Section: Linux System Administration (5)
Updated: 22 October 2012
The rsyslog.conf file is the main configuration file for the rsyslogd?(8) which logs system messages on *nix systems. This file specifies rules for logging. For special features see the rsyslogd?(8) manpage. Rsyslog.conf is backward-compatible with sysklogd's syslog.conf file. So if you migrate from sysklogd you can rename it and it should work.
Note that this version of rsyslog ships with extensive documentation in html format. This is provided in the ./doc subdirectory and probably in a separate package if you installed rsyslog via a packaging system. To use rsyslog's advanced features, you need to look at the html documentation, because the man pages only cover basic aspects of operation.
Rsyslog has a modular design. Consequently, there is a growing number of modules. See the html documentation for their full description.
Please note that the klogd daemon is no longer necessary and consequently no longer provided by the rsyslog package.
in order to receive log messages from local system processes. This config directive should only left out if you know exactly what you are doing.
Lines starting with a hash mark ('#') and empty lines are ignored. Rsyslog.conf should contain following sections (sorted by recommended order in file):
The selector field itself again consists of two parts, a facility and a priority, separated by a period ('.'). Both parts are case insensitive and can also be specified as decimal numbers, but don't do that, you have been warned. Both facilities and priorities are described in syslog?(3). The names mentioned below correspond to the similar LOG_-values in /usr/include/syslog.h.
The facility is one of the following keywords: auth, authpriv, cron, daemon, kern, lpr, mail, mark, news, security (same as auth), syslog, user, uucp and local0 through local7. The keyword security should not be used anymore and mark is only for internal use and therefore should not be used in applications. Anyway, you may want to specify and redirect these messages here. The facility specifies the subsystem that produced the message, i.e. all mail programs log with the mail facility (LOG_MAIL) if they log using syslog.
The priority is one of the following keywords, in ascending order: debug, info, notice, warning, warn (same as warning), err, error (same as err), crit, alert, emerg, panic (same as emerg). The keywords error, warn and panic are deprecated and should not be used anymore. The priority defines the severity of the message.
The behavior of the original BSD syslogd is that all messages of the specified priority and higher are logged according to the given action. Rsyslogd behaves the same, but has some extensions.
In addition to the above mentioned names the rsyslogd?(8) understands the following extensions: An asterisk ('*') stands for all facilities or all priorities, depending on where it is used (before or after the period). The keyword none stands for no priority of the given facility.
You can specify multiple facilities with the same priority pattern in one statement using the comma (',') operator. You may specify as much facilities as you want. Remember that only the facility part from such a statement is taken, a priority part would be skipped.
Multiple selectors may be specified for a single action using the semicolon (';') separator. Remember that each selector in the selector field is capable to overwrite the preceding ones. Using this behavior you can exclude some priorities from the pattern.
Rsyslogd has a syntax extension to the original BSD source, that makes its use more intuitively. You may precede every priority with an equals sign ('=') to specify only this single priority and not any of the above. You may also (both is valid, too) precede the priority with an exclamation mark ('!') to ignore all that priorities, either exact this one or this and any higher priority. If you use both extensions than the exclamation mark must occur before the equals sign, just use it intuitively.
The action field of a rule describes what to do with the message. In general, message content is written to a kind of "logfile". But also other actions might be done, like writing to a database table or forwarding to another host.
Typically messages are logged to real files. The file has to be specified with full pathname, beginning with a slash ('/').
Note: if you would like to use high-precision timestamps in your log files, just remove the ";RSYSLOG_TraditionalFormat". That will select the default template, which, if not changed, uses RFC 3339 timestamps.
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.
If the file you specified is a tty, special tty-handling is done, same with /dev/console.
There are three ways to forward message: the traditional UDP transport, which is extremely lossy but standard, the plain TCP based transport which loses messages only during certain situations but is widely available and the RELP transport which does not lose messages but is currently available only as part of rsyslogd 3.15.0 and above.
To forward messages to another host via UDP, prepend the hostname with the at sign ("@"). To forward it via plain tcp, prepend two at signs ("@@"). To forward via RELP, prepend the string ":omrelp:" in front of the hostname.
In the example above, messages are forwarded via UDP to the machine 192.168.0.1, the destination port defaults to 514. Due to the nature of UDP, you will probably lose some messages in transit. If you expect high traffic volume, you can expect to lose a quite noticeable number of messages (the higher the traffic, the more likely and severe is message loss).
If you would like to prevent message loss, use RELP:
Note that a port number was given as there is no standard port for relp.
Keep in mind that you need to load the correct input and output plugins (see "Modules" above).
Please note that rsyslogd offers a variety of options in regarding to remote forwarding. For full details, please see the html documentation.
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. You may specify more than one user by separating them with commas (','). If they're logged in they get the message (for example: ":omusrmsg:root,user1,user2").
This allows logging of the message to a database table. 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.
See the html documentation for further details on database logging.
If the discard action is carried out, the received message is immediately discarded. 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 single tilde character with no further parameters.
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.
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.
The program-to-execute can be any valid executable. It receives the template string as a single parameter (argv).
Rsyslog offers three different types "filter conditions":
* "traditional" severity and facility based selectors
* property-based filters
* expression-based filters
Selectors are the traditional way of filtering syslog messages. They have been kept in rsyslog with their original syntax, because it is well-known, highly effective and also needed for compatibility with stock syslogd configuration files. If you just need to filter based on priority and facility, you should do this with selector lines. They are not second-class citizens in rsyslog and offer the best performance for this job.
Property-based filters are unique to rsyslogd. They allow to filter on any property, like HOSTNAME, syslogtag and msg.
A property-based filter must start with a colon in column 0. This tells rsyslogd that it is the new filter type. The colon must be followed by the property name, a comma, the name of the compare operation to carry out, another comma and then the value to compare against. This value must be quoted. There can be spaces and tabs between the commas. Property names and compare operations are case-sensitive, so "msg" works, while "MSG" is an invalid property name. In brief, the syntax is as follows:
The following compare-operations are currently supported:
See the html documentation for this feature.
Every output in rsyslog uses templates - this holds true for files, user messages and so on. Templates compatible with the stock syslogd formats are hardcoded into rsyslogd. If no template is specified, we use one of these hardcoded templates. Search for "template_" in syslogd.c and you will find the hardcoded ones.
A template consists of a template directive, a name, the actual template text and optional options. A sample is:
<DD CLASS="c3|$template MyTemplateName,\\7Text some more text\\n,<options>:
The "$template" is the template directive. It tells rsyslog that this line contains a template. The backslash is an escape character. For example, \7 rings the bell (this is an ASCII value), \n is a new line. The set in rsyslog is a bit restricted currently.
All text in the template is used literally, except for things within percent signs. These are properties and allow you access to the contents of the syslog message. Properties are accessed via the property replacer and it can for example pick a substring or do date-specific formatting. More on this is the PROPERTY REPLACER section of this manpage.
% = \%
\ = \\ --> '\' is used to escape (as in C)
$template TraditionalFormat," \n"
Properties can be accessed by the property replacer (see there for details).
Please note that templates can also by used to generate selector lines with dynamic file names. For example, if you would like to split syslog messages from different hosts to different files (one per host), you can define the following template:
<DD CLASS="c3|$template DynFile,/var/log/system-.log:
This template can then be used when defining an output selector line. It will result in something like "/var/log/system-localhost.log"
The <options> part is optional. It carries options influencing the template as whole. See details below. Be sure NOT to mistake template options with property options - the later ones are processed by the property replacer and apply to a SINGLE property, only (and not the whole template).
Template options are case-insensitive. Currently defined are:
Either the sql or stdsql option MUST be specified when a template is used for writing to a database, otherwise injection might occur. Please note that due to the unfortunate fact that several vendors have violated the sql standard and introduced their own escape methods, it is impossible to have a single option doing all the work. So you yourself must make sure you are using the right format. If you choose the wrong one, you are still vulnerable to sql injection.
Please note that the database writer *checks* that the sql option is present in the template. If it is not present, the write database action is disabled. This is to guard you against accidental forgetting it and then becoming vulnerable to SQL injection. The sql option can also be useful with files - especially if you want to import them into a database on another machine for performance reasons. However, do NOT use it if you do not have a real need for it - among others, it takes some toll on the processing time. Not much, but on a really busy system you might notice it ;)
The default template for the write to database action has the sql option set.
Please note that the samples are split across multiple lines. A template MUST NOT actually be split across multiple lines.
A template that resembles traditional syslogd file output:
A template that tells you a little more about the message:
A template for RFC 3164 format:
A template for the format traditionally used for user messages:
And a template with the traditional wall-message format:
A template that can be used for writing to a database (please note the SQL template option)
NOTE 1: This template is embedded into core application under name StdDBFmt , so you don't need to define it.
NOTE 2: You have to have MySQL module installed to use this template.
Output Channels are a new concept first introduced in rsyslog 0.9.0. As of this writing, it is most likely that they will be replaced by something different in the future. So if you use them, be prepared to change you configuration file syntax when you upgrade to a later release.
Output channels are defined via an $outchannel directive. It's syntax is as follows:
<DD CLASS="c3|$outchannel name,file-name,max-size,action-on-max-size:
name is the name of the output channel (not the file), file-name is the file name to be written to, max-size the maximum allowed size and action-on-max-size a command to be issued when the max size is reached. This command always has exactly one parameter. The binary is that part of action-on-max-size before the first space, its parameter is everything behind that space.
Keep in mind that $outchannel just defines a channel with "name". It does not activate it. To do so, you must use a selector line (see below). That selector line includes the channel name plus ":omfile:$" in front of it. A sample might be:
The property replacer is a core component in rsyslogd's output system. A syslog message has a number of well-defined properties (see below). Each of this properties can be accessed and manipulated by the property replacer. With it, it is easy to use only part of a property value or manipulate the value, e.g. by converting all characters to lower case.
Syslog message properties are used inside templates. They are accessed by putting them between percent signs. Properties can be modified by the property replacer. The full syntax is as follows:
propname is the name of the property to access. It is case-sensitive.
Properties starting with a $-sign are so-called system properties. These do NOT stem from the message but are rather internally-generated.
FromChar and toChar are used to build substrings. They specify the offset within the string that should be copied. Offset counting starts at 1, so if you need to obtain the first 2 characters of the message text, you can use this syntax: "". If you do not wish to specify from and to, but you want to specify options, you still need to include the colons. For example, if you would like to convert the full message text to lower case, use "". If you would like to extract from a position until the end of the string, you can place a dollar-sign ("$") in toChar (e.g. %msg:10:$%, which will extract from position 10 to the end of the string).
There is also support for regular expressions. To use them, you need to place a "R" into FromChar. This tells rsyslog that a regular expression instead of position-based extraction is desired. The actual regular expression must then be provided in toChar. The regular expression must be followed by the string "--end". It denotes the end of the regular expression and will not become part of it. If you are using regular expressions, the property replacer will return the part of the property text that matches the regular expression. An example for a property replacer sequence with a regular expression is: "%msg:R:.*Sev:. \(.*\) \[.*--end%"
Also, extraction can be done based on so-called "fields". To do so, place a "F" into FromChar. A field in its current definition is anything that is delimited by a delimiter character. The delimiter by default is TAB (US-ASCII value 9). However, if can be changed to any other US-ASCII character by specifying a comma and the decimal US-ASCII value of the delimiter immediately after the "F". For example, to use comma (",") as a delimiter, use this field specifier: "F,44". If your syslog data is delimited, this is a quicker way to extract than via regular expressions (actually, a *much* quicker way). Field counting starts at 1. Field zero is accepted, but will always lead to a "field not found" error. The same happens if a field number higher than the number of fields in the property is requested. The field number must be placed in the "ToChar" parameter. An example where the 3rd field (delimited by TAB) from the msg property is extracted is as follows: "". The same example with semicolon as delimiter is "".
Please note that the special characters "F" and "R" are case-sensitive. Only upper case works, lower case will return an error. There are no white spaces permitted inside the sequence (that will lead to error messages and will NOT provide the intended result).
Property options are case-insensitive. Currently, the following options are defined:
Rsyslogd supports queued operations to handle offline outputs (like remote syslogd's or database servers being down). When running in queued mode, rsyslogd buffers messages to memory and optionally to disk (on an as-needed basis). Queues survive rsyslogd restarts.
It is highly suggested to use remote forwarding and database writing in queued mode, only.
To learn more about queued operations, see the html documentation.
The complete documentation can be found in the doc folder of the rsyslog distribution or online at
Please note that the man page reflects only a subset of the configuration options. Be sure to read the html documentation for all features and details. This is especially vital if you plan to set up a more-then-extremely-simple system.
rsyslogd is taken from sysklogd sources, which have been heavily modified by Rainer Gerhards ([email protected]) and others.
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