Use this documentation with care! It describes
the heavily outdated version 5, which was actively
developed around 2010 and is considered dead by the
rsyslog team for many years now.
This documentation reflects the latest update of the previously existing (now removed) v5-stable branch. It describes the 5.10.2 version, which was never released. As such, it contains some content that does not apply to any released version.
To obtain the doc that properly matches your installed v5 version, obtain the doc set from your distro. Each version of rsyslog contained the version that exactly matches it.
As general advise, it is strongly suggested to upgrade to the current version supported by the rsyslog project. The current version can always be found on the right-hand side info box on the rsyslog web site.
Note that there is no rsyslog community support available for this heavily outdated version. If you need to stick with it, please ask your distribution for support.
This section describes how rsyslog configuration basically works. Think of rsyslog as a big logging and event processing toolset. It can be considered as a framework with some basic processing that is fixed in the way data flows, but is highly custumizable 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. write it to a file or database or forward it to a remote host.
- 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 of 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) must explicitely be executed. If discard is executed, message processing immediately stops, without eveluating any further rules.
- an action list constains 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)
- $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.
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 similiar facility.
Rsyslog supports two different types of configuration statements concurrently:
- sysklogd - this is the plain old format, thaught 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.
- rsyslog - these are statements that begin with a dollar sign. They set some configuration parameters and modify e.g. the way actions operate.
The rsyslog.conf files consists of statements. Each statement must be on one line (and not be split among lines).
Directives are processed from the top of rsyslog.conf to the bottom. Sequence matters. For example, if you stop processing of a message, obviously all statements after the stop statement are never evaluated.
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.
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.
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 detail documentation available for rsyslog rulesets.