Basic Structure — rsyslog 8.24.0 documentation

# Basic Structure

This section describes how rsyslog configuration basically works. Think of rsyslog as a big logging and event processing toolset. It can be considered a framework with some basic processing that is fixed in the way data flows, but is highly customizable in the details of this message flow. During configuration, this customization is done by defining and customizing the rsyslog objects.

## Quick overview of message flow and objects

Messages enter rsyslog with the help of input modules. Then, they are passed to ruleset, where rules are conditionally applied. When a rule matches, the message is transferred to an action, which then does something to the message, e.g. writes it to a file, database or forwards it to a remote host.

## Processing Principles

- inputs submit received messages to rulesets
  - if the ruleset is not specifically bound, the default ruleset is used
- by default, there is one ruleset (RSYSLOG_DefaultRuleset)
- additional rulesets can be user-defined
- each ruleset contains zero or many rules
  - while it is permitted to have zero rules inside a ruleset, this obviously makes no sense
- a rule consists of a filter and an action list
- filters provide yes/no decisions and thus control-of-flow capability
- if a filter “matches” (filter says “yes”), the corresponding action list is executed. If it does not match, nothing special happens
- rules are evaluated in sequence from the first to the last rule inside the given ruleset. No rules from unrelated rulesets are ever executed.
- all rules are always fully evaluated, no matter if a filter matches or not (so we do not stop at the first match). If message processing shall stop, the “discard” action (represented by the tilde character or the stop command) must explicitly be executed. If discard is executed, message processing immediately stops, without evaluating any further rules.
- an action list contains one or many actions
- inside an action list no further filters are possible
- to have more than one action inside a list, the ampersand character must be placed in the position of the filter, and this must immediately follow the previous action
- actions consist of the action call itself (e.g. ”:omusrmsg:”) as well as all action-defining configuration statements ($Action... directives)
- if legacy format is used (see below), $Action... directives must be specified in front of the action they are intended to configure
- some config directives automatically refer to their previous values after being applied, others not. See the respective doc for details. Be warned that this is currently not always properly documented.
- in general, rsyslog v5 is heavily outdated and its native config language is a pain. The rsyslog project strongly recommends using at least version 7, where these problems are solved and configuration is much easier.
- legacy configuration statements (those starting with $) do not affect RainerScript objects (e.g. actions).

## Configuration File

Upon startup, rsyslog reads its configuration from the `rsyslog.conf` file by default. This file may contain references to include other config files.

A different “root” configuration file can be specified via the `-f <file>` rsyslogd command line option. This is usually done within some init script or similar facility.

## Statement Types

Rsyslog supports three different types of configuration statements concurrently:

- sysklogd - this is the plain old format, taught everywhere and still pretty useful for simple use cases. Note that some very few constructs are no longer supported because they are incompatible with newer features. These are mentioned in the compatibility docs.
- legacy rsyslog - these are statements that begin with a dollar sign. They set some configuration parameters and modify e.g. the way actions operate. This is the only format supported in pre-v6 versions of rsyslog. It is still fully supported in v6 and above. Note that some plugins and features may still only be available through legacy format (because plugins need to be explicitly upgraded to use the new style format, and this hasn’t happened to all plugins).
- RainerScript - the new style format. This is the best and most precise format to be used for more complex cases. The rest of this page assumes RainerScript based rsyslog.conf.

The rsyslog.conf files consists of statements. For old style (sysklogd & legacy rsyslog), lines do matter. For new style (RainerScript) line spacing is irrelevant. Most importantly, this means with new style actions and all other objects can split across lines as users want to.

### Recommended use of Statement Types

In general it is recommended to use RainerScript type statements, as these provide clean and easy to read control-of-flow as well as no doubt about which parameters are active. They also have no side-effects with include files, which can be a major obstacle with legacy rsyslog statements.

For very simple things sysklogd statement types are still suggested, especially if the full config consists of such simple things. The classical sample is writing to files (or forwarding) via priority. In sysklogd, this looks like:

    mail.info /var/log/mail.log
    mail.err @server.example.net

This is hard to beat in simplicity, still being taught in courses and a lot of people know this syntax. It is perfectly fine to use these constructs even in newly written config files.

As a rule of thumb, RainerScript config statements should be used when

- configuration parameters are required (e.g. the `Action...` type of legacy statements)
- more elaborate control-of-flow is required (e.g. when multiple actions must be nested under the same condition)

It is usually not recommended to use rsyslog legacy config format (those directives starting with a dollar sign). However, a few settings and modules have not yet been converted to RainerScript. In those cases, the legacy syntax must be used.

## Comments

There are two types of comments:

- #-Comments - start with a hash sign (#) and run to the end of the line
- C-style Comments - start with /* and end with */, just like in the C programming language. They can be used to comment out multiple lines at once. Comment nesting is not supported, but #-Comments can be contained inside a C-style comment.

## Processing Order

Directives are processed from the top of rsyslog.conf to the bottom. Order matters. For example, if you stop processing of a message, obviously all statements after the stop statement are never evaluated.

### Flow Control Statements

Flow control is provided by:

- Control Structures
- Filter Conditions

### Data Manipulation Statements

Data manipulation is achieved by set, unset and reset statements which are documented here in detail.

## Inputs

Every input requires an input module to be loaded and a listener defined for it. Full details can be found inside the rsyslog modules documentation. Once loaded, inputs are defined via the input() object.

## Outputs

Outputs are also called “actions”. A small set of actions is pre-loaded (like the output file writer, which is used in almost every rsyslog.conf), others must be loaded just like inputs.

An action is invoked via the action(type=”type” ...) object. Type is mandatory and must contain the name of the plugin to be called (e.g. “omfile” or “ommongodb”). Other parameters may be present. Their type and use depends on the output plugin in question.

## Rulesets and Rules

Rulesets and rules form the basis of rsyslog processing. In short, a rule is a way how rsyslog shall process a specific message. Usually, there is a type of filter (if-statement) in front of the rule. Complex nesting of rules is possible, much like in a programming language.

Rulesets are containers for rules. A single ruleset can contain many rules. In the programming language analogy, one may think of a ruleset like being a program. A ruleset can be “bound” (assigned) to a specific input. In the analogy, this means that when a message comes in via that input, the “program” (ruleset) bound to it will be executed (but not any other!).

There is detailed documentation available for rsyslog rulesets.

For quick reference, rulesets are defined as follows:

    ruleset(name="rulesetname") {
        action(type="omfile" file="/path/to/file")
        action(type="..." ...)
        /* and so on... */
    }