:omusrmsg:root,:omusrmsg:rger”, this is invalid). If they’re logged in they get the message.

Everyone logged on

Emergency messages often go to all users currently online to notify them that something strange is happening with the system. To specify this wall(1)-feature use an asterisk as the user message destination(“:omusrmsg:*’’).

Call Plugin

This is a generic way to call an output plugin. The plugin must support this functionality. Actual parameters depend on the module, so see the module’s doc on what to supply. The general syntax is as follows:

:modname:params;template

Currently, the ommysql database output module supports this syntax (in addition to the “>” syntax it traditionally supported). For ommysql, the module name is “ommysql” and the params are the traditional ones. The ;template part is not module specific, it is generic rsyslog functionality available to all modules.

As an example, the ommysql module may be called as follows:

:ommysql:dbhost,dbname,dbuser,dbpassword;dbtemplate

For details, please see the “Database Table” section of this documentation.

Note: as of this writing, the “:modname:” part is hardcoded into the module. So the name to use is not necessarily the name the module’s plugin file is called.

Database Table

This allows logging of the message to a database table. Currently, only MariaDB/MySQL databases are supported. However, other database drivers will most probably be developed as plugins. By default, a MonitorWare-compatible schema is required for this to work. You can create that schema with the createDB.SQL file that came with the rsyslog package. You can also use any other schema of your liking - you just need to define a proper template and assign this template to the action. The database writer is called by specifying a greater-then sign (“>”) in front of the database connect information. Immediately after that sign the database host name must be given, a comma, the database name, another comma, the database user, a comma and then the user’s password. If a specific template is to be used, a semicolon followed by the template name can follow the connect information. This is as follows: >dbhost,dbname,dbuser,dbpassword;dbtemplate

Important: to use the database functionality, the MariaDB/MySQL output module must be loaded in the config file BEFORE the first database table action is used. This is done by placing the

$ModLoad ommysql

directive some place above the first use of the database write (we recommend doing at the beginning of the config file).

Discard / Stop

If the discard action is carried out, the received message is immediately discarded. No further processing of it occurs. Discard has primarily been added to filter out messages before carrying on any further processing. For obvious reasons, the results of “discard” are depending on where in the configuration file it is being used. Please note that once a message has been discarded there is no way to retrieve it in later configuration file lines.

Discard can be highly effective if you want to filter out some annoying messages that otherwise would fill your log files. To do that, place the discard actions early in your log files. This often plays well with property-based filters, giving you great freedom in specifying what you do not want.

Discard is just the word “stop” with no further parameters:

stop

For example,

*.*   stop

discards everything (ok, you can achieve the same by not running rsyslogd at all…).

Note that in legacy configuration the tilde character “~” can also be used instead of the word “stop”.

Output Channel

Binds an output channel definition (see there for details) to this action. Output channel actions must start with a $-sign, e.g. if you would like to bind your output channel definition “mychannel” to the action, use “$mychannel”. Output channels support template definitions like all all other actions.

Shell Execute

NOTE: This action is only supported for backwards compatibility. For new configs, use omprog instead. It provides a more solid and secure solution with higher performance.

This executes a program in a subshell. The program is passed the template-generated message as the only command line parameter. Rsyslog waits until the program terminates and only then continues to run.

^program-to-execute;template

The program-to-execute can be any valid executable. It receives the template string as a single parameter (argv[1]).

WARNING: The Shell Execute action was added to serve an urgent need. While it is considered reasonable save when used with some thinking, its implications must be considered. The current implementation uses a system() call to execute the command. This is not the best way to do it (and will hopefully changed in further releases). Also, proper escaping of special characters is done to prevent command injection. However, attackers always find smart ways to circumvent escaping, so we can not say if the escaping applied will really safe you from all hassles. Lastly, rsyslog will wait until the shell command terminates. Thus, a program error in it (e.g. an infinite loop) can actually disable rsyslog. Even without that, during the programs run-time no messages are processed by rsyslog. As the IP stacks buffers are quickly overflowed, this bears an increased risk of message loss. You must be aware of these implications. Even though they are severe, there are several cases where the “shell execute” action is very useful. This is the reason why we have included it in its current form. To mitigate its risks, always a) test your program thoroughly, b) make sure its runtime is as short as possible (if it requires a longer run-time, you might want to spawn your own sub-shell asynchronously), c) apply proper firewalling so that only known senders can send syslog messages to rsyslog. Point c) is especially important: if rsyslog is accepting message from any hosts, chances are much higher that an attacker might try to exploit the “shell execute” action.

Template Name

Every ACTION can be followed by a template name. If so, that template is used for message formatting. If no name is given, a hard-coded default template is used for the action. There can only be one template name for each given action. The default template is specific to each action. For a description of what a template is and what you can do with it, see the template documentation.

Copyright 2008-2023 Rainer Gerhards (Großrinderfeld), and Others.