This is a part of the rsyslog.conf documentation.


Filter Conditions

Rsyslog offers three different types "filter conditions":

  • RainerScript-based filters
  • "traditional" severity and facility based selectors
  • property-based filters

RainerScript-Based Filters

RainerScript based filters are the prime means of creating complex rsyslog configuration. The permit filtering on arbitrary complex expressions, which can include boolean, arithmetic and string operations. They also support full nesting of filters, just as you know from other scripting environments.
Scripts based filters are indicated by the keyword "if", as usual. They have this format:

if expr then block else block
"If" and "then" are fixed keywords that mus be present. "expr" is a (potentially quite complex) expression. So the expression documentation for details. The keyword "else" and its associated block is optional. Note that a block can contain either a single action (chain), or an arbitrary complex script enclosed in curly braces, e.g.:
if $programname == 'prog1' then {
   action(type="omfile" file="/var/log/prog1.log")
   if $msg contains 'test' then
     action(type="omfile" file="/var/log/prog1test.log")
     action(type="omfile" file="/var/log/prog1notest.log")

Other types of filtes can also be combined with the pure RainerScript ones. This makes it particularly easy to migrate from early config files to RainerScript. Also, the traditional syslog PRI-based filters are a good and easy to use addition. While they are legacy, we still recommend there use where they are up to the job. We do NOT, however, recommend property-based filters any longer. As an example, the following is perfectly valid:
if $fromhost == 'host1' then {
   mail.* action(type="omfile" file="/var/log/host1/mail.log")
   *.err /var/log/host1/errlog # this is also still valid
   # more "old-style rules" ...
} else {
   mail.* action(type="omfile" file="/var/log/mail.log")
   *.err /var/log/errlog
   # more "old-style rules" ...

Right now, you need to specify numerical values if you would like to check for facilities and severity. These can be found in RFC 3164. If you don't like that, you can of course also use the textual property - just be sure to use the right one. As expression support is enhanced, this will change. For example, if you would like to filter on message that have facility local0, start with "DEVNAME" and have either "error1" or "error0" in their message content, you could use the following filter:

if $syslogfacility-text == 'local0' and $msg startswith 'DEVNAME' and ($msg contains 'error1' or $msg contains 'error0') then /var/log/somelog

Please note that the above must all be on one line! And if you would like to store all messages except those that contain "error1" or "error0", you just need to add a "not":

if $syslogfacility-text == 'local0' and $msg startswith 'DEVNAME' and not ($msg contains 'error1' or $msg contains 'error0') then /var/log/somelog

If you would like to do case-insensitive comparisons, use "contains_i" instead of "contains" and "startswith_i" instead of "startswith".

Regular expressions are supported via functions (see function list).


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.

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.

Property-Based Filters

Property-based filters are unique to rsyslogd. They allow to filter on any property, like HOSTNAME, syslogtag and msg. A list of all currently-supported properties can be found in the property replacer documentation (but keep in mind that only the properties, not the replacer is supported). With this filter, each properties can be checked against a specified value, using a specified compare operation.

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:

:property, [!]compare-operation, "value"

The following compare-operations are currently supported:

contains Checks if the string provided in value is contained in the property. There must be an exact match, wildcards are not supported.
isempty Checks if the property is empty. The value is discarded. This is especially useful when working with normalized data, where some fields may be populated based on normalization result. Available since 6.6.2.
isequal Compares the "value" string provided and the property contents. These two values must be exactly equal to match. The difference to contains is that contains searches for the value anywhere inside the property value, whereas all characters must be identical for isequal. As such, isequal is most useful for fields like syslogtag or FROMHOST, where you probably know the exact contents.
startswith Checks if the value is found exactly at the beginning of the property value. For example, if you search for "val" with

:msg, startswith, "val"

it will be a match if msg contains "values are in this message" but it won't match if the msg contains "There are values in this message" (in the later case, contains would match). Please note that "startswith" is by far faster than regular expressions. So it makes very much sense (performance-wise) to use "startswith".

Note: when processing syslog messages, please note that $msg usually starts with a space. The reason for this is RFC3164. Please read the detail description of what that means to you. In short, you need to make sure that you include the first space if you use "startswith", otherwise you will not get matches.

regex Compares the property against the provided POSIX BRE regular expression.
ereregex Compares the property against the provided POSIX ERE regular expression.

You can use the bang-character (!) immediately in front of a compare-operation, the outcome of this operation is negated. For example, if msg contains "This is an informative message", the following sample would not match:

:msg, contains, "error"

but this one matches:

:msg, !contains, "error"

Using negation can be useful if you would like to do some generic processing but exclude some specific events. You can use the discard action in conjunction with that. A sample would be:

*.* /var/log/allmsgs-including-informational.log
:msg, contains, "informational"  ~
*.* /var/log/allmsgs-but-informational.log

Do not overlook the red tilde in line 2! In this sample, all messages are written to the file allmsgs-including-informational.log. Then, all messages containing the string "informational" are discarded. That means the config file lines below the "discard line" (number 2 in our sample) will not be applied to this message. Then, all remaining lines will also be written to the file allmsgs-but-informational.log.

Value is a quoted string. It supports some escape sequences:

\" - the quote character (e.g. "String with \"Quotes\"")
\\ - the backslash character (e.g. "C:\\tmp")

Escape sequences always start with a backslash. Additional escape sequences might be added in the future. Backslash characters must be escaped. Any other sequence then those outlined above is invalid and may lead to unpredictable results.

Probably, "msg" is the most prominent use case of property based filters. It is the actual message text. If you would like to filter based on some message content (e.g. the presence of a specific code), this can be done easily by:

:msg, contains, "ID-4711"

This filter will match when the message contains the string "ID-4711". Please note that the comparison is case-sensitive, so it would not match if "id-4711" would be contained in the message.

:msg, regex, "fatal .* error"

This filter uses a POSIX regular expression. It matches when the string contains the words "fatal" and "error" with anything in between (e.g. "fatal net error" and "fatal lib error" but not "fatal error" as two spaces are required by the regular expression!).

Getting property-based filters right can sometimes be challenging. In order to help you do it with as minimal effort as possible, rsyslogd spits out debug information for all property-based filters during their evaluation. To enable this, run rsyslogd in foreground and specify the "-d" option.

Boolean operations inside property based filters (like 'message contains "ID17" or message contains "ID18"') are currently not supported (except for "not" as outlined above). Please note that while it is possible to query facility and severity via property-based filters, it is far more advisable to use classic selectors (see above) for those cases.

See Also

[manual index] [rsyslog.conf] [rsyslog site]

This documentation is part of the rsyslog project.
Copyright © 2008 by Rainer Gerhards and Adiscon. Released under the GNU GPL version 2 or higher.